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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 non-zero).
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
The following list shows the capabilities implemented on Linux, and the
operations or behaviors that each capability permits:
CAP_AUDIT_CONTROL (since Linux 2.6.11)
Enable and disable kernel auditing; change auditing filter
rules; retrieve auditing status and filtering rules.
CAP_AUDIT_WRITE (since Linux 2.6.11)
Write records to kernel auditing log.
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 file read permission checks and directory read and
execute permission checks.
* Bypass permission checks on operations that normally require
the file system 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 extended file attributes (see chattr(1)) on arbitrary
* set Access Control Lists (ACLs) on arbitrary files;
* ignore directory sticky bit on file deletion;
* specify O_NOATIME for arbitrary files in open(2) and fcntl(2).
Don’t clear set-user-ID and set-group-ID permission bits when a
file is modified; set the set-group-ID bit for a file whose GID
does not match the file system or any of the supplementary GIDs
of the calling process.
Lock memory (mlock(2), mlockall(2), mmap(2), shmctl(2)).
Bypass permission checks for operations on System V IPC objects.
Bypass permission checks for sending signals (see kill(2)).
This includes use of the ioctl(2) KDSIGACCEPT operation.
CAP_LEASE (since Linux 2.4)
Establish leases on arbitrary files (see fcntl(2)).
Set the FS_APPEND_FL and FS_IMMUTABLE_FL i-node flags (see
CAP_MAC_ADMIN (since Linux 2.6.25)
Override Mandatory Access Control (MAC). Implemented for the
Smack Linux Security Module (LSM).
CAP_MAC_OVERRIDE (since Linux 2.6.25)
Allow MAC configuration or state changes. Implemented for the
CAP_MKNOD (since Linux 2.4)
Create special files using mknod(2).
Perform various network-related operations (e.g., setting
privileged socket options, enabling multicasting, interface
configuration, modifying routing tables).
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.
Make arbitrary manipulations of process GIDs and supplementary
GID list; forge GID when passing socket credentials via Unix
CAP_SETFCAP (since Linux 2.6.24)
Set file capabilities.
If file capabilities are not supported: 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.)
If file capabilities are supported: 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 securebits flags.
Make arbitrary manipulations of process UIDs (setuid(2),
setreuid(2), setresuid(2), setfsuid(2)); make forged UID when
passing socket credentials via Unix domain sockets.
* Perform a range of system administration operations including:
quotactl(2), mount(2), umount(2), swapon(2), swapoff(2),
sethostname(2), and setdomainname(2);
* perform IPC_SET and IPC_RMID operations on arbitrary System V
* perform operations on trusted and security Extended Attributes
* use lookup_dcookie(2);
* use ioprio_set(2) to assign IOPRIO_CLASS_RT and (before Linux
2.6.25) IOPRIO_CLASS_IDLE I/O scheduling classes;
* forge UID when passing socket credentials;
* exceed /proc/sys/fs/file-max, the system-wide limit on the
number of open files, in system calls that open files (e.g.,
accept(2), execve(2), open(2), pipe(2));
* employ CLONE_NEWNS flag with clone(2) and unshare(2);
* perform KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2) operations.
Use reboot(2) and kexec_load(2).
Load and unload kernel modules (see init_module(2) and
delete_module(2)); in kernels before 2.6.25: drop capabilities
from the system-wide capability bounding set.
* Raise process nice value (nice(2), setpriority(2)) and change
the nice value for arbitrary processes;
* set real-time scheduling policies for calling process, and set
scheduling policies and priorities for arbitrary processes
* set CPU affinity for arbitrary processes
* set I/O scheduling class and priority for arbitrary processes
* apply migrate_pages(2) to arbitrary processes and allow
processes to be migrated to arbitrary nodes;
* apply move_pages(2) to arbitrary processes;
* use the MPOL_MF_MOVE_ALL flag with mbind(2) and move_pages(2).
Trace arbitrary processes using ptrace(2)
Perform I/O port operations (iopl(2) and ioperm(2)); access
* Use reserved space on ext2 file systems;
* make ioctl(2) calls controlling ext3 journaling;
* override disk quota limits;
* increase resource limits (see setrlimit(2));
* override RLIMIT_NPROC resource limit;
* raise msg_qbytes limit for a System V message queue above the
limit in /proc/sys/kernel/msgmnb (see msgop(2) and msgctl(2)).
Set system clock (settimeofday(2), stime(2), adjtimex(2)); set
real-time (hardware) clock.
Past and Current Implementation
A full implementation of capabilities requires that:
1. For all privileged operations, the kernel must check whether the
thread has the required capability in its effective set.
2. The kernel must provide system calls allowing a thread’s capability
sets to be changed and retrieved.
3. The file system must support attaching capabilities to an executable
file, so that a process gains those capabilities when the file is
Before kernel 2.6.24, only the first two of these requirements are met;
since kernel 2.6.24, all three requirements are met.
Thread Capability Sets
Each thread has three capability sets containing zero or more of the
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
If a thread drops a capability from its permitted set, it can
never re-acquire 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). It
provides a mechanism for a process to assign capabilities to the
permitted set of the new program during an execve(2).
This is the set of capabilities used by the kernel to perform
permission checks for the thread.
A child created via fork(2) inherits copies of its parent’s capability
sets. See below for a discussion of the treatment of capabilities
Using capset(2), a thread may manipulate its own capability sets (see
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)) 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 forced):
These capabilities are automatically permitted to the thread,
regardless of the thread’s inheritable capabilities.
Inheritable (formerly known as allowed):
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 transormation 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.
Transformation of Capabilities During execve()
During an execve(2), the kernel calculates the new capabilities of the
process using the following algorithm:
P’(permitted) = (P(inheritable) & F(inheritable)) |
(F(permitted) & cap_bset)
P’(effective) = F(effective) ? P’(permitted) : 0
P’(inheritable) = P(inheritable) [i.e., unchanged]
P denotes the value of a thread capability set before the
P’ denotes the value of a capability set after the execve(2)
F denotes a file capability set
cap_bset is the value of the capability bounding set (described
Capabilities and execution of programs by root
In order to provide an all-powerful root using capability sets, during
1. If a set-user-ID-root program is being executed, or the real 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).
2. If a set-user-ID-root program is being executed, 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 that 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. This provides semantics that are the same as those
provided by traditional Unix systems.
Capability bounding set
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 the capability
is not in the bounding set, then a thread can’t add one of its
permitted capabilities to its inheritable set and thereby have that
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 inherited capabilities. If a thread maintains a capability in
its inherited 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 inherited 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: programs 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 onwards
From Linux 2.6.25, the capability bounding set is a per-thread
attribute. (There is no longer a system-wide capability bounding set.)
The bounding set is inherited at fork(2) from the thread’s parent, and
is preserved across an execve(2).
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
Removing capabilities from the bounding set is only supported if file
capabilities are compiled into the kernel
(CONFIG_SECURITY_FILE_CAPABILITIES). In that case, 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 inherited set. However it does prevent the capability from
being added back into the thread’s inherited set in the future.
Effect of User ID Changes on Capabilities
To preserve the traditional semantics for transitions between 0 and
non-zero 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 file system user IDs (using setuid(2), setresuid(2), or similar):
1. 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 non-zero value, then all capabilities are cleared from the
permitted and effective capability sets.
2. If the effective user ID is changed from 0 to non-zero, then all
capabilities are cleared from the effective set.
3. If the effective user ID is changed from non-zero to 0, then the
permitted set is copied to the effective set.
4. If the file system user ID is changed from 0 to non-zero (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, and CAP_MAC_OVERRIDE. If the file system
UID is changed from non-zero to 0, then any of these capabilities
that are enabled in the permitted set are enabled in the effective
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 non-zero values, it can do so using the prctl(2)
Programmatically adjusting capability sets
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:
1. 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.
2. (Since kernel 2.6.25) The new inheritable set must be a subset of
the combination of the existing inheritable set and the capability
3. 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).
4. The new effective set must be a subset of the new permitted set.
The "securebits" flags: establishing a capabilities-only environment
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 its capabilities when it switches all of its UIDs to a
non-zero value. If this flag is not set, then such a UID switch
causes the thread to lose all capabilities. This flag is always
cleared on an execve(2). (This flag provides the same
functionality as the older prctl(2) PR_SET_KEEPCAPS operation.)
Setting this flag stops the kernel from adjusting capability
sets when the threads’s effective and file system UIDs are
switched between zero and non-zero 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.)
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: SECURE_KEEP_CAPS_LOCKED,
SECURE_NO_SETUID_FIXUP_LOCKED, and SECURE_NOROOT_LOCKED.
The 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.
The securebits flags are inherited by child processes. During an
execve(2), all of the flags are preserved, except SECURE_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
1 << SECURE_KEEP_CAPS_LOCKED |
1 << SECURE_NO_SETUID_FIXUP |
1 << SECURE_NO_SETUID_FIXUP_LOCKED |
1 << SECURE_NOROOT |
1 << SECURE_NOROOT_LOCKED);
No standards govern capabilities, but the Linux capability
implementation is based on the withdrawn POSIX.1e draft standard; see
Since kernel 2.5.27, capabilities are an optional kernel component, and
can be enabled/disabled via the CONFIG_SECURITY_CAPABILITIES kernel
The /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.
The 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 since kernel 2.6.24 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
* 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.
capget(2), 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), credentials(7), pthreads(7), getcap(8),
include/linux/capability.h in the kernel source
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be found at http://www.kernel.org/doc/man-pages/.