1CAPABILITIES(7) Linux Programmer's Manual CAPABILITIES(7)
2
6 capabilities - overview of Linux capabilities
7
9 For the purpose of performing permission checks, traditional UNIX
10 implementations distinguish two categories of processes: privileged
11 processes (whose effective user ID is 0, referred to as superuser or
12 root), and unprivileged processes (whose effective UID is nonzero).
13 Privileged processes bypass all kernel permission checks, while unpriv‐
14 ileged processes are subject to full permission checking based on the
15 process's credentials (usually: effective UID, effective GID, and sup‐
16 plementary group list).
17 Starting with kernel 2.2, Linux divides the privileges traditionally
19 associated with superuser into distinct units, known as capabilities,
20 which can be independently enabled and disabled. Capabilities are a
21 per-thread attribute.
22 Capabilities list
24 The following list shows the capabilities implemented on Linux, and the
25 operations or behaviors that each capability permits:
26 CAP_AUDIT_CONTROL (since Linux 2.6.11)
28 Enable and disable kernel auditing; change auditing filter
29 rules; retrieve auditing status and filtering rules.
30 CAP_AUDIT_READ (since Linux 3.16)
32 Allow reading the audit log via a multicast netlink socket.
33 CAP_AUDIT_WRITE (since Linux 2.6.11)
35 Write records to kernel auditing log.
36 CAP_BLOCK_SUSPEND (since Linux 3.5)
38 Employ features that can block system suspend (epoll(7) EPOLL‐
39 WAKEUP, /proc/sys/wake_lock).
40 CAP_CHOWN
42 Make arbitrary changes to file UIDs and GIDs (see chown(2)).
43 CAP_DAC_OVERRIDE
45 Bypass file read, write, and execute permission checks. (DAC is
46 an abbreviation of "discretionary access control".)
47 CAP_DAC_READ_SEARCH
49 * Bypass file read permission checks and directory read and exe‐
50 cute permission checks;
51 * invoke open_by_handle_at(2);
52 * use the linkat(2) AT_EMPTY_PATH flag to create a link to a
53 file referred to by a file descriptor.
54 CAP_FOWNER
56 * Bypass permission checks on operations that normally require
57 the filesystem UID of the process to match the UID of the file
58 (e.g., chmod(2), utime(2)), excluding those operations covered
59 by CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH;
60 * set inode flags (see ioctl_iflags(2)) on arbitrary files;
61 * set Access Control Lists (ACLs) on arbitrary files;
62 * ignore directory sticky bit on file deletion;
63 * specify O_NOATIME for arbitrary files in open(2) and fcntl(2).
64 CAP_FSETID
66 * Don't clear set-user-ID and set-group-ID mode bits when a file
67 is modified;
68 * set the set-group-ID bit for a file whose GID does not match
69 the filesystem or any of the supplementary GIDs of the calling
70 process.
71 CAP_IPC_LOCK
73 Lock memory (mlock(2), mlockall(2), mmap(2), shmctl(2)).
74 CAP_IPC_OWNER
76 Bypass permission checks for operations on System V IPC objects.
77 CAP_KILL
79 Bypass permission checks for sending signals (see kill(2)).
80 This includes use of the ioctl(2) KDSIGACCEPT operation.
81 CAP_LEASE (since Linux 2.4)
83 Establish leases on arbitrary files (see fcntl(2)).
84 CAP_LINUX_IMMUTABLE
86 Set the FS_APPEND_FL and FS_IMMUTABLE_FL inode flags (see
87 ioctl_iflags(2)).
88 CAP_MAC_ADMIN (since Linux 2.6.25)
90 Allow MAC configuration or state changes. Implemented for the
91 Smack Linux Security Module (LSM).
92 CAP_MAC_OVERRIDE (since Linux 2.6.25)
94 Override Mandatory Access Control (MAC). Implemented for the
95 Smack LSM.
96 CAP_MKNOD (since Linux 2.4)
98 Create special files using mknod(2).
99 CAP_NET_ADMIN
101 Perform various network-related operations:
102 * interface configuration;
103 * administration of IP firewall, masquerading, and accounting;
104 * modify routing tables;
105 * bind to any address for transparent proxying;
106 * set type-of-service (TOS)
107 * clear driver statistics;
108 * set promiscuous mode;
109 * enabling multicasting;
110 * use setsockopt(2) to set the following socket options:
111 SO_DEBUG, SO_MARK, SO_PRIORITY (for a priority outside the
112 range 0 to 6), SO_RCVBUFFORCE, and SO_SNDBUFFORCE.
113 CAP_NET_BIND_SERVICE
115 Bind a socket to Internet domain privileged ports (port numbers
116 less than 1024).
117 CAP_NET_BROADCAST
119 (Unused) Make socket broadcasts, and listen to multicasts.
120 CAP_NET_RAW
122 * Use RAW and PACKET sockets;
123 * bind to any address for transparent proxying.
124 CAP_SETGID
126 * Make arbitrary manipulations of process GIDs and supplementary
127 GID list;
128 * forge GID when passing socket credentials via UNIX domain
129 sockets;
130 * write a group ID mapping in a user namespace (see user_names‐
131 paces(7)).
132 CAP_SETFCAP (since Linux 2.6.24)
134 Set arbitrary capabilities on a file.
135 CAP_SETPCAP
137 If file capabilities are supported (i.e., since Linux 2.6.24):
138 add any capability from the calling thread's bounding set to its
139 inheritable set; drop capabilities from the bounding set (via
140 prctl(2) PR_CAPBSET_DROP); make changes to the securebits flags.
141 If file capabilities are not supported (i.e., kernels before
143 Linux 2.6.24): grant or remove any capability in the caller's
144 permitted capability set to or from any other process. (This
145 property of CAP_SETPCAP is not available when the kernel is con‐
146 figured to support file capabilities, since CAP_SETPCAP has
147 entirely different semantics for such kernels.)
148 CAP_SETUID
150 * Make arbitrary manipulations of process UIDs (setuid(2),
151 setreuid(2), setresuid(2), setfsuid(2));
152 * forge UID when passing socket credentials via UNIX domain
153 sockets;
154 * write a user ID mapping in a user namespace (see user_names‐
155 paces(7)).
156 CAP_SYS_ADMIN
158 Note: this capability is overloaded; see Notes to kernel devel‐
159 opers, below.
160 * Perform a range of system administration operations including:
162 quotactl(2), mount(2), umount(2), swapon(2), swapoff(2),
163 sethostname(2), and setdomainname(2);
164 * perform privileged syslog(2) operations (since Linux 2.6.37,
165 CAP_SYSLOG should be used to permit such operations);
166 * perform VM86_REQUEST_IRQ vm86(2) command;
167 * perform IPC_SET and IPC_RMID operations on arbitrary System V
168 IPC objects;
169 * override RLIMIT_NPROC resource limit;
170 * perform operations on trusted and security Extended Attributes
171 (see xattr(7));
172 * use lookup_dcookie(2);
173 * use ioprio_set(2) to assign IOPRIO_CLASS_RT and (before Linux
174 2.6.25) IOPRIO_CLASS_IDLE I/O scheduling classes;
175 * forge PID when passing socket credentials via UNIX domain
176 sockets;
177 * exceed /proc/sys/fs/file-max, the system-wide limit on the
178 number of open files, in system calls that open files (e.g.,
179 accept(2), execve(2), open(2), pipe(2));
180 * employ CLONE_* flags that create new namespaces with clone(2)
181 and unshare(2) (but, since Linux 3.8, creating user namespaces
182 does not require any capability);
183 * call perf_event_open(2);
184 * access privileged perf event information;
185 * call setns(2) (requires CAP_SYS_ADMIN in the target names‐
186 pace);
187 * call fanotify_init(2);
188 * call bpf(2);
189 * perform privileged KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2)
190 operations;
191 * use ptrace(2) PTRACE_SECCOMP_GET_FILTER to dump a tracees sec‐
192 comp filters;
193 * perform madvise(2) MADV_HWPOISON operation;
194 * employ the TIOCSTI ioctl(2) to insert characters into the
195 input queue of a terminal other than the caller's controlling
196 terminal;
197 * employ the obsolete nfsservctl(2) system call;
198 * employ the obsolete bdflush(2) system call;
199 * perform various privileged block-device ioctl(2) operations;
200 * perform various privileged filesystem ioctl(2) operations;
201 * perform privileged ioctl(2) operations on the /dev/random
202 device (see random(4));
203 * install a seccomp(2) filter without first having to set the
204 no_new_privs thread attribute;
205 * modify allow/deny rules for device control groups;
206 * employ the ptrace(2) PTRACE_SECCOMP_GET_FILTER operation to
207 dump tracee's seccomp filters;
208 * employ the ptrace(2) PTRACE_SETOPTIONS operation to suspend
209 the tracee's seccomp protections (i.e., the PTRACE_O_SUS‐
210 PEND_SECCOMP flag).
211 * perform administrative operations on many device drivers.
212 CAP_SYS_BOOT
214 Use reboot(2) and kexec_load(2).
215 CAP_SYS_CHROOT
217 Use chroot(2).
218 CAP_SYS_MODULE
220 * Load and unload kernel modules (see init_module(2) and
221 delete_module(2));
222 * in kernels before 2.6.25: drop capabilities from the system-
223 wide capability bounding set.
224 CAP_SYS_NICE
226 * Raise process nice value (nice(2), setpriority(2)) and change
227 the nice value for arbitrary processes;
228 * set real-time scheduling policies for calling process, and set
229 scheduling policies and priorities for arbitrary processes
230 (sched_setscheduler(2), sched_setparam(2), shed_setattr(2));
231 * set CPU affinity for arbitrary processes (sched_setaffin‐
232 ity(2));
233 * set I/O scheduling class and priority for arbitrary processes
234 (ioprio_set(2));
235 * apply migrate_pages(2) to arbitrary processes and allow pro‐
236 cesses to be migrated to arbitrary nodes;
237 * apply move_pages(2) to arbitrary processes;
238 * use the MPOL_MF_MOVE_ALL flag with mbind(2) and move_pages(2).
239 CAP_SYS_PACCT
241 Use acct(2).
242 CAP_SYS_PTRACE
244 * Trace arbitrary processes using ptrace(2);
245 * apply get_robust_list(2) to arbitrary processes;
246 * transfer data to or from the memory of arbitrary processes
247 using process_vm_readv(2) and process_vm_writev(2);
248 * inspect processes using kcmp(2).
249 CAP_SYS_RAWIO
251 * Perform I/O port operations (iopl(2) and ioperm(2));
252 * access /proc/kcore;
253 * employ the FIBMAP ioctl(2) operation;
254 * open devices for accessing x86 model-specific registers (MSRs,
255 see msr(4));
256 * update /proc/sys/vm/mmap_min_addr;
257 * create memory mappings at addresses below the value specified
258 by /proc/sys/vm/mmap_min_addr;
259 * map files in /proc/bus/pci;
260 * open /dev/mem and /dev/kmem;
261 * perform various SCSI device commands;
262 * perform certain operations on hpsa(4) and cciss(4) devices;
263 * perform a range of device-specific operations on other
264 devices.
265 CAP_SYS_RESOURCE
267 * Use reserved space on ext2 filesystems;
268 * make ioctl(2) calls controlling ext3 journaling;
269 * override disk quota limits;
270 * increase resource limits (see setrlimit(2));
271 * override RLIMIT_NPROC resource limit;
272 * override maximum number of consoles on console allocation;
273 * override maximum number of keymaps;
274 * allow more than 64hz interrupts from the real-time clock;
275 * raise msg_qbytes limit for a System V message queue above the
276 limit in /proc/sys/kernel/msgmnb (see msgop(2) and msgctl(2));
277 * allow the RLIMIT_NOFILE resource limit on the number of "in-
278 flight" file descriptors to be bypassed when passing file
279 descriptors to another process via a UNIX domain socket (see
280 unix(7));
281 * override the /proc/sys/fs/pipe-size-max limit when setting the
282 capacity of a pipe using the F_SETPIPE_SZ fcntl(2) command.
283 * use F_SETPIPE_SZ to increase the capacity of a pipe above the
284 limit specified by /proc/sys/fs/pipe-max-size;
285 * override /proc/sys/fs/mqueue/queues_max limit when creating
286 POSIX message queues (see mq_overview(7));
287 * employ the prctl(2) PR_SET_MM operation;
288 * set /proc/[pid]/oom_score_adj to a value lower than the value
289 last set by a process with CAP_SYS_RESOURCE.
290 CAP_SYS_TIME
292 Set system clock (settimeofday(2), stime(2), adjtimex(2)); set
293 real-time (hardware) clock.
294 CAP_SYS_TTY_CONFIG
296 Use vhangup(2); employ various privileged ioctl(2) operations on
297 virtual terminals.
298 CAP_SYSLOG (since Linux 2.6.37)
300 * Perform privileged syslog(2) operations. See syslog(2) for
301 information on which operations require privilege.
302 * View kernel addresses exposed via /proc and other interfaces
303 when /proc/sys/kernel/kptr_restrict has the value 1. (See the
304 discussion of the kptr_restrict in proc(5).)
305 CAP_WAKE_ALARM (since Linux 3.0)
307 Trigger something that will wake up the system (set CLOCK_REAL‐
308 TIME_ALARM and CLOCK_BOOTTIME_ALARM timers).
309 Past and current implementation
311 A full implementation of capabilities requires that:
312 1. For all privileged operations, the kernel must check whether the
314 thread has the required capability in its effective set.
315 2. The kernel must provide system calls allowing a thread's capability
317 sets to be changed and retrieved.
318 3. The filesystem must support attaching capabilities to an executable
320 file, so that a process gains those capabilities when the file is
321 executed.
322 Before kernel 2.6.24, only the first two of these requirements are met;
324 since kernel 2.6.24, all three requirements are met.
325 Notes to kernel developers
327 When adding a new kernel feature that should be governed by a capabil‐
328 ity, consider the following points.
329 * The goal of capabilities is divide the power of superuser into
331 pieces, such that if a program that has one or more capabilities is
332 compromised, its power to do damage to the system would be less than
333 the same program running with root privilege.
334 * You have the choice of either creating a new capability for your new
336 feature, or associating the feature with one of the existing capa‐
337 bilities. In order to keep the set of capabilities to a manageable
338 size, the latter option is preferable, unless there are compelling
339 reasons to take the former option. (There is also a technical
340 limit: the size of capability sets is currently limited to 64 bits.)
341 * To determine which existing capability might best be associated with
343 your new feature, review the list of capabilities above in order to
344 find a "silo" into which your new feature best fits. One approach
345 to take is to determine if there are other features requiring capa‐
346 bilities that will always be use along with the new feature. If the
347 new feature is useless without these other features, you should use
348 the same capability as the other features.
349 * Don't choose CAP_SYS_ADMIN if you can possibly avoid it! A vast
351 proportion of existing capability checks are associated with this
352 capability (see the partial list above). It can plausibly be called
353 "the new root", since on the one hand, it confers a wide range of
354 powers, and on the other hand, its broad scope means that this is
355 the capability that is required by many privileged programs. Don't
356 make the problem worse. The only new features that should be asso‐
357 ciated with CAP_SYS_ADMIN are ones that closely match existing uses
358 in that silo.
359 * If you have determined that it really is necessary to create a new
361 capability for your feature, don't make or name it as a "single-use"
362 capability. Thus, for example, the addition of the highly specific
363 CAP_SYS_PACCT was probably a mistake. Instead, try to identify and
364 name your new capability as a broader silo into which other related
365 future use cases might fit.
366 Thread capability sets
368 Each thread has three capability sets containing zero or more of the
369 above capabilities:
370 Permitted:
372 This is a limiting superset for the effective capabilities that
373 the thread may assume. It is also a limiting superset for the
374 capabilities that may be added to the inheritable set by a
375 thread that does not have the CAP_SETPCAP capability in its
376 effective set.
377 If a thread drops a capability from its permitted set, it can
379 never reacquire that capability (unless it execve(2)s either a
380 set-user-ID-root program, or a program whose associated file
381 capabilities grant that capability).
382 Inheritable:
384 This is a set of capabilities preserved across an execve(2).
385 Inheritable capabilities remain inheritable when executing any
386 program, and inheritable capabilities are added to the permitted
387 set when executing a program that has the corresponding bits set
388 in the file inheritable set.
389 Because inheritable capabilities are not generally preserved
391 across execve(2) when running as a non-root user, applications
392 that wish to run helper programs with elevated capabilities
393 should consider using ambient capabilities, described below.
394 Effective:
396 This is the set of capabilities used by the kernel to perform
397 permission checks for the thread.
398 Ambient (since Linux 4.3):
400 This is a set of capabilities that are preserved across an
401 execve(2) of a program that is not privileged. The ambient
402 capability set obeys the invariant that no capability can ever
403 be ambient if it is not both permitted and inheritable.
404 The ambient capability set can be directly modified using
406 prctl(2). Ambient capabilities are automatically lowered if
407 either of the corresponding permitted or inheritable capabili‐
408 ties is lowered.
409 Executing a program that changes UID or GID due to the set-user-
411 ID or set-group-ID bits or executing a program that has any file
412 capabilities set will clear the ambient set. Ambient capabili‐
413 ties are added to the permitted set and assigned to the effec‐
414 tive set when execve(2) is called.
415 A child created via fork(2) inherits copies of its parent's capability
417 sets. See below for a discussion of the treatment of capabilities dur‐
418 ing execve(2).
419 Using capset(2), a thread may manipulate its own capability sets (see
421 below).
422 Since Linux 3.2, the file /proc/sys/kernel/cap_last_cap exposes the
424 numerical value of the highest capability supported by the running ker‐
425 nel; this can be used to determine the highest bit that may be set in a
426 capability set.
427 File capabilities
429 Since kernel 2.6.24, the kernel supports associating capability sets
430 with an executable file using setcap(8). The file capability sets are
431 stored in an extended attribute (see setxattr(2) and xattr(7)) named
432 security.capability. Writing to this extended attribute requires the
433 CAP_SETFCAP capability. The file capability sets, in conjunction with
434 the capability sets of the thread, determine the capabilities of a
435 thread after an execve(2).
436 The three file capability sets are:
438 Permitted (formerly known as forced):
440 These capabilities are automatically permitted to the thread,
441 regardless of the thread's inheritable capabilities.
442 Inheritable (formerly known as allowed):
444 This set is ANDed with the thread's inheritable set to determine
445 which inheritable capabilities are enabled in the permitted set
446 of the thread after the execve(2).
447 Effective:
449 This is not a set, but rather just a single bit. If this bit is
450 set, then during an execve(2) all of the new permitted capabili‐
451 ties for the thread are also raised in the effective set. If
452 this bit is not set, then after an execve(2), none of the new
453 permitted capabilities is in the new effective set.
454 Enabling the file effective capability bit implies that any file
456 permitted or inheritable capability that causes a thread to
457 acquire the corresponding permitted capability during an
458 execve(2) (see the transformation rules described below) will
459 also acquire that capability in its effective set. Therefore,
460 when assigning capabilities to a file (setcap(8),
461 cap_set_file(3), cap_set_fd(3)), if we specify the effective
462 flag as being enabled for any capability, then the effective
463 flag must also be specified as enabled for all other capabili‐
464 ties for which the corresponding permitted or inheritable flags
465 is enabled.
466 Transformation of capabilities during execve()
468 During an execve(2), the kernel calculates the new capabilities of the
469 process using the following algorithm:
470 P'(ambient) = (file is privileged) ? 0 : P(ambient)
472 P'(permitted) = (P(inheritable) & F(inheritable)) |
474 (F(permitted) & cap_bset) | P'(ambient)
475 P'(effective) = F(effective) ? P'(permitted) : P'(ambient)
477 P'(inheritable) = P(inheritable) [i.e., unchanged]
479 where:
481 P denotes the value of a thread capability set before the
483 execve(2)
484 P' denotes the value of a thread capability set after the
486 execve(2)
487 F denotes a file capability set
489 cap_bset is the value of the capability bounding set (described
491 below).
492 A privileged file is one that has capabilities or has the set-user-ID
494 or set-group-ID bit set.
495 Note: the capability transitions described above may not be performed
497 (i.e., file capabilities may be ignored) for the same reasons that the
498 set-user-ID and set-group-ID bits are ignored; see execve(2).
499 Note: according to the rules above, if a process with nonzero user IDs
501 performs an execve(2) then any capabilities that are present in its
502 permitted and effective sets will be cleared. For the treatment of
503 capabilities when a process with a user ID of zero performs an
504 execve(2), see below under Capabilities and execution of programs by
505 root.
506 Safety checking for capability-dumb binaries
508 A capability-dumb binary is an application that has been marked to have
509 file capabilities, but has not been converted to use the libcap(3) API
510 to manipulate its capabilities. (In other words, this is a traditional
511 set-user-ID-root program that has been switched to use file capabili‐
512 ties, but whose code has not been modified to understand capabilities.)
513 For such applications, the effective capability bit is set on the file,
514 so that the file permitted capabilities are automatically enabled in
515 the process effective set when executing the file. The kernel recog‐
516 nizes a file which has the effective capability bit set as capability-
517 dumb for the purpose of the check described here.
518 When executing a capability-dumb binary, the kernel checks if the
520 process obtained all permitted capabilities that were specified in the
521 file permitted set, after the capability transformations described
522 above have been performed. (The typical reason why this might not
523 occur is that the capability bounding set masked out some of the capa‐
524 bilities in the file permitted set.) If the process did not obtain the
525 full set of file permitted capabilities, then execve(2) fails with the
526 error EPERM. This prevents possible security risks that could arise
527 when a capability-dumb application is executed with less privilege that
528 it needs. Note that, by definition, the application could not itself
529 recognize this problem, since it does not employ the libcap(3) API.
530 Capabilities and execution of programs by root
532 In order to provide an all-powerful root using capability sets, during
533 an execve(2):
534 1. If a set-user-ID-root program is being executed, or the real or
536 effective user ID of the process is 0 (root) then the file inherita‐
537 ble and permitted sets are defined to be all ones (i.e., all capa‐
538 bilities enabled).
539 2. If a set-user-ID-root program is being executed, or the effective
541 user ID of the process is 0 (root) then the file effective bit is
542 defined to be one (enabled).
543 The upshot of the above rules, combined with the capabilities transfor‐
545 mations described above, is as follows:
546 * When a process execve(2)s a set-user-ID-root program, or when a
548 process with an effective UID of 0 execve(2)s a program, it gains
549 all capabilities in its permitted and effective capability sets,
550 except those masked out by the capability bounding set.
551 * When a process with a real UID of 0 execve(2)s a program, it gains
553 all capabilities in its permitted capability set, except those
554 masked out by the capability bounding set.
555 The above steps yield semantics that are the same as those provided by
557 traditional UNIX systems.
558 Set-user-ID-root programs that have file capabilities
560 Executing a program that is both set-user-ID root and has file capabil‐
561 ities will cause the process to gain just the capabilities granted by
562 the program (i.e., not all capabilities, as would occur when executing
563 a set-user-ID-root program that does not have any associated file capa‐
564 bilities). Note that one can assign empty capability sets to a program
565 file, and thus it is possible to create a set-user-ID-root program that
566 changes the effective and saved set-user-ID of the process that exe‐
567 cutes the program to 0, but confers no capabilities to that process.
568 Capability bounding set
570 The capability bounding set is a security mechanism that can be used to
571 limit the capabilities that can be gained during an execve(2). The
572 bounding set is used in the following ways:
573 * During an execve(2), the capability bounding set is ANDed with the
575 file permitted capability set, and the result of this operation is
576 assigned to the thread's permitted capability set. The capability
577 bounding set thus places a limit on the permitted capabilities that
578 may be granted by an executable file.
579 * (Since Linux 2.6.25) The capability bounding set acts as a limiting
581 superset for the capabilities that a thread can add to its inherita‐
582 ble set using capset(2). This means that if a capability is not in
583 the bounding set, then a thread can't add this capability to its
584 inheritable set, even if it was in its permitted capabilities, and
585 thereby cannot have this capability preserved in its permitted set
586 when it execve(2)s a file that has the capability in its inheritable
587 set.
588 Note that the bounding set masks the file permitted capabilities, but
590 not the inheritable capabilities. If a thread maintains a capability
591 in its inheritable set that is not in its bounding set, then it can
592 still gain that capability in its permitted set by executing a file
593 that has the capability in its inheritable set.
594 Depending on the kernel version, the capability bounding set is either
596 a system-wide attribute, or a per-process attribute.
597 Capability bounding set prior to Linux 2.6.25
599 In kernels before 2.6.25, the capability bounding set is a system-wide
601 attribute that affects all threads on the system. The bounding set is
602 accessible via the file /proc/sys/kernel/cap-bound. (Confusingly, this
603 bit mask parameter is expressed as a signed decimal number in
604 /proc/sys/kernel/cap-bound.)
605 Only the init process may set capabilities in the capability bounding
607 set; other than that, the superuser (more precisely: a process with the
608 CAP_SYS_MODULE capability) may only clear capabilities from this set.
609 On a standard system the capability bounding set always masks out the
611 CAP_SETPCAP capability. To remove this restriction (dangerous!), mod‐
612 ify the definition of CAP_INIT_EFF_SET in include/linux/capability.h
613 and rebuild the kernel.
614 The system-wide capability bounding set feature was added to Linux
616 starting with kernel version 2.2.11.
617 Capability bounding set from Linux 2.6.25 onward
619 From Linux 2.6.25, the capability bounding set is a per-thread
621 attribute. (There is no longer a system-wide capability bounding set.)
622 The bounding set is inherited at fork(2) from the thread's parent, and
624 is preserved across an execve(2).
625 A thread may remove capabilities from its capability bounding set using
627 the prctl(2) PR_CAPBSET_DROP operation, provided it has the CAP_SETPCAP
628 capability. Once a capability has been dropped from the bounding set,
629 it cannot be restored to that set. A thread can determine if a capa‐
630 bility is in its bounding set using the prctl(2) PR_CAPBSET_READ opera‐
631 tion.
632 Removing capabilities from the bounding set is supported only if file
634 capabilities are compiled into the kernel. In kernels before Linux
635 2.6.33, file capabilities were an optional feature configurable via the
636 CONFIG_SECURITY_FILE_CAPABILITIES option. Since Linux 2.6.33, the con‐
637 figuration option has been removed and file capabilities are always
638 part of the kernel. When file capabilities are compiled into the ker‐
639 nel, the init process (the ancestor of all processes) begins with a
640 full bounding set. If file capabilities are not compiled into the ker‐
641 nel, then init begins with a full bounding set minus CAP_SETPCAP,
642 because this capability has a different meaning when there are no file
643 capabilities.
644 Removing a capability from the bounding set does not remove it from the
646 thread's inheritable set. However it does prevent the capability from
647 being added back into the thread's inheritable set in the future.
648 Effect of user ID changes on capabilities
650 To preserve the traditional semantics for transitions between 0 and
651 nonzero user IDs, the kernel makes the following changes to a thread's
652 capability sets on changes to the thread's real, effective, saved set,
653 and filesystem user IDs (using setuid(2), setresuid(2), or similar):
654 1. If one or more of the real, effective or saved set user IDs was pre‐
656 viously 0, and as a result of the UID changes all of these IDs have
657 a nonzero value, then all capabilities are cleared from the permit‐
658 ted, effective, and ambient capability sets.
659 2. If the effective user ID is changed from 0 to nonzero, then all
661 capabilities are cleared from the effective set.
662 3. If the effective user ID is changed from nonzero to 0, then the per‐
664 mitted set is copied to the effective set.
665 4. If the filesystem user ID is changed from 0 to nonzero (see setf‐
667 suid(2)), then the following capabilities are cleared from the
668 effective set: CAP_CHOWN, CAP_DAC_OVERRIDE, CAP_DAC_READ_SEARCH,
669 CAP_FOWNER, CAP_FSETID, CAP_LINUX_IMMUTABLE (since Linux 2.6.30),
670 CAP_MAC_OVERRIDE, and CAP_MKNOD (since Linux 2.6.30). If the
671 filesystem UID is changed from nonzero to 0, then any of these capa‐
672 bilities that are enabled in the permitted set are enabled in the
673 effective set.
674 If a thread that has a 0 value for one or more of its user IDs wants to
676 prevent its permitted capability set being cleared when it resets all
677 of its user IDs to nonzero values, it can do so using the
678 SECBIT_KEEP_CAPS securebits flag described below.
679 Programmatically adjusting capability sets
681 A thread can retrieve and change its capability sets using the
682 capget(2) and capset(2) system calls. However, the use of
683 cap_get_proc(3) and cap_set_proc(3), both provided in the libcap pack‐
684 age, is preferred for this purpose. The following rules govern changes
685 to the thread capability sets:
686 1. If the caller does not have the CAP_SETPCAP capability, the new
688 inheritable set must be a subset of the combination of the existing
689 inheritable and permitted sets.
690 2. (Since Linux 2.6.25) The new inheritable set must be a subset of the
692 combination of the existing inheritable set and the capability
693 bounding set.
694 3. The new permitted set must be a subset of the existing permitted set
696 (i.e., it is not possible to acquire permitted capabilities that the
697 thread does not currently have).
698 4. The new effective set must be a subset of the new permitted set.
700 The securebits flags: establishing a capabilities-only environment
702 Starting with kernel 2.6.26, and with a kernel in which file capabili‐
703 ties are enabled, Linux implements a set of per-thread securebits flags
704 that can be used to disable special handling of capabilities for UID 0
705 (root). These flags are as follows:
706 SECBIT_KEEP_CAPS
708 Setting this flag allows a thread that has one or more 0 UIDs to
709 retain capabilities in its permitted and effective sets when it
710 switches all of its UIDs to nonzero values. If this flag is not
711 set, then such a UID switch causes the thread to lose all capa‐
712 bilities in those sets. This flag is always cleared on an
713 execve(2).
714 The setting of the SECBIT_KEEP_CAPS flag is ignored if the
716 SECBIT_NO_SETUID_FIXUP flag is set. (The latter flag provides a
717 superset of the effect of the former flag.)
718 This flag provides the same functionality as the older prctl(2)
720 PR_SET_KEEPCAPS operation.
721 SECBIT_NO_SETUID_FIXUP
723 Setting this flag stops the kernel from adjusting the process's
724 permitted, effective, and ambient capability sets when the
725 thread's effective and filesystem UIDs are switched between zero
726 and nonzero values. (See the subsection Effect of user ID
727 changes on capabilities.)
728 SECBIT_NOROOT
730 If this bit is set, then the kernel does not grant capabilities
731 when a set-user-ID-root program is executed, or when a process
732 with an effective or real UID of 0 calls execve(2). (See the
733 subsection Capabilities and execution of programs by root.)
734 SECBIT_NO_CAP_AMBIENT_RAISE
736 Setting this flag disallows raising ambient capabilities via the
737 prctl(2) PR_CAP_AMBIENT_RAISE operation.
738 Each of the above "base" flags has a companion "locked" flag. Setting
740 any of the "locked" flags is irreversible, and has the effect of pre‐
741 venting further changes to the corresponding "base" flag. The locked
742 flags are: SECBIT_KEEP_CAPS_LOCKED, SECBIT_NO_SETUID_FIXUP_LOCKED,
743 SECBIT_NOROOT_LOCKED, and SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED.
744 The securebits flags can be modified and retrieved using the prctl(2)
746 PR_SET_SECUREBITS and PR_GET_SECUREBITS operations. The CAP_SETPCAP
747 capability is required to modify the flags.
748 The securebits flags are inherited by child processes. During an
750 execve(2), all of the flags are preserved, except SECBIT_KEEP_CAPS
751 which is always cleared.
752 An application can use the following call to lock itself, and all of
754 its descendants, into an environment where the only way of gaining
755 capabilities is by executing a program with associated file capabili‐
756 ties:
757 prctl(PR_SET_SECUREBITS,
759 /* SECBIT_KEEP_CAPS off */
760 SECBIT_KEEP_CAPS_LOCKED |
761 SECBIT_NO_SETUID_FIXUP |
762 SECBIT_NO_SETUID_FIXUP_LOCKED |
763 SECBIT_NOROOT |
764 SECBIT_NOROOT_LOCKED);
765 /* Setting/locking SECBIT_NO_CAP_AMBIENT_RAISE
766 is not required */
767 Interaction with user namespaces
769 For a discussion of the interaction of capabilities and user names‐
770 paces, see user_namespaces(7).
771
773 No standards govern capabilities, but the Linux capability implementa‐
774 tion is based on the withdrawn POSIX.1e draft standard; see
775 ⟨http://wt.tuxomania.net/publications/posix.1e/⟩.
776
778 From kernel 2.5.27 to kernel 2.6.26, capabilities were an optional ker‐
779 nel component, and could be enabled/disabled via the CONFIG_SECU‐
780 RITY_CAPABILITIES kernel configuration option.
781 The /proc/[pid]/task/TID/status file can be used to view the capability
783 sets of a thread. The /proc/[pid]/status file shows the capability
784 sets of a process's main thread. Before Linux 3.8, nonexistent capa‐
785 bilities were shown as being enabled (1) in these sets. Since Linux
786 3.8, all nonexistent capabilities (above CAP_LAST_CAP) are shown as
787 disabled (0).
788 The libcap package provides a suite of routines for setting and getting
790 capabilities that is more comfortable and less likely to change than
791 the interface provided by capset(2) and capget(2). This package also
792 provides the setcap(8) and getcap(8) programs. It can be found at
793 ⟨http://www.kernel.org/pub/linux/libs/security/linux-privs⟩.
794 Before kernel 2.6.24, and from kernel 2.6.24 to kernel 2.6.32 if file
796 capabilities are not enabled, a thread with the CAP_SETPCAP capability
797 can manipulate the capabilities of threads other than itself. However,
798 this is only theoretically possible, since no thread ever has CAP_SETP‐
799 CAP in either of these cases:
800 * In the pre-2.6.25 implementation the system-wide capability bounding
802 set, /proc/sys/kernel/cap-bound, always masks out this capability,
803 and this can not be changed without modifying the kernel source and
804 rebuilding.
805 * If file capabilities are disabled in the current implementation, then
807 init starts out with this capability removed from its per-process
808 bounding set, and that bounding set is inherited by all other pro‐
809 cesses created on the system.
810
812 capsh(1), setpriv(1), prctl(2), setfsuid(2), cap_clear(3),
813 cap_copy_ext(3), cap_from_text(3), cap_get_file(3), cap_get_proc(3),
814 cap_init(3), capgetp(3), capsetp(3), libcap(3), proc(5), creden‐
815 tials(7), pthreads(7), user_namespaces(7), captest(8), filecap(8), get‐
816 cap(8), netcap(8), pscap(8), setcap(8)
817 include/linux/capability.h in the Linux kernel source tree
819
821 This page is part of release 4.15 of the Linux man-pages project. A
822 description of the project, information about reporting bugs, and the
823 latest version of this page, can be found at
824 https://www.kernel.org/doc/man-pages/.
825Linux 2018-02-02 CAPABILITIES(7)