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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 unpriv-
ileged processes are subject to full permission checking based on the
process's credentials (usually: effective UID, effective GID, and sup-
plementary 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.
CAP_BLOCK_SUSPEND (since Linux 3.5)
Employ features that can block system suspend (epoll(7) EPOLL-
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 exe-
cute permission checks;
* Invoke open_by_handle_at(2).
* 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 extended file attributes (see chattr(1)) on arbitrary
* set Access Control Lists (ACLs) on arbitrary files;
* ignore directory sticky bit on file deletion;
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:
* interface configuration;
* 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;
* enabling multicasting;
* 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
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 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
* 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_* flags that create new namespaces with clone(2)
* call perf_event_open(2);
* access privileged perf event information;
* call setns(2);
* call fanotify_init(2);
* perform KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2) operations;
* 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
* 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;
* perform administrative operations on many device drivers.
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.
* use the MPOL_MF_MOVE_ALL flag with mbind(2) and move_pages(2).
Trace arbitrary processes using ptrace(2); apply
get_robust_list(2) to arbitrary processes; inspect processes
* Perform I/O port operations (iopl(2) and ioperm(2));
* access /proc/kcore;
* employ the FIBMAP ioctl(2) operation;
* open devices for accessing x86 model-specific registers (MSRs,
* update /proc/sys/vm/mmap_min_addr;
* create memory mappings at addresses below the value specified
* map files in /proc/bus/pci;
* open /dev/mem and /dev/kmem;
* perform various SCSI device commands;
* perform certain operations on hpsa(4) and cciss(4) devices;
* perform a range of device-specific operations on other
* 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;
* raise msg_qbytes limit for a System V message queue above the
limit in /proc/sys/kernel/msgmnb (see msgop(2) and msgctl(2));
* override the /proc/sys/fs/pipe-size-max limit 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/pipe-max-size;
* override /proc/sys/fs/mqueue/queues_max limit when creating
POSIX message queues (see mq_overview(7));
* employ prctl(2) PR_SET_MM operation;
* set /proc/PID/oom_score_adj to a value lower than the value
last set by a process with CAP_SYS_RESOURCE.
Set system clock (settimeofday(2), stime(2), adjtimex(2)); set
real-time (hardware) clock.
TIME_ALARM and CLOCK_BOOTTIME_ALARM timers).
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 filesystem 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 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). 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 dur-
Using capset(2), a thread may manipulate its own capability sets (see
Since Linux 3.2, the file /proc/sys/kernel/cap_last_cap exposes the
numerical value of the highest capability supported by the running ker-
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 capabili-
ties 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 capabili-
ties for which the corresponding permitted or inheritable flags
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
effective capability sets, except those masked out by the capability
bounding set. This provides semantics that are the same as those pro-
vided 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 inherita-
ble 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
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!), mod-
ify 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.
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 con-
figuration option has been removed and file capabilities are always
part of the kernel. When file capabilities are compiled into the ker-
nel, the init process (the ancestor of all processes) begins with a
full bounding set. If file capabilities are not compiled into the ker-
nel, then init begins with a full bounding set minus CAP_SETPCAP,
because this capability has a different meaning when there are no file
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
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):
1. If one or more of the real, effective or saved set user IDs was pre-
viously 0, and as a result of the UID changes all of these IDs have
a nonzero value, then all capabilities are cleared from the permit-
ted and effective capability sets.
2. If the effective user ID is changed from 0 to nonzero, then all
capabilities are cleared from the effective set.
3. If the effective user ID is changed from nonzero to 0, then the per-
mitted set is copied to the effective set.
4. If the filesystem user ID is changed from 0 to nonzero (see setf-
suid(2)) then the following capabilities are cleared from the effec-
tive 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 capa-
bilities that are enabled in the permitted set are enabled in the
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 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
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 capabili-
ties 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
nonzero 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 function-
ality 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 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.)
Each of the above "base" flags has a companion "locked" flag. Setting
any of the "locked" flags is irreversible, and has the effect of pre-
venting further changes to the corresponding "base" flag. The locked
flags are: SECBIT_KEEP_CAPS_LOCKED, SECBIT_NO_SETUID_FIXUP_LOCKED, and
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 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 capabili-
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
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 pro-
cesses 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), libcap(3), credentials(7), pthreads(7), get-
include/linux/capability.h in the Linux kernel source tree
Linux 2013-09-27 CAPABILITIES(7)