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CAPABILITIES(7) Linux Programmer's Manual CAPABILITIES(7)
capabilities - overview of Linux capabilities
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.
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_WRITE (since Linux 2.6.11)
Write records to kernel auditing log.
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.
CAP_FOWNER
* Bypass permission checks on operations that normally require the
file system 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 extended file attributes (see chattr(1)) on arbitrary files;
* set Access Control Lists (ACLs) on arbitrary files;
* ignore directory sticky bit on file deletion;
* specify O_NOATIME for arbitrary files in open(2) and fcntl(2).
CAP_FSETID
Don't clear set-user-ID and set-group-ID permission bits when a file
is modified; set the set-group-ID bit for a file whose GID does not
match the file system 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 i-node flags (see
chattr(1)).
CAP_MAC_ADMIN (since Linux 2.6.25)
Override Mandatory Access Control (MAC). Implemented for the Smack
Linux Security Module (LSM).
CAP_MAC_OVERRIDE (since Linux 2.6.25)
Allow MAC configuration or state changes. 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_SETGID
Make arbitrary manipulations of process GIDs and supplementary GID
list; forge GID when passing socket credentials via UNIX domain
sockets.
CAP_SETFCAP (since Linux 2.6.24)
Set file capabilities.
CAP_SETPCAP
If file capabilities are not supported: 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.)
If file capabilities are supported: 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.
CAP_SETUID
Make arbitrary manipulations of process UIDs (setuid(2),
setreuid(2), setresuid(2), setfsuid(2)); make forged UID when
passing socket credentials via UNIX domain sockets.
CAP_SYS_ADMIN
* Perform a range of system administration operations including:
quotactl(2), mount(2), umount(2), swapon(2), swapoff(2),
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;
* perform IPC_SET and IPC_RMID operations on arbitrary System V IPC
objects;
* perform operations on trusted and security Extended Attributes
(see attr(5));
* 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 UID when passing socket credentials;
* 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);
* call perf_event_open(2);
* access privileged perf event information;
* call setns(2);
* call fanotify_init(2);
* perform 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 file-system ioctl(2) operations;
* perform administrative operations on many device drivers.
CAP_SYS_BOOT
Use reboot(2) and kexec_load(2).
CAP_SYS_CHROOT
Use chroot(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
* Raise 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));
* 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.
CAP_SYS_RAWIO
Perform I/O port operations (iopl(2) and ioperm(2)); access
/proc/kcore; employ the FIBMAP ioctl(2) operation.
CAP_SYS_RESOURCE
* Use reserved space on ext2 file systems;
* 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));
* 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 limit when creating POSIX
message queues (see mq_overview(7));
* employ prctl(2) PR_SET_MM operation.
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.
CAP_WAKE_ALARM (since Linux 3.0)
Trigger something that will wake up the system (set
CLOCK_REALTIME_ALARM and CLOCK_BOOTTIME_ALARM timers).
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 file system 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.
Each thread has three 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). It
provides a mechanism for a process to assign capabilities to the
permitted set of the new program during an execve(2).
Effective:
This is the set of capabilities used by the kernel to perform
permission checks for the thread.
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 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)) 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.
During an execve(2), the kernel calculates the new capabilities of the
process using the following algorithm:
P'(permitted) = (P(inheritable) & F(inheritable)) |
(F(permitted) & cap_bset)
P'(effective) = F(effective) ? P'(permitted) : 0
P'(inheritable) = P(inheritable) [i.e., unchanged]
where:
P denotes the value of a thread capability set before the
execve(2)
P' denotes the value of a capability set after the execve(2)
F denotes a file capability set
cap_bset is the value of the capability bounding set (described
below).
In order to provide an all-powerful root using capability sets, during an
execve(2):
1. If a set-user-ID-root program is being executed, or the real user ID of
the process is 0 (root) then the file inheritable and permitted sets are
defined to be all ones (i.e., all capabilities enabled).
2. If a set-user-ID-root program is being executed, then the file effective
bit is defined to be one (enabled).
The upshot of the above rules, combined with the capabilities
transformations described above, is that when a process execve(2)s a set-
user-ID-root program, or when a process with an effective UID of 0
execve(2)s a program, it gains all capabilities in its permitted and
effective capability sets, except those masked out by the capability
bounding set. This provides semantics that are the same as those provided
by traditional UNIX systems.
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 inherited capabilities. If a thread maintains a capability in its
inherited 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 inherited set.
Depending on the kernel version, the capability bounding set is either a
system-wide attribute, or a per-process attribute.
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: programs 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.
Capability bounding set from Linux 2.6.25 onward
From Linux 2.6.25, the capability bounding set is a per-thread attribute.
(There is no longer a system-wide capability bounding set.)
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 only supported 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 inherited set. However it does prevent the capability from being
added back into the thread's inherited set in the future.
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 file system
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 and
effective 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 file system 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 2.2.30),
CAP_MAC_OVERRIDE, and CAP_MKNOD (since Linux 2.2.30). If the file
system 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 prctl(2)
PR_SET_KEEPCAPS operation.
A thread can retrieve and change its 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 kernel 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.
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 its capabilities when it switches all of its UIDs to a
nonzero value. If this flag is not set, then such a UID switch
causes the thread to lose all capabilities. This flag is always
cleared on an execve(2). (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 capability sets
when the threads's effective and file system 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.)
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, and
SECBIT_NOROOT_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.
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_LOCKED |
SECBIT_NO_SETUID_FIXUP |
SECBIT_NO_SETUID_FIXUP_LOCKED |
SECBIT_NOROOT |
SECBIT_NOROOT_LOCKED);
No standards govern capabilities, but the Linux capability implementation
is based on the withdrawn POSIX.1e draft standard; see
http://wt.xpilot.org/publications/posix.1e/.
Since kernel 2.5.27, capabilities are an optional kernel component, and can
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.
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
http://www.kernel.org/pub/linux/libs/security/linux-privs.
Before kernel 2.6.24, and since kernel 2.6.24 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 this capability, and this
can not be changed without modifying the kernel source and rebuilding.
* If file capabilities are disabled in the current implementation, then
init starts out with this capability removed from its per-process
bounding set, and that bounding set is inherited by all other processes
created on the system.
capget(2), 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), credentials(7), pthreads(7), getcap(8),
setcap(8)
Comments on the purposes of various capabilities in
include/linux/capability.h in the kernel source
This page is part of release 3.41 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be
found at http://www.kernel.org/doc/man-pages/.
Linux 2012-04-15 CAPABILITIES(7)
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