cap_drop_bound(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

CAP_GET_PROC(3)           Linux Programmer's Manual          CAP_GET_PROC(3)

NAME         top

       cap_get_proc,  cap_set_proc,  capgetp, cap_get_bound, cap_drop_bound,
       cap_get_ambient, cap_set_ambient, cap_reset_ambient, cap_get_secbits,
       cap_set_secbits,     cap_get_mode,    cap_set_mode,    cap_mode_name,
       cap_get_pid, cap_setuid, cap_setgroups - capability  manipulation  on
       processes

SYNOPSIS         top

       #include <sys/capability.h>

       cap_t cap_get_proc(void);

       int cap_set_proc(cap_t cap_p);

       int cap_get_bound(cap_value_t cap);

       CAP_IS_SUPPORTED(cap_value_t cap);

       int cap_drop_bound(cap_value_t cap);

       int cap_get_ambient(cap_value_t cap);

       int cap_set_ambient(cap_value_t cap, cap_flag_value_t value);

       int cap_reset_ambient(void);

       CAP_AMBIENT_SUPPORTED();

       unsigned cap_get_secbits(void);

       int cap_set_secbits(unsigned bits);

       cap_mode_t cap_get_mode(void);

       const char *cap_mode_name(cap_mode_t mode);

       int cap_set_mode(cap_mode_t mode);

       #include <sys/types.h>

       cap_t cap_get_pid(pid_t pid);

       int cap_setuid(uid_t uid);

       int cap_setgroups(gid_t gid, size_t ngroups, const gid_t groups);

       Link with -lcap.

DESCRIPTION         top

       cap_get_proc() allocates a capability state in working storage, sets
       its state to that of the calling process, and returns a pointer to
       this newly created capability state.  The caller should free any
       releasable memory, when the capability state in working storage is no
       longer required, by calling cap_free() with the cap_t as an argument.

       cap_set_proc() sets the values for all capability flags for all
       capabilities to the capability state identified by cap_p.  The new
       capability state of the process will be completely determined by the
       contents of cap_p upon successful return from this function.  If any
       flag in cap_p is set for any capability not currently permitted for
       the calling process, the function will fail, and the capability state
       of the process will remain unchanged.

       cap_get_pid() returns cap_t, see cap_init(3), with the process
       capabilities of the process indicated by pid.  (If pid is 0, then the
       calling process's capabilities are returned.)  This information can
       also be obtained from the /proc/<pid>/status file.

       cap_get_bound() with a cap as an argument returns the current value
       of this bounding set capability flag in effect for the calling
       process. This operation is unprivileged. Note, a macro function
       CAP_IS_SUPPORTED(cap_value_t cap) is provided that evaluates to true
       (1) if the system supports the specified capability, cap.  If the
       system does not support the capability, this function returns 0. This
       macro works by testing for an error condition with cap_get_bound().

       cap_drop_bound() can be used to lower the specified bounding set
       capability, cap.  To complete successfully, the prevailing effective
       capability set must have a raised CAP_SETPCAP.

       cap_get_ambient() returns the prevailing value of the specified
       ambient capability, or -1 if the capability is not supported by the
       running kernel.  A macro CAP_AMBIENT_SUPPORTED() uses this function
       to determine if ambient capabilities are supported by the kernel.

       cap_set_ambient() sets the specified ambient capability to a specific
       value. To complete successfully, the prevailing effective capability
       set must have a raised CAP_SETPCAP.  Further, to raise a specific
       ambient capability the inheritable and permitted sets of the calling
       process must contain the specified capability, and raised ambient
       bits will only be retained as long as this remains true.

       cap_reset_ambient() resets all of the ambient capabilities for the
       calling process to their lowered value. To complete successfully, the
       prevailing effective capability set must have a raised CAP_SETPCAP.
       Note, the ambient set is intended to operate in a legacy environment
       where the application has limited awareness of capabilities in
       general. Executing a file with associated filesystem capabilities,
       the kernel will implicitly reset the ambient set of the process.
       Also, changes to the inheritable set by the program code without
       explicitly fixing up the ambient set can also drop ambient bits.

       cap_get_secbits() returns the securebits of the calling process.
       These bits affect the way in which the calling process implements
       things like setuid-root fixup and ambient capabilities.

       cap_set_secbits() attempts to modify the securebits of the calling
       process. Note CAP_SETPCAP must be in the effective capability set for
       this to be effective. Some settings lock the sub-states of the
       securebits, so attempts to set values may be denied by the kernel
       even when the CAP_SETPCAP capability is raised.

       To help manage the complexity of the securebits, libcap provides a
       combined securebit and capability set concept called a libcap mode.
       cap_get_mode() attempts to summarize the prevailing security
       environment in the form of a numerical cap_mode_t value. A text
       representation of the mode can be obtained via the cap_mode_name()
       function. The vast majority of combinations of these values are not
       well defined in terms of a libcap mode, and for these states
       cap_get_mode() returns (cap_mode_t)0 which cap_get_name() identifies
       as ``UNCERTAIN''.  Supported modes are: CAP_MODE_NOPRIV,
       CAP_MODE_PURE1E_INIT and CAP_MODE_PURE1E.

       cap_set_mode() can be used to set the desired mode. The permitted
       capability CAP_SETPCAP is required for this function to succeed.

       cap_setuid() is a convenience function for the setuid(2) system call.
       Where cap_setuid() arranges for the right effective capability to be
       raised in order to perform the system call, and also arranges to
       preserve the availability of permitted capabilities after the uid has
       changed. Following this call all effective capabilities are lowered.

       cap_setgroups() is a convenience function for performing both
       setgid(2) and setgroups(2) calls in one call. The cap_setgroups()
       call raises the right effective capability for the duration of the
       call, and empties the effective capability set before returning.

RETURN VALUE         top

       The functions cap_get_proc() and cap_get_pid() return a non-NULL
       value on success, and NULL on failure.

       The function cap_get_bound() returns -1 if the requested capability
       is unknown, otherwise the return value reflects the current state of
       that capability in the prevailing bounding set. Note, a macro
       function,

       The all of the setting functions such as cap_set_proc() and
       cap_drop_bound() return zero for success, and -1 on failure.

       On failure, errno is set to EINVAL, EPERM, or ENOMEM.

CONFORMING TO         top

       cap_set_proc() and cap_get_proc() are specified in the withdrawn
       POSIX.1e draft specification.  cap_get_pid() is a Linux extension.

NOTES         top

       Neither glibc, nor the Linux kernel honors POSIX semantics for
       setting capabilities and securebits in the presence of pthreads. That
       is, changing capability sets, by default, only affect the running
       thread. To be meaningfully secure, however, the capability sets
       should be mirrored by all threads within a common program because
       threads are not memory isolated. As a workaround for this, libcap is
       packaged with a separate POSIX semantics system call library: libpsx.
       If your program uses POSIX threads, to achieve meaningful POSIX
       semantics capability manipulation, you should link your program with:

       ld ... -lcap -lpsx -lpthread --wrap=pthread_create

       or,

       gcc ... -lcap -lpsx -lpthread -Wl,-wrap,pthread_create

       When linked this way, due to linker magic, libcap uses psx_syscall(3)
       and psx_syscall6(3) to perform state setting system calls.

   capgetp() and capsetp()
       The library also supports the deprecated functions:

       int capgetp(pid_t pid, cap_t cap_d);

       int capsetp(pid_t pid, cap_t cap_d);

       capgetp() attempts to obtain the capabilities of some other process;
       storing the capabilities in a pre-allocated cap_d.  See cap_init()
       for information on allocating an empty capability set. This function
       is deprecated; you should use cap_get_pid().

       capsetp() attempts to set the capabilities of the calling porcess or
       of some other process(es), pid.  Note that setting capabilities of
       another process is only possible on older kernels that do not provide
       VFS support for setting file capabilities.  See capset(2) for
       information on which kernels provide such support.

       If pid is positive it refers to a specific process;  if it is zero,
       it refers to the calling process; -1 refers to all processes other
       than the calling process and process '1' (typically init(8)); other
       negative values refer to the -pid process group.

       In order to use this function, the kernel must support it and the
       calling process must have CAP_SETPCAP raised in its Effective
       capability set. The capabilities set in the target process(es) are
       those contained in cap_d.

       Kernels that support filesystem capabilities redefine the semantics
       of CAP_SETPCAP and on such systems, capsetp() will always fail for
       any target not equal to the calling process.  capsetp() returns zero
       for success, and -1 on failure.

       On kernels where it is (was) supported, capsetp() should be used with
       care.  It existed, primarily, to overcome an early lack of support
       for capabilities in the filesystems supported by Linux.  Note that on
       older kernels where capsetp() could be used to set the capabilities
       of another process, the only processes that had CAP_SETPCAP available
       to them by default were processes started as kernel threads.
       (Typically this includes init(8), kflushd and kswapd.) A kernel
       recompilation was needed to modify this default.

EXAMPLE         top

       The code segment below raises the CAP_FOWNER and CAP_SETFCAP
       effective capabilities for the caller:

           ...
           cap_t caps;
           const cap_value_t cap_list[2] = {CAP_FOWNER, CAP_SETFCAP};

           if (!CAP_IS_SUPPORTED(CAP_SETFCAP))
               /* handle error */

           caps = cap_get_proc();
           if (caps == NULL)
               /* handle error */;

           if (cap_set_flag(caps, CAP_EFFECTIVE, 2, cap_list, CAP_SET) == -1)
               /* handle error */;

           if (cap_set_proc(caps) == -1)
               /* handle error */;

           if (cap_free(caps) == -1)
               /* handle error */;
           ...

       Alternatively, to completely drop privilege in a program launched
       setuid-root but wanting to run as a specific user ID etc. in such a
       way that neither it, nor any of its children can acquire privilege
       again:

           ...
           uid_t nobody = 65534;
           const gid_t groups[] = {65534};

           if (cap_setgroups(groups[0], 1, groups) != 0)
               /* handle error */;
           if (cap_setuid(nobody) != 0)
               /* handle error */;

           /*
            * privilege is still available here
            */

           if (cap_set_mode(CAP_MODE_NOPRIV) != 0)
               /* handle error */
           ...

       Note, the above sequence can be performed by the capsh tool as
       follows:

       sudo /sbin/capsh --user=nobody --mode=NOPRIV --print

       where --print displays the resulting privilege state.

SEE ALSO         top

       libcap(3), libpsx(3), capsh(1), cap_clear(3), cap_copy_ext(3),
       cap_from_text(3), cap_get_file(3), cap_init(3), psx_syscall(3),
       capabilities(7).

COLOPHON         top

       This page is part of the libcap (capabilities commands and library)
       project.  Information about the project can be found at 
       ⟨https://git.kernel.org/pub/scm/libs/libcap/libcap.git/⟩.  If you have
       a bug report for this manual page, send it to morgan@kernel.org
       (please put "libcap" in the Subject line).  This page was obtained
       from the project's upstream Git repository
       ⟨https://git.kernel.org/pub/scm/libs/libcap/libcap.git/⟩ on
       2020-11-01.  (At that time, the date of the most recent commit that
       was found in the repository was 2020-10-28.)  If you discover any
       rendering problems in this HTML version of the page, or you believe
       there is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to
       man-pages@man7.org

                                 2019-12-21                  CAP_GET_PROC(3)

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