Ubuntu Manpage: cap_get_proc, cap_set_proc, capgetp, cap_get_bound, cap_drop_bound, cap_get

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

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.