Provided by: libcap-dev_2.66-4ubuntu1_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_prctl, cap_prctlw, 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_prctl(long int pr_cmd, long int arg1, long int arg2,
long int arg3, long int arg4, long int arg5);
int cap_prctlw(long int pr_cmd, long int arg1, long int arg2,
long int arg3, long int arg4, long int arg5);
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 a cap_t, see cap_init(3), with the process capabilities of the
process known to the caller as 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.
(The entries in that file can be translated with the capsh --decode=XXX command line.)
When the caller is operating within a (CLONE_NEWPID) namespace, the numerical pid argument
is interpreted in the range of that namespace. As such, the caller's idea of the target
pid may differ from that of the target process when they are operating in different pid
namespaces. See pid_namespaces(7) for details. Further, the returned cap_t value holds
the capabilities that the target pid thinks it has. If the target is operating in a
(CLONE_NEWUSER) namespace, the system wide privilege of those user namespace capabilities
my be substantially reduced. See user_namespaces(7) for details.
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. 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. Further, 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_prctl() can be used to read state via the prctl() system call.
cap_prctlw() can be used to write state via the prctl() system call.
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. Notably, this also ensures that cap_prctlw() can be
used to ensure process control bits are shared over all threads of a single process.
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 process 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 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), namespaces(7), pid_namespaces(7), user_namespaces(7),
psx_syscall(3), capabilities(7).
2022-04-28 CAP_GET_PROC(3)