capabilities(7) — Linux manual page

NAME | DESCRIPTION | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

CAPABILITIES(7)         Linux Programmer's Manual        CAPABILITIES(7)

NAME         top

       capabilities - overview of Linux capabilities

DESCRIPTION         top

       For the purpose of performing permission checks, traditional UNIX
       implementations distinguish two categories of processes:
       privileged processes (whose effective user ID is 0, referred to
       as superuser or root), and unprivileged processes (whose
       effective UID is nonzero).  Privileged processes bypass all
       kernel permission checks, while unprivileged processes are
       subject to full permission checking based on the process's
       credentials (usually: effective UID, effective GID, and
       supplementary group list).

       Starting with kernel 2.2, Linux divides the privileges
       traditionally associated with superuser into distinct units,
       known as capabilities, which can be independently enabled and
       disabled.  Capabilities are a per-thread attribute.

   Capabilities list
       The following list shows the capabilities implemented on Linux,
       and the operations or behaviors that each capability permits:

       CAP_AUDIT_CONTROL (since Linux 2.6.11)
              Enable and disable kernel auditing; change auditing filter
              rules; retrieve auditing status and filtering rules.

       CAP_AUDIT_READ (since Linux 3.16)
              Allow reading the audit log via a multicast netlink
              socket.

       CAP_AUDIT_WRITE (since Linux 2.6.11)
              Write records to kernel auditing log.

       CAP_BLOCK_SUSPEND (since Linux 3.5)
              Employ features that can block system suspend (epoll(7)
              EPOLLWAKEUP, /proc/sys/wake_lock).

       CAP_BPF (since Linux 5.8)
              Employ privileged BPF operations; see bpf(2) and
              bpf-helpers(7).

              This capability was added in Linux 5.8 to separate out BPF
              functionality from the overloaded CAP_SYS_ADMIN
              capability.

       CAP_CHECKPOINT_RESTORE (since Linux 5.9)
              * Update /proc/sys/kernel/ns_last_pid (see
                pid_namespaces(7));
              * employ the set_tid feature of clone3(2);
              * read the contents of the symbolic links in
                /proc/[pid]/map_files for other processes.

              This capability was added in Linux 5.9 to separate out
              checkpoint/restore functionality from the overloaded
              CAP_SYS_ADMIN capability.

       CAP_CHOWN
              Make arbitrary changes to file UIDs and GIDs (see
              chown(2)).

       CAP_DAC_OVERRIDE
              Bypass file read, write, and execute permission checks.
              (DAC is an abbreviation of "discretionary access
              control".)

       CAP_DAC_READ_SEARCH
              * Bypass file read permission checks and directory read
                and execute permission checks;
              * invoke open_by_handle_at(2);
              * use the linkat(2) AT_EMPTY_PATH flag to create a link to
                a file referred to by a file descriptor.

       CAP_FOWNER
              * Bypass permission checks on operations that normally
                require the filesystem UID of the process to match the
                UID of the file (e.g., chmod(2), utime(2)), excluding
                those operations covered by CAP_DAC_OVERRIDE and
                CAP_DAC_READ_SEARCH;
              * set inode flags (see ioctl_iflags(2)) on arbitrary
                files;
              * set Access Control Lists (ACLs) on arbitrary files;
              * ignore directory sticky bit on file deletion;
              * modify user extended attributes on sticky directory
                owned by any user;
              * specify O_NOATIME for arbitrary files in open(2) and
                fcntl(2).

       CAP_FSETID
              * Don't clear set-user-ID and set-group-ID mode bits when
                a file is modified;
              * set the set-group-ID bit for a file whose GID does not
                match the filesystem or any of the supplementary GIDs of
                the calling process.

       CAP_IPC_LOCK
              Lock memory (mlock(2), mlockall(2), mmap(2), shmctl(2)).

       CAP_IPC_OWNER
              Bypass permission checks for operations on System V IPC
              objects.

       CAP_KILL
              Bypass permission checks for sending signals (see
              kill(2)).  This includes use of the ioctl(2) KDSIGACCEPT
              operation.

       CAP_LEASE (since Linux 2.4)
              Establish leases on arbitrary files (see fcntl(2)).

       CAP_LINUX_IMMUTABLE
              Set the FS_APPEND_FL and FS_IMMUTABLE_FL inode flags (see
              ioctl_iflags(2)).

       CAP_MAC_ADMIN (since Linux 2.6.25)
              Allow MAC configuration or state changes.  Implemented for
              the Smack Linux Security Module (LSM).

       CAP_MAC_OVERRIDE (since Linux 2.6.25)
              Override Mandatory Access Control (MAC).  Implemented for
              the Smack LSM.

       CAP_MKNOD (since Linux 2.4)
              Create special files using mknod(2).

       CAP_NET_ADMIN
              Perform various network-related operations:
              * interface configuration;
              * administration of IP firewall, masquerading, and
                accounting;
              * modify routing tables;
              * bind to any address for transparent proxying;
              * set type-of-service (TOS);
              * clear driver statistics;
              * set promiscuous mode;
              * enabling multicasting;
              * use setsockopt(2) to set the following socket options:
                SO_DEBUG, SO_MARK, SO_PRIORITY (for a priority outside
                the range 0 to 6), SO_RCVBUFFORCE, and SO_SNDBUFFORCE.

       CAP_NET_BIND_SERVICE
              Bind a socket to Internet domain privileged ports (port
              numbers less than 1024).

       CAP_NET_BROADCAST
              (Unused)  Make socket broadcasts, and listen to
              multicasts.

       CAP_NET_RAW
              * Use RAW and PACKET sockets;
              * bind to any address for transparent proxying.

       CAP_PERFMON (since Linux 5.8)
              Employ various performance-monitoring mechanisms,
              including:

              * call perf_event_open(2);
              * employ various BPF operations that have performance
                implications.

              This capability was added in Linux 5.8 to separate out
              performance monitoring functionality from the overloaded
              CAP_SYS_ADMIN capability.  See also the kernel source file
              Documentation/admin-guide/perf-security.rst.

       CAP_SETGID
              * Make arbitrary manipulations of process GIDs and
                supplementary GID list;
              * forge GID when passing socket credentials via UNIX
                domain sockets;
              * write a group ID mapping in a user namespace (see
                user_namespaces(7)).

       CAP_SETFCAP (since Linux 2.6.24)
              Set arbitrary capabilities on a file.

       CAP_SETPCAP
              If file capabilities are supported (i.e., since Linux
              2.6.24): add any capability from the calling thread's
              bounding set to its inheritable set; drop capabilities
              from the bounding set (via prctl(2) PR_CAPBSET_DROP); make
              changes to the securebits flags.

              If file capabilities are not supported (i.e., kernels
              before Linux 2.6.24): grant or remove any capability in
              the caller's permitted capability set to or from any other
              process.  (This property of CAP_SETPCAP is not available
              when the kernel is configured to support file
              capabilities, since CAP_SETPCAP has entirely different
              semantics for such kernels.)

       CAP_SETUID
              * Make arbitrary manipulations of process UIDs (setuid(2),
                setreuid(2), setresuid(2), setfsuid(2));
              * forge UID when passing socket credentials via UNIX
                domain sockets;
              * write a user ID mapping in a user namespace (see
                user_namespaces(7)).

       CAP_SYS_ADMIN
              Note: this capability is overloaded; see Notes to kernel
              developers, below.

              * Perform a range of system administration operations
                including: quotactl(2), mount(2), umount(2),
                pivot_root(2), swapon(2), swapoff(2), sethostname(2),
                and setdomainname(2);
              * perform privileged syslog(2) operations (since Linux
                2.6.37, CAP_SYSLOG should be used to permit such
                operations);
              * perform VM86_REQUEST_IRQ vm86(2) command;
              * access the same checkpoint/restore functionality that is
                governed by CAP_CHECKPOINT_RESTORE (but the latter,
                weaker capability is preferred for accessing that
                functionality).
              * perform the same BPF operations as are governed by
                CAP_BPF (but the latter, weaker capability is preferred
                for accessing that functionality).
              * employ the same performance monitoring mechanisms as are
                governed by CAP_PERFMON (but the latter, weaker
                capability is preferred for accessing that
                functionality).
              * perform IPC_SET and IPC_RMID operations on arbitrary
                System V IPC objects;
              * override RLIMIT_NPROC resource limit;
              * perform operations on trusted and security extended
                attributes (see xattr(7));
              * use lookup_dcookie(2);
              * use ioprio_set(2) to assign IOPRIO_CLASS_RT and (before
                Linux 2.6.25) IOPRIO_CLASS_IDLE I/O scheduling classes;
              * forge PID when passing socket credentials via UNIX
                domain sockets;
              * exceed /proc/sys/fs/file-max, the system-wide limit on
                the number of open files, in system calls that open
                files (e.g., accept(2), execve(2), open(2), pipe(2));
              * employ CLONE_* flags that create new namespaces with
                clone(2) and unshare(2) (but, since Linux 3.8, creating
                user namespaces does not require any capability);
              * access privileged perf event information;
              * call setns(2) (requires CAP_SYS_ADMIN in the target
                namespace);
              * call fanotify_init(2);
              * perform privileged KEYCTL_CHOWN and KEYCTL_SETPERM
                keyctl(2) operations;
              * perform madvise(2) MADV_HWPOISON operation;
              * employ the TIOCSTI ioctl(2) to insert characters into
                the input queue of a terminal other than the caller's
                controlling terminal;
              * employ the obsolete nfsservctl(2) system call;
              * employ the obsolete bdflush(2) system call;
              * perform various privileged block-device ioctl(2)
                operations;
              * perform various privileged filesystem ioctl(2)
                operations;
              * perform privileged ioctl(2) operations on the
                /dev/random device (see random(4));
              * install a seccomp(2) filter without first having to set
                the no_new_privs thread attribute;
              * modify allow/deny rules for device control groups;
              * employ the ptrace(2) PTRACE_SECCOMP_GET_FILTER operation
                to dump tracee's seccomp filters;
              * employ the ptrace(2) PTRACE_SETOPTIONS operation to
                suspend the tracee's seccomp protections (i.e., the
                PTRACE_O_SUSPEND_SECCOMP flag);
              * perform administrative operations on many device
                drivers;
              * modify autogroup nice values by writing to
                /proc/[pid]/autogroup (see sched(7)).

       CAP_SYS_BOOT
              Use reboot(2) and kexec_load(2).

       CAP_SYS_CHROOT
              * Use chroot(2);
              * change mount namespaces using setns(2).

       CAP_SYS_MODULE
              * Load and unload kernel modules (see init_module(2) and
                delete_module(2));
              * in kernels before 2.6.25: drop capabilities from the
                system-wide capability bounding set.

       CAP_SYS_NICE
              * Lower the process nice value (nice(2), setpriority(2))
                and change the nice value for arbitrary processes;
              * set real-time scheduling policies for calling process,
                and set scheduling policies and priorities for arbitrary
                processes (sched_setscheduler(2), sched_setparam(2),
                sched_setattr(2));
              * set CPU affinity for arbitrary processes
                (sched_setaffinity(2));
              * set I/O scheduling class and priority for arbitrary
                processes (ioprio_set(2));
              * apply migrate_pages(2) to arbitrary processes and allow
                processes to be migrated to arbitrary nodes;
              * apply move_pages(2) to arbitrary processes;
              * use the MPOL_MF_MOVE_ALL flag with mbind(2) and
                move_pages(2).

       CAP_SYS_PACCT
              Use acct(2).

       CAP_SYS_PTRACE
              * Trace arbitrary processes using ptrace(2);
              * apply get_robust_list(2) to arbitrary processes;
              * transfer data to or from the memory of arbitrary
                processes using process_vm_readv(2) and
                process_vm_writev(2);
              * inspect processes using kcmp(2).

       CAP_SYS_RAWIO
              * Perform I/O port operations (iopl(2) and ioperm(2));
              * access /proc/kcore;
              * employ the FIBMAP ioctl(2) operation;
              * open devices for accessing x86 model-specific registers
                (MSRs, see msr(4));
              * update /proc/sys/vm/mmap_min_addr;
              * create memory mappings at addresses below the value
                specified by /proc/sys/vm/mmap_min_addr;
              * map files in /proc/bus/pci;
              * open /dev/mem and /dev/kmem;
              * perform various SCSI device commands;
              * perform certain operations on hpsa(4) and cciss(4)
                devices;
              * perform a range of device-specific operations on other
                devices.

       CAP_SYS_RESOURCE
              * Use reserved space on ext2 filesystems;
              * make ioctl(2) calls controlling ext3 journaling;
              * override disk quota limits;
              * increase resource limits (see setrlimit(2));
              * override RLIMIT_NPROC resource limit;
              * override maximum number of consoles on console
                allocation;
              * override maximum number of keymaps;
              * allow more than 64hz interrupts from the real-time
                clock;
              * raise msg_qbytes limit for a System V message queue
                above the limit in /proc/sys/kernel/msgmnb (see msgop(2)
                and msgctl(2));
              * allow the RLIMIT_NOFILE resource limit on the number of
                "in-flight" file descriptors to be bypassed when passing
                file descriptors to another process via a UNIX domain
                socket (see unix(7));
              * override the /proc/sys/fs/pipe-size-max limit when
                setting the capacity of a pipe using the F_SETPIPE_SZ
                fcntl(2) command;
              * use F_SETPIPE_SZ to increase the capacity of a pipe
                above the limit specified by /proc/sys/fs/pipe-max-size;
              * override /proc/sys/fs/mqueue/queues_max,
                /proc/sys/fs/mqueue/msg_max, and
                /proc/sys/fs/mqueue/msgsize_max limits when creating
                POSIX message queues (see mq_overview(7));
              * employ the prctl(2) PR_SET_MM operation;
              * set /proc/[pid]/oom_score_adj to a value lower than the
                value last set by a process with CAP_SYS_RESOURCE.

       CAP_SYS_TIME
              Set system clock (settimeofday(2), stime(2), adjtimex(2));
              set real-time (hardware) clock.

       CAP_SYS_TTY_CONFIG
              Use vhangup(2); employ various privileged ioctl(2)
              operations on virtual terminals.

       CAP_SYSLOG (since Linux 2.6.37)
              * Perform privileged syslog(2) operations.  See syslog(2)
                for information on which operations require privilege.
              * View kernel addresses exposed via /proc and other
                interfaces when /proc/sys/kernel/kptr_restrict has the
                value 1.  (See the discussion of the kptr_restrict in
                proc(5).)

       CAP_WAKE_ALARM (since Linux 3.0)
              Trigger something that will wake up the system (set
              CLOCK_REALTIME_ALARM and CLOCK_BOOTTIME_ALARM timers).

   Past and current implementation
       A full implementation of capabilities requires that:

       1. For all privileged operations, the kernel must check whether
          the thread has the required capability in its effective set.

       2. The kernel must provide system calls allowing a thread's
          capability sets to be changed and retrieved.

       3. The filesystem must support attaching capabilities to an
          executable file, so that a process gains those capabilities
          when the file is executed.

       Before kernel 2.6.24, only the first two of these requirements
       are met; since kernel 2.6.24, all three requirements are met.

   Notes to kernel developers
       When adding a new kernel feature that should be governed by a
       capability, consider the following points.

       *  The goal of capabilities is divide the power of superuser into
          pieces, such that if a program that has one or more
          capabilities is compromised, its power to do damage to the
          system would be less than the same program running with root
          privilege.

       *  You have the choice of either creating a new capability for
          your new feature, or associating the feature with one of the
          existing capabilities.  In order to keep the set of
          capabilities to a manageable size, the latter option is
          preferable, unless there are compelling reasons to take the
          former option.  (There is also a technical limit: the size of
          capability sets is currently limited to 64 bits.)

       *  To determine which existing capability might best be
          associated with your new feature, review the list of
          capabilities above in order to find a "silo" into which your
          new feature best fits.  One approach to take is to determine
          if there are other features requiring capabilities that will
          always be used along with the new feature.  If the new feature
          is useless without these other features, you should use the
          same capability as the other features.

       *  Don't choose CAP_SYS_ADMIN if you can possibly avoid it!  A
          vast proportion of existing capability checks are associated
          with this capability (see the partial list above).  It can
          plausibly be called "the new root", since on the one hand, it
          confers a wide range of powers, and on the other hand, its
          broad scope means that this is the capability that is required
          by many privileged programs.  Don't make the problem worse.
          The only new features that should be associated with
          CAP_SYS_ADMIN are ones that closely match existing uses in
          that silo.

       *  If you have determined that it really is necessary to create a
          new capability for your feature, don't make or name it as a
          "single-use" capability.  Thus, for example, the addition of
          the highly specific CAP_SYS_PACCT was probably a mistake.
          Instead, try to identify and name your new capability as a
          broader silo into which other related future use cases might
          fit.

   Thread capability sets
       Each thread has the following capability sets containing zero or
       more of the above capabilities:

       Permitted
              This is a limiting superset for the effective capabilities
              that the thread may assume.  It is also a limiting
              superset for the capabilities that may be added to the
              inheritable set by a thread that does not have the
              CAP_SETPCAP capability in its effective set.

              If a thread drops a capability from its permitted set, it
              can never reacquire that capability (unless it execve(2)s
              either a set-user-ID-root program, or a program whose
              associated file capabilities grant that capability).

       Inheritable
              This is a set of capabilities preserved across an
              execve(2).  Inheritable capabilities remain inheritable
              when executing any program, and inheritable capabilities
              are added to the permitted set when executing a program
              that has the corresponding bits set in the file
              inheritable set.

              Because inheritable capabilities are not generally
              preserved across execve(2) when running as a non-root
              user, applications that wish to run helper programs with
              elevated capabilities should consider using ambient
              capabilities, described below.

       Effective
              This is the set of capabilities used by the kernel to
              perform permission checks for the thread.

       Bounding (per-thread since Linux 2.6.25)
              The capability bounding set is a mechanism that can be
              used to limit the capabilities that are gained during
              execve(2).

              Since Linux 2.6.25, this is a per-thread capability set.
              In older kernels, the capability bounding set was a system
              wide attribute shared by all threads on the system.

              For more details on the capability bounding set, see
              below.

       Ambient (since Linux 4.3)
              This is a set of capabilities that are preserved across an
              execve(2) of a program that is not privileged.  The
              ambient capability set obeys the invariant that no
              capability can ever be ambient if it is not both permitted
              and inheritable.

              The ambient capability set can be directly modified using
              prctl(2).  Ambient capabilities are automatically lowered
              if either of the corresponding permitted or inheritable
              capabilities is lowered.

              Executing a program that changes UID or GID due to the
              set-user-ID or set-group-ID bits or executing a program
              that has any file capabilities set will clear the ambient
              set.  Ambient capabilities are added to the permitted set
              and assigned to the effective set when execve(2) is
              called.  If ambient capabilities cause a process's
              permitted and effective capabilities to increase during an
              execve(2), this does not trigger the secure-execution mode
              described in ld.so(8).

       A child created via fork(2) inherits copies of its parent's
       capability sets.  See below for a discussion of the treatment of
       capabilities during execve(2).

       Using capset(2), a thread may manipulate its own capability sets
       (see below).

       Since Linux 3.2, the file /proc/sys/kernel/cap_last_cap exposes
       the numerical value of the highest capability supported by the
       running kernel; this can be used to determine the highest bit
       that may be set in a capability set.

   File capabilities
       Since kernel 2.6.24, the kernel supports associating capability
       sets with an executable file using setcap(8).  The file
       capability sets are stored in an extended attribute (see
       setxattr(2) and xattr(7)) named security.capability.  Writing to
       this extended attribute requires the CAP_SETFCAP capability.  The
       file capability sets, in conjunction with the capability sets of
       the thread, determine the capabilities of a thread after an
       execve(2).

       The three file capability sets are:

       Permitted (formerly known as forced):
              These capabilities are automatically permitted to the
              thread, regardless of the thread's inheritable
              capabilities.

       Inheritable (formerly known as allowed):
              This set is ANDed with the thread's inheritable set to
              determine which inheritable capabilities are enabled in
              the permitted set of the thread after the execve(2).

       Effective:
              This is not a set, but rather just a single bit.  If this
              bit is set, then during an execve(2) all of the new
              permitted capabilities for the thread are also raised in
              the effective set.  If this bit is not set, then after an
              execve(2), none of the new permitted capabilities is in
              the new effective set.

              Enabling the file effective capability bit implies that
              any file permitted or inheritable capability that causes a
              thread to acquire the corresponding permitted capability
              during an execve(2) (see the transformation rules
              described below) will also acquire that capability in its
              effective set.  Therefore, when assigning capabilities to
              a file (setcap(8), cap_set_file(3), cap_set_fd(3)), if we
              specify the effective flag as being enabled for any
              capability, then the effective flag must also be specified
              as enabled for all other capabilities for which the
              corresponding permitted or inheritable flags is enabled.

   File capability extended attribute versioning
       To allow extensibility, the kernel supports a scheme to encode a
       version number inside the security.capability extended attribute
       that is used to implement file capabilities.  These version
       numbers are internal to the implementation, and not directly
       visible to user-space applications.  To date, the following
       versions are supported:

       VFS_CAP_REVISION_1
              This was the original file capability implementation,
              which supported 32-bit masks for file capabilities.

       VFS_CAP_REVISION_2 (since Linux 2.6.25)
              This version allows for file capability masks that are 64
              bits in size, and was necessary as the number of supported
              capabilities grew beyond 32.  The kernel transparently
              continues to support the execution of files that have
              32-bit version 1 capability masks, but when adding
              capabilities to files that did not previously have
              capabilities, or modifying the capabilities of existing
              files, it automatically uses the version 2 scheme (or
              possibly the version 3 scheme, as described below).

       VFS_CAP_REVISION_3 (since Linux 4.14)
              Version 3 file capabilities are provided to support
              namespaced file capabilities (described below).

              As with version 2 file capabilities, version 3 capability
              masks are 64 bits in size.  But in addition, the root user
              ID of namespace is encoded in the security.capability
              extended attribute.  (A namespace's root user ID is the
              value that user ID 0 inside that namespace maps to in the
              initial user namespace.)

              Version 3 file capabilities are designed to coexist with
              version 2 capabilities; that is, on a modern Linux system,
              there may be some files with version 2 capabilities while
              others have version 3 capabilities.

       Before Linux 4.14, the only kind of file capability extended
       attribute that could be attached to a file was a
       VFS_CAP_REVISION_2 attribute.  Since Linux 4.14, the version of
       the security.capability extended attribute that is attached to a
       file depends on the circumstances in which the attribute was
       created.

       Starting with Linux 4.14, a security.capability extended
       attribute is automatically created as (or converted to) a version
       3 (VFS_CAP_REVISION_3) attribute if both of the following are
       true:

       (1) The thread writing the attribute resides in a noninitial user
           namespace.  (More precisely: the thread resides in a user
           namespace other than the one from which the underlying
           filesystem was mounted.)

       (2) The thread has the CAP_SETFCAP capability over the file
           inode, meaning that (a) the thread has the CAP_SETFCAP
           capability in its own user namespace; and (b) the UID and GID
           of the file inode have mappings in the writer's user
           namespace.

       When a VFS_CAP_REVISION_3 security.capability extended attribute
       is created, the root user ID of the creating thread's user
       namespace is saved in the extended attribute.

       By contrast, creating or modifying a security.capability extended
       attribute from a privileged (CAP_SETFCAP) thread that resides in
       the namespace where the underlying filesystem was mounted (this
       normally means the initial user namespace) automatically results
       in the creation of a version 2 (VFS_CAP_REVISION_2) attribute.

       Note that the creation of a version 3 security.capability
       extended attribute is automatic.  That is to say, when a user-
       space application writes (setxattr(2)) a security.capability
       attribute in the version 2 format, the kernel will automatically
       create a version 3 attribute if the attribute is created in the
       circumstances described above.  Correspondingly, when a version 3
       security.capability attribute is retrieved (getxattr(2)) by a
       process that resides inside a user namespace that was created by
       the root user ID (or a descendant of that user namespace), the
       returned attribute is (automatically) simplified to appear as a
       version 2 attribute (i.e., the returned value is the size of a
       version 2 attribute and does not include the root user ID).
       These automatic translations mean that no changes are required to
       user-space tools (e.g., setcap(1) and getcap(1)) in order for
       those tools to be used to create and retrieve version 3
       security.capability attributes.

       Note that a file can have either a version 2 or a version 3
       security.capability extended attribute associated with it, but
       not both: creation or modification of the security.capability
       extended attribute will automatically modify the version
       according to the circumstances in which the extended attribute is
       created or modified.

   Transformation of capabilities during execve()
       During an execve(2), the kernel calculates the new capabilities
       of the process using the following algorithm:

           P'(ambient)     = (file is privileged) ? 0 : P(ambient)

           P'(permitted)   = (P(inheritable) & F(inheritable)) |
                             (F(permitted) & P(bounding)) | P'(ambient)

           P'(effective)   = F(effective) ? P'(permitted) : P'(ambient)

           P'(inheritable) = P(inheritable)    [i.e., unchanged]

           P'(bounding)    = P(bounding)       [i.e., unchanged]

       where:

           P()   denotes the value of a thread capability set before the
                 execve(2)

           P'()  denotes the value of a thread capability set after the
                 execve(2)

           F()   denotes a file capability set

       Note the following details relating to the above capability
       transformation rules:

       *  The ambient capability set is present only since Linux 4.3.
          When determining the transformation of the ambient set during
          execve(2), a privileged file is one that has capabilities or
          has the set-user-ID or set-group-ID bit set.

       *  Prior to Linux 2.6.25, the bounding set was a system-wide
          attribute shared by all threads.  That system-wide value was
          employed to calculate the new permitted set during execve(2)
          in the same manner as shown above for P(bounding).

       Note: during the capability transitions described above, file
       capabilities may be ignored (treated as empty) for the same
       reasons that the set-user-ID and set-group-ID bits are ignored;
       see execve(2).  File capabilities are similarly ignored if the
       kernel was booted with the no_file_caps option.

       Note: according to the rules above, if a process with nonzero
       user IDs performs an execve(2) then any capabilities that are
       present in its permitted and effective sets will be cleared.  For
       the treatment of capabilities when a process with a user ID of
       zero performs an execve(2), see below under Capabilities and
       execution of programs by root.

   Safety checking for capability-dumb binaries
       A capability-dumb binary is an application that has been marked
       to have file capabilities, but has not been converted to use the
       libcap(3) API to manipulate its capabilities.  (In other words,
       this is a traditional set-user-ID-root program that has been
       switched to use file capabilities, but whose code has not been
       modified to understand capabilities.)  For such applications, the
       effective capability bit is set on the file, so that the file
       permitted capabilities are automatically enabled in the process
       effective set when executing the file.  The kernel recognizes a
       file which has the effective capability bit set as capability-
       dumb for the purpose of the check described here.

       When executing a capability-dumb binary, the kernel checks if the
       process obtained all permitted capabilities that were specified
       in the file permitted set, after the capability transformations
       described above have been performed.  (The typical reason why
       this might not occur is that the capability bounding set masked
       out some of the capabilities in the file permitted set.)  If the
       process did not obtain the full set of file permitted
       capabilities, then execve(2) fails with the error EPERM.  This
       prevents possible security risks that could arise when a
       capability-dumb application is executed with less privilege that
       it needs.  Note that, by definition, the application could not
       itself recognize this problem, since it does not employ the
       libcap(3) API.

   Capabilities and execution of programs by root
       In order to mirror traditional UNIX semantics, the kernel
       performs special treatment of file capabilities when a process
       with UID 0 (root) executes a program and when a set-user-ID-root
       program is executed.

       After having performed any changes to the process effective ID
       that were triggered by the set-user-ID mode bit of the binary—
       e.g., switching the effective user ID to 0 (root) because a set-
       user-ID-root program was executed—the kernel calculates the file
       capability sets as follows:

       1. If the real or effective user ID of the process is 0 (root),
          then the file inheritable and permitted sets are ignored;
          instead they are notionally considered to be all ones (i.e.,
          all capabilities enabled).  (There is one exception to this
          behavior, described below in Set-user-ID-root programs that
          have file capabilities.)

       2. If the effective user ID of the process is 0 (root) or the
          file effective bit is in fact enabled, then the file effective
          bit is notionally defined to be one (enabled).

       These notional values for the file's capability sets are then
       used as described above to calculate the transformation of the
       process's capabilities during execve(2).

       Thus, when a process with nonzero UIDs execve(2)s a set-user-ID-
       root program that does not have capabilities attached, or when a
       process whose real and effective UIDs are zero execve(2)s a
       program, the calculation of the process's new permitted
       capabilities simplifies to:

           P'(permitted)   = P(inheritable) | P(bounding)

           P'(effective)   = P'(permitted)

       Consequently, the process gains all capabilities in its permitted
       and effective capability sets, except those masked out by the
       capability bounding set.  (In the calculation of P'(permitted),
       the P'(ambient) term can be simplified away because it is by
       definition a proper subset of P(inheritable).)

       The special treatments of user ID 0 (root) described in this
       subsection can be disabled using the securebits mechanism
       described below.

   Set-user-ID-root programs that have file capabilities
       There is one exception to the behavior described under
       Capabilities and execution of programs by root.  If (a) the
       binary that is being executed has capabilities attached and (b)
       the real user ID of the process is not 0 (root) and (c) the
       effective user ID of the process is 0 (root), then the file
       capability bits are honored (i.e., they are not notionally
       considered to be all ones).  The usual way in which this
       situation can arise is when executing a set-UID-root program that
       also has file capabilities.  When such a program is executed, the
       process gains just the capabilities granted by the program (i.e.,
       not all capabilities, as would occur when executing a set-user-
       ID-root program that does not have any associated file
       capabilities).

       Note that one can assign empty capability sets to a program file,
       and thus it is possible to create a set-user-ID-root program that
       changes the effective and saved set-user-ID of the process that
       executes the program to 0, but confers no capabilities to that
       process.

   Capability bounding set
       The capability bounding set is a security mechanism that can be
       used to limit the capabilities that can be gained during an
       execve(2).  The bounding set is used in the following ways:

       * During an execve(2), the capability bounding set is ANDed with
         the file permitted capability set, and the result of this
         operation is assigned to the thread's permitted capability set.
         The capability bounding set thus places a limit on the
         permitted capabilities that may be granted by an executable
         file.

       * (Since Linux 2.6.25) The capability bounding set acts as a
         limiting superset for the capabilities that a thread can add to
         its inheritable set using capset(2).  This means that if a
         capability is not in the bounding set, then a thread can't add
         this capability to its inheritable set, even if it was in its
         permitted capabilities, and thereby cannot have this capability
         preserved in its permitted set when it execve(2)s a file that
         has the capability in its inheritable set.

       Note that the bounding set masks the file permitted capabilities,
       but not the inheritable capabilities.  If a thread maintains a
       capability in its inheritable set that is not in its bounding
       set, then it can still gain that capability in its permitted set
       by executing a file that has the capability in its inheritable
       set.

       Depending on the kernel version, the capability bounding set is
       either a system-wide attribute, or a per-process attribute.

       Capability bounding set from Linux 2.6.25 onward

       From Linux 2.6.25, the capability bounding set is a per-thread
       attribute.  (The system-wide capability bounding set described
       below no longer exists.)

       The bounding set is inherited at fork(2) from the thread's
       parent, and is preserved across an execve(2).

       A thread may remove capabilities from its capability bounding set
       using the prctl(2) PR_CAPBSET_DROP operation, provided it has the
       CAP_SETPCAP capability.  Once a capability has been dropped from
       the bounding set, it cannot be restored to that set.  A thread
       can determine if a capability is in its bounding set using the
       prctl(2) PR_CAPBSET_READ operation.

       Removing capabilities from the bounding set is supported only if
       file capabilities are compiled into the kernel.  In kernels
       before Linux 2.6.33, file capabilities were an optional feature
       configurable via the CONFIG_SECURITY_FILE_CAPABILITIES option.
       Since Linux 2.6.33, the configuration option has been removed and
       file capabilities are always part of the kernel.  When file
       capabilities are compiled into the kernel, the init process (the
       ancestor of all processes) begins with a full bounding set.  If
       file capabilities are not compiled into the kernel, then init
       begins with a full bounding set minus CAP_SETPCAP, because this
       capability has a different meaning when there are no file
       capabilities.

       Removing a capability from the bounding set does not remove it
       from the thread's inheritable set.  However it does prevent the
       capability from being added back into the thread's inheritable
       set in the future.

       Capability bounding set prior to Linux 2.6.25

       In kernels before 2.6.25, the capability bounding set is a
       system-wide attribute that affects all threads on the system.
       The bounding set is accessible via the file /proc/sys/kernel/cap-
       bound.  (Confusingly, this bit mask parameter is expressed as a
       signed decimal number in /proc/sys/kernel/cap-bound.)

       Only the init process may set capabilities in the capability
       bounding set; other than that, the superuser (more precisely: a
       process with the CAP_SYS_MODULE capability) may only clear
       capabilities from this set.

       On a standard system the capability bounding set always masks out
       the CAP_SETPCAP capability.  To remove this restriction
       (dangerous!), modify the definition of CAP_INIT_EFF_SET in
       include/linux/capability.h and rebuild the kernel.

       The system-wide capability bounding set feature was added to
       Linux starting with kernel version 2.2.11.

   Effect of user ID changes on capabilities
       To preserve the traditional semantics for transitions between 0
       and nonzero user IDs, the kernel makes the following changes to a
       thread's capability sets on changes to the thread's real,
       effective, saved set, and filesystem user IDs (using setuid(2),
       setresuid(2), or similar):

       1. If one or more of the real, effective or saved set user IDs
          was previously 0, and as a result of the UID changes all of
          these IDs have a nonzero value, then all capabilities are
          cleared from the permitted, effective, and ambient capability
          sets.

       2. If the effective user ID is changed from 0 to nonzero, then
          all capabilities are cleared from the effective set.

       3. If the effective user ID is changed from nonzero to 0, then
          the permitted set is copied to the effective set.

       4. If the filesystem user ID is changed from 0 to nonzero (see
          setfsuid(2)), then the following capabilities are cleared from
          the effective set: CAP_CHOWN, CAP_DAC_OVERRIDE,
          CAP_DAC_READ_SEARCH, CAP_FOWNER, CAP_FSETID,
          CAP_LINUX_IMMUTABLE (since Linux 2.6.30), CAP_MAC_OVERRIDE,
          and CAP_MKNOD (since Linux 2.6.30).  If the filesystem UID is
          changed from nonzero to 0, then any of these capabilities that
          are enabled in the permitted set are enabled in the effective
          set.

       If a thread that has a 0 value for one or more of its user IDs
       wants to prevent its permitted capability set being cleared when
       it resets all of its user IDs to nonzero values, it can do so
       using the SECBIT_KEEP_CAPS securebits flag described below.

   Programmatically adjusting capability sets
       A thread can retrieve and change its permitted, effective, and
       inheritable capability sets using the capget(2) and capset(2)
       system calls.  However, the use of cap_get_proc(3) and
       cap_set_proc(3), both provided in the libcap package, is
       preferred for this purpose.  The following rules govern changes
       to the thread capability sets:

       1. If the caller does not have the CAP_SETPCAP capability, the
          new inheritable set must be a subset of the combination of the
          existing inheritable and permitted sets.

       2. (Since Linux 2.6.25) The new inheritable set must be a subset
          of the combination of the existing inheritable set and the
          capability bounding set.

       3. The new permitted set must be a subset of the existing
          permitted set (i.e., it is not possible to acquire permitted
          capabilities that the thread does not currently have).

       4. The new effective set must be a subset of the new permitted
          set.

   The securebits flags: establishing a capabilities-only environment
       Starting with kernel 2.6.26, and with a kernel in which file
       capabilities are enabled, Linux implements a set of per-thread
       securebits flags that can be used to disable special handling of
       capabilities for UID 0 (root).  These flags are as follows:

       SECBIT_KEEP_CAPS
              Setting this flag allows a thread that has one or more 0
              UIDs to retain capabilities in its permitted set when it
              switches all of its UIDs to nonzero values.  If this flag
              is not set, then such a UID switch causes the thread to
              lose all permitted capabilities.  This flag is always
              cleared on an execve(2).

              Note that even with the SECBIT_KEEP_CAPS flag set, the
              effective capabilities of a thread are cleared when it
              switches its effective UID to a nonzero value.  However,
              if the thread has set this flag and its effective UID is
              already nonzero, and the thread subsequently switches all
              other UIDs to nonzero values, then the effective
              capabilities will not be cleared.

              The setting of the SECBIT_KEEP_CAPS flag is ignored if the
              SECBIT_NO_SETUID_FIXUP flag is set.  (The latter flag
              provides a superset of the effect of the former flag.)

              This flag provides the same functionality as the older
              prctl(2) PR_SET_KEEPCAPS operation.

       SECBIT_NO_SETUID_FIXUP
              Setting this flag stops the kernel from adjusting the
              process's permitted, effective, and ambient capability
              sets when the thread's effective and filesystem UIDs are
              switched between zero and nonzero values.  (See the
              subsection Effect of user ID changes on capabilities.)

       SECBIT_NOROOT
              If this bit is set, then the kernel does not grant
              capabilities when a set-user-ID-root program is executed,
              or when a process with an effective or real UID of 0 calls
              execve(2).  (See the subsection Capabilities and execution
              of programs by root.)

       SECBIT_NO_CAP_AMBIENT_RAISE
              Setting this flag disallows raising ambient capabilities
              via the prctl(2) PR_CAP_AMBIENT_RAISE operation.

       Each of the above "base" flags has a companion "locked" flag.
       Setting any of the "locked" flags is irreversible, and has the
       effect of preventing further changes to the corresponding "base"
       flag.  The locked flags are: SECBIT_KEEP_CAPS_LOCKED,
       SECBIT_NO_SETUID_FIXUP_LOCKED, SECBIT_NOROOT_LOCKED, and
       SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED.

       The securebits flags can be modified and retrieved using the
       prctl(2) PR_SET_SECUREBITS and PR_GET_SECUREBITS operations.  The
       CAP_SETPCAP capability is required to modify the flags.  Note
       that the SECBIT_* constants are available only after including
       the <linux/securebits.h> header file.

       The securebits flags are inherited by child processes.  During an
       execve(2), all of the flags are preserved, except
       SECBIT_KEEP_CAPS which is always cleared.

       An application can use the following call to lock itself, and all
       of its descendants, into an environment where the only way of
       gaining capabilities is by executing a program with associated
       file capabilities:

           prctl(PR_SET_SECUREBITS,
                   /* SECBIT_KEEP_CAPS off */
                   SECBIT_KEEP_CAPS_LOCKED |
                   SECBIT_NO_SETUID_FIXUP |
                   SECBIT_NO_SETUID_FIXUP_LOCKED |
                   SECBIT_NOROOT |
                   SECBIT_NOROOT_LOCKED);
                   /* Setting/locking SECBIT_NO_CAP_AMBIENT_RAISE
                      is not required */

   Per-user-namespace "set-user-ID-root" programs
       A set-user-ID program whose UID matches the UID that created a
       user namespace will confer capabilities in the process's
       permitted and effective sets when executed by any process inside
       that namespace or any descendant user namespace.

       The rules about the transformation of the process's capabilities
       during the execve(2) are exactly as described in the subsections
       Transformation of capabilities during execve() and Capabilities
       and execution of programs by root, with the difference that, in
       the latter subsection, "root" is the UID of the creator of the
       user namespace.

   Namespaced file capabilities
       Traditional (i.e., version 2) file capabilities associate only a
       set of capability masks with a binary executable file.  When a
       process executes a binary with such capabilities, it gains the
       associated capabilities (within its user namespace) as per the
       rules described above in "Transformation of capabilities during
       execve()".

       Because version 2 file capabilities confer capabilities to the
       executing process regardless of which user namespace it resides
       in, only privileged processes are permitted to associate
       capabilities with a file.  Here, "privileged" means a process
       that has the CAP_SETFCAP capability in the user namespace where
       the filesystem was mounted (normally the initial user namespace).
       This limitation renders file capabilities useless for certain use
       cases.  For example, in user-namespaced containers, it can be
       desirable to be able to create a binary that confers capabilities
       only to processes executed inside that container, but not to
       processes that are executed outside the container.

       Linux 4.14 added so-called namespaced file capabilities to
       support such use cases.  Namespaced file capabilities are
       recorded as version 3 (i.e., VFS_CAP_REVISION_3)
       security.capability extended attributes.  Such an attribute is
       automatically created in the circumstances described above under
       "File capability extended attribute versioning".  When a version
       3 security.capability extended attribute is created, the kernel
       records not just the capability masks in the extended attribute,
       but also the namespace root user ID.

       As with a binary that has VFS_CAP_REVISION_2 file capabilities, a
       binary with VFS_CAP_REVISION_3 file capabilities confers
       capabilities to a process during execve().  However, capabilities
       are conferred only if the binary is executed by a process that
       resides in a user namespace whose UID 0 maps to the root user ID
       that is saved in the extended attribute, or when executed by a
       process that resides in a descendant of such a namespace.

   Interaction with user namespaces
       For further information on the interaction of capabilities and
       user namespaces, see user_namespaces(7).

CONFORMING TO         top

       No standards govern capabilities, but the Linux capability
       implementation is based on the withdrawn POSIX.1e draft standard;
       see ⟨https://archive.org/details/posix_1003.1e-990310⟩.

NOTES         top

       When attempting to strace(1) binaries that have capabilities (or
       set-user-ID-root binaries), you may find the -u <username> option
       useful.  Something like:

           $ sudo strace -o trace.log -u ceci ./myprivprog

       From kernel 2.5.27 to kernel 2.6.26, capabilities were an
       optional kernel component, and could be enabled/disabled via the
       CONFIG_SECURITY_CAPABILITIES kernel configuration option.

       The /proc/[pid]/task/TID/status file can be used to view the
       capability sets of a thread.  The /proc/[pid]/status file shows
       the capability sets of a process's main thread.  Before Linux
       3.8, nonexistent capabilities were shown as being enabled (1) in
       these sets.  Since Linux 3.8, all nonexistent capabilities (above
       CAP_LAST_CAP) are shown as disabled (0).

       The libcap package provides a suite of routines for setting and
       getting capabilities that is more comfortable and less likely to
       change than the interface provided by capset(2) and capget(2).
       This package also provides the setcap(8) and getcap(8) programs.
       It can be found at
       ⟨https://git.kernel.org/pub/scm/libs/libcap/libcap.git/refs/⟩.

       Before kernel 2.6.24, and from kernel 2.6.24 to kernel 2.6.32 if
       file capabilities are not enabled, a thread with the CAP_SETPCAP
       capability can manipulate the capabilities of threads other than
       itself.  However, this is only theoretically possible, since no
       thread ever has CAP_SETPCAP in either of these cases:

       * In the pre-2.6.25 implementation the system-wide capability
         bounding set, /proc/sys/kernel/cap-bound, always masks out the
         CAP_SETPCAP capability, and this can not be changed without
         modifying the kernel source and rebuilding the kernel.

       * If file capabilities are disabled (i.e., the kernel
         CONFIG_SECURITY_FILE_CAPABILITIES option is disabled), then
         init starts out with the CAP_SETPCAP capability removed from
         its per-process bounding set, and that bounding set is
         inherited by all other processes created on the system.

SEE ALSO         top

       capsh(1), setpriv(1), prctl(2), setfsuid(2), cap_clear(3),
       cap_copy_ext(3), cap_from_text(3), cap_get_file(3),
       cap_get_proc(3), cap_init(3), capgetp(3), capsetp(3), libcap(3),
       proc(5), credentials(7), pthreads(7), user_namespaces(7),
       captest(8), filecap(8), getcap(8), getpcaps(8), netcap(8),
       pscap(8), setcap(8)

       include/linux/capability.h in the Linux kernel source tree

COLOPHON         top

       This page is part of release 5.10 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2020-08-13                CAPABILITIES(7)

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