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

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

#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);

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.

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.

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.

cap_set_proc() and cap_get_proc() are specified in the withdrawn POSIX.1e draft specification.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

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 meaninful POSIX sematics 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.

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.

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