cap_drop_bound

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



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       cap_set_proc() and cap_get_proc() are specified in the withdrawn POSIX.1e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

NOTES
       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
       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
       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).




                                   2019-12-21                    CAP_GET_PROC(3)