focal (3) cap_get_bound.3.gz

Provided by: libcap-dev_2.32-1ubuntu0.1_amd64 bug

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.  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  current  process.  This  operation  is   unpriveged.   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 current 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 current 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 current process. These bits affect the way in  which  the
       current process implements things like setuid-root fixup and ambient capabilities.

       cap_set_secbits()  attempts  to modify the securebits of the current 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() declares
       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.

       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,
       capgetp(), is deprecated, you should use cap_get_pid().

       capsetp()  attempts to set the capabilities of some other process(es), pid.  If pid is positive it refers
       to a specific process;  if it is zero, it refers to the current process; -1 refers to all processes other
       than  the  current  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  current  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 this function will always fail for any target not equal to the current
       process.  capsetp() returns zero for success, and -1 on failure.

       Where supported by the kernel, the function 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,
       by default, the only processes that have CAP_SETPCAP available to them are processes started as a  kernel
       thread.   (Typically this includes init(8), kflushd and kswapd). You will need to recompile the kernel 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)