capabilities - overview of Linux capabilities
For the purpose of performing permission checks, traditional UNIX implementations distinguish two categories of processes:
privileged processes (whose effective user ID is 0, referred to as superuser or root), and
unprivileged processes (whose effective UID is nonzero). Privileged processes bypass all kernel permission checks, while unprivileged processes are subject to full permission checking based on the process's credentials (usually: effective UID, effective GID, and supplementary group list).
Starting with kernel 2.2, Linux divides the privileges traditionally associated with superuser into distinct units, known as
capabilities, which can be independently enabled and disabled. Capabilities are a per-thread attribute. The following list shows the capabilities implemented on Linux, and the operations or behaviors that each capability permits:
Enable and disable kernel auditing; change auditing filter rules; retrieve auditing status and filtering rules.
Allow reading the audit log via a multicast netlink socket.
Write records to kernel auditing log.
Employ features that can block system suspend (epoll(7) EPOLLWAKEUP,
Make arbitrary changes to file UIDs and GIDs (see chown(2)).
Bypass file read, write, and execute permission checks. (DAC is an abbreviation of "discretionary access control".)
Bypass permission checks on operations that normally require the filesystem UID of the process to match the UID of the file (e.g., chmod(2), utime(2)), excluding those operations covered by CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH;
set inode flags (see ioctl_iflags(2)) on arbitrary files;
set Access Control Lists (ACLs) on arbitrary files;
ignore directory sticky bit on file deletion;
Don't clear set-user-ID and set-group-ID mode bits when a file is modified;
set the set-group-ID bit for a file whose GID does not match the filesystem or any of the supplementary GIDs of the calling process.
Bypass permission checks for operations on System V IPC objects.
Establish leases on arbitrary files (see fcntl(2)).
Set the FS_APPEND_FL and FS_IMMUTABLE_FL inode flags (see ioctl_iflags(2)).
Allow MAC configuration or state changes. Implemented for the Smack Linux Security Module (LSM).
Override Mandatory Access Control (MAC). Implemented for the Smack LSM.
Create special files using mknod(2).
Perform various network-related operations:
administration of IP firewall, masquerading, and accounting;
modify routing tables;
bind to any address for transparent proxying;
set type-of-service (TOS)
clear driver statistics;
set promiscuous mode;
use setsockopt(2) to set the following socket options: SO_DEBUG, SO_MARK, SO_PRIORITY (for a priority outside the range 0 to 6), SO_RCVBUFFORCE, and SO_SNDBUFFORCE.
Bind a socket to Internet domain privileged ports (port numbers less than 1024).
(Unused) Make socket broadcasts, and listen to multicasts.
Use RAW and PACKET sockets;
bind to any address for transparent proxying.
Make arbitrary manipulations of process GIDs and supplementary GID list;
forge GID when passing socket credentials via UNIX domain sockets;
write a group ID mapping in a user namespace (see user_namespaces(7)).
Set arbitrary capabilities on a file.
If file capabilities are supported (i.e., since Linux 2.6.24): add any capability from the calling thread's bounding set to its inheritable set; drop capabilities from the bounding set (via prctl(2) PR_CAPBSET_DROP); make changes to the
If file capabilities are not supported (i.e., kernels before Linux 2.6.24): grant or remove any capability in the caller's permitted capability set to or from any other process. (This property of CAP_SETPCAP is not available when the kernel is configured to support file capabilities, since CAP_SETPCAP has entirely different semantics for such kernels.)
Note: this capability is overloaded; see
Notes to kernel developers, below.
perform privileged syslog(2) operations (since Linux 2.6.37, CAP_SYSLOG should be used to permit such operations);
perform VM86_REQUEST_IRQ vm86(2) command;
perform IPC_SET and IPC_RMID operations on arbitrary System V IPC objects;
override RLIMIT_NPROC resource limit;
perform operations on
securityExtended Attributes (see xattr(7));
use ioprio_set(2) to assign IOPRIO_CLASS_RT and (before Linux 2.6.25) IOPRIO_CLASS_IDLE I/O scheduling classes;
forge PID when passing socket credentials via UNIX domain sockets;
call setns(2) (requires CAP_SYS_ADMIN in the
perform privileged KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2) operations;
use ptrace(2) PTRACE_SECCOMP_GET_FILTER to dump a tracees seccomp filters;
perform madvise(2) MADV_HWPOISON operation;
employ the TIOCSTI ioctl(2) to insert characters into the input queue of a terminal other than the caller's controlling terminal;
employ the obsolete nfsservctl(2) system call;
employ the obsolete bdflush(2) system call;
perform various privileged block-device ioctl(2) operations;
perform various privileged filesystem ioctl(2) operations;
install a seccomp(2) filter without first having to set the
modify allow/deny rules for device control groups;
employ the ptrace(2) PTRACE_SECCOMP_GET_FILTER operation to dump tracee's seccomp filters;
employ the ptrace(2) PTRACE_SETOPTIONS operation to suspend the tracee's seccomp protections (i.e., the PTRACE_O_SUSPEND_SECCOMP flag).
perform administrative operations on many device drivers.
set CPU affinity for arbitrary processes (sched_setaffinity(2));
set I/O scheduling class and priority for arbitrary processes (ioprio_set(2));
apply migrate_pages(2) to arbitrary processes and allow processes to be migrated to arbitrary nodes;
apply move_pages(2) to arbitrary processes;
employ the FIBMAP ioctl(2) operation;
open devices for accessing x86 model-specific registers (MSRs, see msr(4));
create memory mappings at addresses below the value specified by
map files in
perform various SCSI device commands;
perform a range of device-specific operations on other devices.
Use reserved space on ext2 filesystems;
make ioctl(2) calls controlling ext3 journaling;
override disk quota limits;
increase resource limits (see setrlimit(2));
override RLIMIT_NPROC resource limit;
override maximum number of consoles on console allocation;
override maximum number of keymaps;
allow more than 64hz interrupts from the real-time clock;
allow the RLIMIT_NOFILE resource limit on the number of "in-flight" file descriptors to be bypassed when passing file descriptors to another process via a UNIX domain socket (see unix(7));
/proc/sys/fs/pipe-size-maxlimit when setting the capacity of a pipe using the F_SETPIPE_SZ fcntl(2) command.
use F_SETPIPE_SZ to increase the capacity of a pipe above the limit specified by
/proc/sys/fs/mqueue/queues_maxlimit when creating POSIX message queues (see mq_overview(7));
employ the prctl(2) PR_SET_MM operation;
/proc/[pid]/oom_score_adjto a value lower than the value last set by a process with CAP_SYS_RESOURCE.
Trigger something that will wake up the system (set CLOCK_REALTIME_ALARM and CLOCK_BOOTTIME_ALARM timers). A full implementation of capabilities requires that:
For all privileged operations, the kernel must check whether the thread has the required capability in its effective set.
The kernel must provide system calls allowing a thread's capability sets to be changed and retrieved.
The filesystem must support attaching capabilities to an executable file, so that a process gains those capabilities when the file is executed.
Before kernel 2.6.24, only the first two of these requirements are met; since kernel 2.6.24, all three requirements are met. When adding a new kernel feature that should be governed by a capability, consider the following points.
The goal of capabilities is divide the power of superuser into pieces, such that if a program that has one or more capabilities is compromised, its power to do damage to the system would be less than the same program running with root privilege.
You have the choice of either creating a new capability for your new feature, or associating the feature with one of the existing capabilities. In order to keep the set of capabilities to a manageable size, the latter option is preferable, unless there are compelling reasons to take the former option. (There is also a technical limit: the size of capability sets is currently limited to 64 bits.)
To determine which existing capability might best be associated with your new feature, review the list of capabilities above in order to find a "silo" into which your new feature best fits. One approach to take is to determine if there are other features requiring capabilities that will always be use along with the new feature. If the new feature is useless without these other features, you should use the same capability as the other features.
Don't choose CAP_SYS_ADMIN if you can possibly avoid it! A vast proportion of existing capability checks are associated with this capability (see the partial list above). It can plausibly be called "the new root", since on the one hand, it confers a wide range of powers, and on the other hand, its broad scope means that this is the capability that is required by many privileged programs. Don't make the problem worse. The only new features that should be associated with CAP_SYS_ADMIN are ones that
closely match existing uses in that silo.
If you have determined that it really is necessary to create a new capability for your feature, don't make or name it as a "single-use" capability. Thus, for example, the addition of the highly specific CAP_SYS_PACCT was probably a mistake. Instead, try to identify and name your new capability as a broader silo into which other related future use cases might fit. Each thread has three capability sets containing zero or more of the above capabilities:
This is a limiting superset for the effective capabilities that the thread may assume. It is also a limiting superset for the capabilities that may be added to the inheritable set by a thread that does not have the CAP_SETPCAP capability in its effective set.
If a thread drops a capability from its permitted set, it can never reacquire that capability (unless it execve(2)s either a set-user-ID-root program, or a program whose associated file capabilities grant that capability).
This is a set of capabilities preserved across an execve(2). Inheritable capabilities remain inheritable when executing any program, and inheritable capabilities are added to the permitted set when executing a program that has the corresponding bits set in the file inheritable set.
Because inheritable capabilities are not generally preserved across execve(2) when running as a non-root user, applications that wish to run helper programs with elevated capabilities should consider using ambient capabilities, described below.
This is the set of capabilities used by the kernel to perform permission checks for the thread.
Ambient(since Linux 4.3):
This is a set of capabilities that are preserved across an execve(2) of a program that is not privileged. The ambient capability set obeys the invariant that no capability can ever be ambient if it is not both permitted and inheritable.
The ambient capability set can be directly modified using prctl(2). Ambient capabilities are automatically lowered if either of the corresponding permitted or inheritable capabilities is lowered.
Executing a program that changes UID or GID due to the set-user-ID or set-group-ID bits or executing a program that has any file capabilities set will clear the ambient set. Ambient capabilities are added to the permitted set and assigned to the effective set when execve(2) is called.
Using capset(2), a thread may manipulate its own capability sets (see below).
Since Linux 3.2, the file
/proc/sys/kernel/cap_last_cap exposes the numerical value of the highest capability supported by the running kernel; this can be used to determine the highest bit that may be set in a capability set. Since kernel 2.6.24, the kernel supports associating capability sets with an executable file using setcap(8). The file capability sets are stored in an extended attribute (see setxattr(2) and xattr(7)) named
security.capability. Writing to this extended attribute requires the CAP_SETFCAP capability. The file capability sets, in conjunction with the capability sets of the thread, determine the capabilities of a thread after an execve(2).
The three file capability sets are:
Permitted(formerly known as
These capabilities are automatically permitted to the thread, regardless of the thread's inheritable capabilities.
Inheritable(formerly known as
This set is ANDed with the thread's inheritable set to determine which inheritable capabilities are enabled in the permitted set of the thread after the execve(2).
This is not a set, but rather just a single bit. If this bit is set, then during an execve(2) all of the new permitted capabilities for the thread are also raised in the effective set. If this bit is not set, then after an execve(2), none of the new permitted capabilities is in the new effective set.
Enabling the file effective capability bit implies that any file permitted or inheritable capability that causes a thread to acquire the corresponding permitted capability during an execve(2) (see the transformation rules described below) will also acquire that capability in its effective set. Therefore, when assigning capabilities to a file (setcap(8), cap_set_file(3), cap_set_fd(3)), if we specify the effective flag as being enabled for any capability, then the effective flag must also be specified as enabled for all other capabilities for which the corresponding permitted or inheritable flags is enabled.
During an execve(2), the kernel calculates the new capabilities of the process using the following algorithm:
P'(ambient) = (file is privileged) ? 0 : P(ambient) P'(permitted) = (P(inheritable) & F(inheritable)) | (F(permitted) & cap_bset) | P'(ambient) P'(effective) = F(effective) ? P'(permitted) : P'(ambient) P'(inheritable) = P(inheritable) [i.e., unchanged]
denotes the value of a thread capability set before the execve(2)
denotes the value of a thread capability set after the execve(2)
denotes a file capability set
is the value of the capability bounding set (described below).
A privileged file is one that has capabilities or has the set-user-ID or set-group-ID bit set.
Note: the capability transitions described above may
not be performed (i.e., file capabilities may be ignored) for the same reasons that the set-user-ID and set-group-ID bits are ignored; see execve(2).
Note: according to the rules above, if a process with nonzero user IDs performs an execve(2) then any capabilities that are present in its permitted and effective sets will be cleared. For the treatment of capabilities when a process with a user ID of zero performs an execve(2), see below under
Capabilities and execution of programs by root. A capability-dumb binary is an application that has been marked to have file capabilities, but has not been converted to use the libcap(3) API to manipulate its capabilities. (In other words, this is a traditional set-user-ID-root program that has been switched to use file capabilities, but whose code has not been modified to understand capabilities.) For such applications, the effective capability bit is set on the file, so that the file permitted capabilities are automatically enabled in the process effective set when executing the file. The kernel recognizes a file which has the effective capability bit set as capability-dumb for the purpose of the check described here.
When executing a capability-dumb binary, the kernel checks if the process obtained all permitted capabilities that were specified in the file permitted set, after the capability transformations described above have been performed. (The typical reason why this might
not occur is that the capability bounding set masked out some of the capabilities in the file permitted set.) If the process did not obtain the full set of file permitted capabilities, then execve(2) fails with the error EPERM. This prevents possible security risks that could arise when a capability-dumb application is executed with less privilege that it needs. Note that, by definition, the application could not itself recognize this problem, since it does not employ the libcap(3) API. In order to provide an all-powerful
root using capability sets, during an execve(2):
If a set-user-ID-root program is being executed, or the real or effective user ID of the process is 0 (root) then the file inheritable and permitted sets are defined to be all ones (i.e., all capabilities enabled).
If a set-user-ID-root program is being executed, or the effective user ID of the process is 0 (root) then the file effective bit is defined to be one (enabled).
The upshot of the above rules, combined with the capabilities transformations described above, is as follows:
When a process execve(2)s a set-user-ID-root program, or when a process with an effective UID of 0 execve(2)s a program, it gains all capabilities in its permitted and effective capability sets, except those masked out by the capability bounding set.
When a process with a real UID of 0 execve(2)s a program, it gains all capabilities in its permitted capability set, except those masked out by the capability bounding set.
The above steps yield semantics that are the same as those provided by traditional UNIX systems. Executing a program that is both set-user-ID root and has file capabilities will cause the process to gain just the capabilities granted by the program (i.e., not all capabilities, as would occur when executing a set-user-ID-root program that does not have any associated file capabilities). Note that one can assign empty capability sets to a program file, and thus it is possible to create a set-user-ID-root program that changes the effective and saved set-user-ID of the process that executes the program to 0, but confers no capabilities to that process. The capability bounding set is a security mechanism that can be used to limit the capabilities that can be gained during an execve(2). The bounding set is used in the following ways:
During an execve(2), the capability bounding set is ANDed with the file permitted capability set, and the result of this operation is assigned to the thread's permitted capability set. The capability bounding set thus places a limit on the permitted capabilities that may be granted by an executable file.
(Since Linux 2.6.25) The capability bounding set acts as a limiting superset for the capabilities that a thread can add to its inheritable set using capset(2). This means that if a capability is not in the bounding set, then a thread can't add this capability to its inheritable set, even if it was in its permitted capabilities, and thereby cannot have this capability preserved in its permitted set when it execve(2)s a file that has the capability in its inheritable set.
Note that the bounding set masks the file permitted capabilities, but not the inheritable capabilities. If a thread maintains a capability in its inheritable set that is not in its bounding set, then it can still gain that capability in its permitted set by executing a file that has the capability in its inheritable set.
Depending on the kernel version, the capability bounding set is either a system-wide attribute, or a per-process attribute.
Capability bounding set prior to Linux 2.6.25
In kernels before 2.6.25, the capability bounding set is a system-wide attribute that affects all threads on the system. The bounding set is accessible via the file
/proc/sys/kernel/cap-bound. (Confusingly, this bit mask parameter is expressed as a signed decimal number in
Only the init process may set capabilities in the capability bounding set; other than that, the superuser (more precisely: a process with the CAP_SYS_MODULE capability) may only clear capabilities from this set.
On a standard system the capability bounding set always masks out the CAP_SETPCAP capability. To remove this restriction (dangerous!), modify the definition of CAP_INIT_EFF_SET in
include/linux/capability.h and rebuild the kernel.
The system-wide capability bounding set feature was added to Linux starting with kernel version 2.2.11. Capability bounding set from Linux 2.6.25 onward
From Linux 2.6.25, the
capability bounding set is a per-thread attribute. (There is no longer a system-wide capability bounding set.)
A thread may remove capabilities from its capability bounding set using the prctl(2) PR_CAPBSET_DROP operation, provided it has the CAP_SETPCAP capability. Once a capability has been dropped from the bounding set, it cannot be restored to that set. A thread can determine if a capability is in its bounding set using the prctl(2) PR_CAPBSET_READ operation.
Removing capabilities from the bounding set is supported only if file capabilities are compiled into the kernel. In kernels before Linux 2.6.33, file capabilities were an optional feature configurable via the CONFIG_SECURITY_FILE_CAPABILITIES option. Since Linux 2.6.33, the configuration option has been removed and file capabilities are always part of the kernel. When file capabilities are compiled into the kernel, the init process (the ancestor of all processes) begins with a full bounding set. If file capabilities are not compiled into the kernel, then init begins with a full bounding set minus CAP_SETPCAP, because this capability has a different meaning when there are no file capabilities.
Removing a capability from the bounding set does not remove it from the thread's inheritable set. However it does prevent the capability from being added back into the thread's inheritable set in the future.
To preserve the traditional semantics for transitions between 0 and nonzero user IDs, the kernel makes the following changes to a thread's capability sets on changes to the thread's real, effective, saved set, and filesystem user IDs (using setuid(2), setresuid(2), or similar):
If one or more of the real, effective or saved set user IDs was previously 0, and as a result of the UID changes all of these IDs have a nonzero value, then all capabilities are cleared from the permitted, effective, and ambient capability sets.
If the effective user ID is changed from 0 to nonzero, then all capabilities are cleared from the effective set.
If the effective user ID is changed from nonzero to 0, then the permitted set is copied to the effective set.
If the filesystem user ID is changed from 0 to nonzero (see setfsuid(2)), then the following capabilities are cleared from the effective set: CAP_CHOWN, CAP_DAC_OVERRIDE, CAP_DAC_READ_SEARCH, CAP_FOWNER, CAP_FSETID, CAP_LINUX_IMMUTABLE (since Linux 2.6.30), CAP_MAC_OVERRIDE, and CAP_MKNOD (since Linux 2.6.30). If the filesystem UID is changed from nonzero to 0, then any of these capabilities that are enabled in the permitted set are enabled in the effective set.
If a thread that has a 0 value for one or more of its user IDs wants to prevent its permitted capability set being cleared when it resets all of its user IDs to nonzero values, it can do so using the SECBIT_KEEP_CAPS securebits flag described below. A thread can retrieve and change its capability sets using the capget(2) and capset(2) system calls. However, the use of cap_get_proc(3) and cap_set_proc(3), both provided in the
libcap package, is preferred for this purpose. The following rules govern changes to the thread capability sets:
If the caller does not have the CAP_SETPCAP capability, the new inheritable set must be a subset of the combination of the existing inheritable and permitted sets.
(Since Linux 2.6.25) The new inheritable set must be a subset of the combination of the existing inheritable set and the capability bounding set.
The new permitted set must be a subset of the existing permitted set (i.e., it is not possible to acquire permitted capabilities that the thread does not currently have).
The new effective set must be a subset of the new permitted set.
Starting with kernel 2.6.26, and with a kernel in which file capabilities are enabled, Linux implements a set of per-thread
securebits flags that can be used to disable special handling of capabilities for UID 0 (
root). These flags are as follows:
Setting this flag allows a thread that has one or more 0 UIDs to retain capabilities in its permitted and effective sets when it switches all of its UIDs to nonzero values. If this flag is not set, then such a UID switch causes the thread to lose all capabilities in those sets. This flag is always cleared on an execve(2).
The setting of the SECBIT_KEEP_CAPS flag is ignored if the SECBIT_NO_SETUID_FIXUP flag is set. (The latter flag provides a superset of the effect of the former flag.)
This flag provides the same functionality as the older prctl(2) PR_SET_KEEPCAPS operation.
Setting this flag stops the kernel from adjusting the process's permitted, effective, and ambient capability sets when the thread's effective and filesystem UIDs are switched between zero and nonzero values. (See the subsection
Effect of user ID changes on capabilities.)
If this bit is set, then the kernel does not grant capabilities when a set-user-ID-root program is executed, or when a process with an effective or real UID of 0 calls execve(2). (See the subsection
Capabilities and execution of programs by root.)
Setting this flag disallows raising ambient capabilities via the prctl(2) PR_CAP_AMBIENT_RAISE operation.
Each of the above "base" flags has a companion "locked" flag. Setting any of the "locked" flags is irreversible, and has the effect of preventing further changes to the corresponding "base" flag. The locked flags are: SECBIT_KEEP_CAPS_LOCKED, SECBIT_NO_SETUID_FIXUP_LOCKED, SECBIT_NOROOT_LOCKED, and SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED.
securebits flags can be modified and retrieved using the prctl(2) PR_SET_SECUREBITS and PR_GET_SECUREBITS operations. The CAP_SETPCAP capability is required to modify the flags.
securebits flags are inherited by child processes. During an execve(2), all of the flags are preserved, except SECBIT_KEEP_CAPS which is always cleared.
An application can use the following call to lock itself, and all of its descendants, into an environment where the only way of gaining capabilities is by executing a program with associated file capabilities:
prctl(PR_SET_SECUREBITS, /* SECBIT_KEEP_CAPS off */ SECBIT_KEEP_CAPS_LOCKED | SECBIT_NO_SETUID_FIXUP | SECBIT_NO_SETUID_FIXUP_LOCKED | SECBIT_NOROOT | SECBIT_NOROOT_LOCKED); /* Setting/locking SECBIT_NO_CAP_AMBIENT_RAISE is not required */
For a discussion of the interaction of capabilities and user namespaces, see user_namespaces(7).
No standards govern capabilities, but the Linux capability implementation is based on the withdrawn POSIX.1e draft standard; see http://wt.tuxomania.net/publications/posix.1e/">.
From kernel 2.5.27 to kernel 2.6.26, capabilities were an optional kernel component, and could be enabled/disabled via the CONFIG_SECURITY_CAPABILITIES kernel configuration option.
/proc/[pid]/task/TID/status file can be used to view the capability sets of a thread. The
/proc/[pid]/status file shows the capability sets of a process's main thread. Before Linux 3.8, nonexistent capabilities were shown as being enabled (1) in these sets. Since Linux 3.8, all nonexistent capabilities (above CAP_LAST_CAP) are shown as disabled (0).
libcap package provides a suite of routines for setting and getting capabilities that is more comfortable and less likely to change than the interface provided by capset(2) and capget(2). This package also provides the setcap(8) and getcap(8) programs. It can be found at
Before kernel 2.6.24, and from kernel 2.6.24 to kernel 2.6.32 if file capabilities are not enabled, a thread with the CAP_SETPCAP capability can manipulate the capabilities of threads other than itself. However, this is only theoretically possible, since no thread ever has CAP_SETPCAP in either of these cases:
In the pre-2.6.25 implementation the system-wide capability bounding set,
/proc/sys/kernel/cap-bound, always masks out this capability, and this can not be changed without modifying the kernel source and rebuilding.
If file capabilities are disabled in the current implementation, then init starts out with this capability removed from its per-process bounding set, and that bounding set is inherited by all other processes created on the system.
capsh(1), setpriv(1), prctl(2), setfsuid(2), cap_clear(3), cap_copy_ext(3), cap_from_text(3), cap_get_file(3), cap_get_proc(3), cap_init(3), capgetp(3), capsetp(3), libcap(3), proc(5), credentials(7), pthreads(7), user_namespaces(7), captest(8), filecap(8), getcap(8), netcap(8), pscap(8), setcap(8)
include/linux/capability.h in the Linux kernel source tree
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