cap_get_proc(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
       2021-04-01.  (At that time, the date of the most recent commit
       that was found in the repository was 2021-03-30.)  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

                               2021-03-06                CAP_GET_PROC(3)

Pages that refer to this page: capsh(1)capget(2)prctl(2)cap_clear(3)cap_copy_ext(3)cap_from_text(3)cap_get_file(3)cap_iab(3)cap_init(3)cap_launch(3)libcap(3)capabilities(7)