возможность манипулирования процессами (capability manipulation on processes)
Дубль
(статьи:
cap_drop_bound - возможность манипулирования процессами )
Имя (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
Синопсис (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.
Описание (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_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.
Возвращаемое значение (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.
Стандарты (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.
Примечание (Note)
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.
Примеры (Examples)
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.
Смотри также (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).
