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),
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:
- 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_READ (since Linux 3.16)
- Allow reading the audit log via a multicast netlink socket.
- 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)
- 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
- invoke open_by_handle_at(2);
- use the linkat(2) AT_EMPTY_PATH flag to create a link to a
file referred to by a file descriptor.
- 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;
- modify user extended attributes on sticky directory owned by any
- specify O_NOATIME for arbitrary files in open(2) and
- Don't clear set-user-ID and set-group-ID mode bits when a file is
- 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.
- Lock memory (mlock(2), mlockall(2), mmap(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 inode flags (see
- CAP_MAC_ADMIN (since Linux 2.6.25)
- Allow MAC configuration or state changes. Implemented for the Smack Linux
Security Module (LSM).
- CAP_MAC_OVERRIDE (since Linux 2.6.25)
- Override Mandatory Access Control (MAC). Implemented for the Smack
- 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
- Bind a socket to Internet domain privileged ports (port numbers less than
- (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
- forge GID when passing socket credentials via UNIX domain sockets;
- write a group ID mapping in a user namespace (see
- CAP_SETFCAP (since Linux 2.6.24)
- 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 securebits flags.
- 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.)
- Make arbitrary manipulations of process UIDs (setuid(2),
setreuid(2), setresuid(2), setfsuid(2));
- forge UID when passing socket credentials via UNIX domain sockets;
- write a user ID mapping in a user namespace (see
- Note: this capability is overloaded; see Notes to kernel
- Perform a range of system administration operations including:
quotactl(2), mount(2), umount(2),
pivot_root(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 IPC objects;
- override RLIMIT_NPROC resource limit;
- perform operations on trusted and security Extended
Attributes (see xattr(7));
- 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 PID when passing socket credentials via UNIX domain sockets;
- 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) and unshare(2) (but, since Linux 3.8, creating user
namespaces does not require any capability);
- call perf_event_open(2);
- access privileged perf event information;
- call setns(2) (requires CAP_SYS_ADMIN in the target
- call fanotify_init(2);
- call bpf(2);
- perform privileged KEYCTL_CHOWN and KEYCTL_SETPERM
- 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 privileged ioctl(2) operations on the /dev/random
device (see random(4));
- install a seccomp(2) filter without first having to set the
no_new_privs thread attribute;
- 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
- perform administrative operations on many device drivers.
- Modify autogroup nice values by writing to /proc/[pid]/autogroup
- Use reboot(2) and kexec_load(2).
- Use chroot(2);
- change mount namespaces using setns(2).
- Load and unload kernel modules (see init_module(2) and
- 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
- Use acct(2).
- Trace arbitrary processes using ptrace(2);
- apply get_robust_list(2) to arbitrary processes;
- transfer data to or from the memory of arbitrary processes using
process_vm_readv(2) and process_vm_writev(2);
- inspect processes using kcmp(2).
- 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, see
- update /proc/sys/vm/mmap_min_addr;
- create memory mappings at addresses below the value specified by
- 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)
- 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;
- raise msg_qbytes limit for a System V message queue above the limit
in /proc/sys/kernel/msgmnb (see msgop(2) and
- 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
- override the /proc/sys/fs/pipe-size-max limit when setting the
capacity of a pipe using the F_SETPIPE_SZ fcntl(2)
- 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 the 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.
- Use vhangup(2); employ various privileged ioctl(2)
operations on virtual terminals.
- CAP_SYSLOG (since Linux 2.6.37)
- Perform privileged syslog(2) operations. See syslog(2) for
information on which operations require privilege.
- View kernel addresses exposed via /proc and other interfaces when
/proc/sys/kernel/kptr_restrict has the value 1. (See the discussion
of the kptr_restrict in proc(5).)
- CAP_WAKE_ALARM (since Linux 3.0)
- 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 used 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
- 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 the following 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 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
- 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.
- Bounding (per-thread since Linux 2.6.25)
- The capability bounding set is a mechanism that can be used to limit the
capabilities that are gained during execve(2).
- Since Linux 2.6.25, this is a per-thread capability set. In older kernels,
the capability bounding set was a system wide attribute shared by all
threads on the system.
- For more details on the capability bounding set, see below.
- 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. If ambient capabilities cause a process's permitted and effective
capabilities to increase during an execve(2), this does not trigger
the secure-execution mode described in ld.so(8).
A child created via fork
(2) inherits copies of its parent's capability
sets. See below for a discussion of the treatment of capabilities during
(2), a thread may manipulate its own capability sets (see
Since Linux 3.2, the file /proc/sys/kernel/cap_last_cap
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
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
. Writing to this extended attribute requires the
capability. The file capability sets, in conjunction with
the capability sets of the thread, determine the capabilities of a thread
after an execve
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
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
To allow extensibility, the kernel supports a scheme to encode a version number
inside the security.capability
extended attribute that is used to
implement file capabilities. These version numbers are internal to the
implementation, and not directly visible to user-space applications. To date,
the following versions are supported:
- This was the original file capability implementation, which supported
32-bit masks for file capabilities.
- VFS_CAP_REVISION_2 (since Linux 2.6.25)
- This version allows for file capability masks that are 64 bits in size,
and was necessary as the number of supported capabilities grew beyond 32.
The kernel transparently continues to support the execution of files that
have 32-bit version 1 capability masks, but when adding capabilities to
files that did not previously have capabilities, or modifying the
capabilities of existing files, it automatically uses the version 2 scheme
(or possibly the version 3 scheme, as described below).
- VFS_CAP_REVISION_3 (since Linux 4.14)
- Version 3 file capabilities are provided to support namespaced file
capabilities (described below).
- As with version 2 file capabilities, version 3 capability masks are 64
bits in size. But in addition, the root user ID of namespace is encoded in
the security.capability extended attribute. (A namespace's root
user ID is the value that user ID 0 inside that namespace maps to in the
initial user namespace.)
- Version 3 file capabilities are designed to coexist with version 2
capabilities; that is, on a modern Linux system, there may be some files
with version 2 capabilities while others have version 3 capabilities.
Before Linux 4.14, the only kind of file capability extended attribute that
could be attached to a file was a VFS_CAP_REVISION_2
Linux 4.14, the version of the security.capability
that is attached to a file depends on the circumstances in which the attribute
Starting with Linux 4.14, a security.capability
extended attribute is
automatically created as (or converted to) a version 3
) attribute if both of the following are true:
- The thread writing the attribute resides in a noninitial user namespace.
(More precisely: the thread resides in a user namespace other than the one
from which the underlying filesystem was mounted.)
- The thread has the CAP_SETFCAP capability over the file inode,
meaning that (a) the thread has the CAP_SETFCAP capability in its
own user namespace; and (b) the UID and GID of the file inode have
mappings in the writer's user namespace.
When a VFS_CAP_REVISION_3 security.capability
is created, the root user ID of the creating thread's user namespace is saved
in the extended attribute.
By contrast, creating or modifying a security.capability
attribute from a privileged (CAP_SETFCAP
) thread that resides in the
namespace where the underlying filesystem was mounted (this normally means the
initial user namespace) automatically results in the creation of a version 2
Note that the creation of a version 3 security.capability
attribute is automatic. That is to say, when a user-space application writes
(2)) a security.capability
attribute in the version 2
format, the kernel will automatically create a version 3 attribute if the
attribute is created in the circumstances described above. Correspondingly,
when a version 3 security.capability
attribute is retrieved
(2)) by a process that resides inside a user namespace that
was created by the root user ID (or a descendant of that user namespace), the
returned attribute is (automatically) simplified to appear as a version 2
attribute (i.e., the returned value is the size of a version 2 attribute and
does not include the root user ID). These automatic translations mean that no
changes are required to user-space tools (e.g., setcap
(1)) in order for those tools to be used to create and retrieve
version 3 security.capability
Note that a file can have either a version 2 or a version 3
extended attribute associated with it, but not
both: creation or modification of the security.capability
attribute will automatically modify the version according to the circumstances
in which the extended attribute is created or modified.
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) & P(bounding)) | P'(ambient)
P'(effective) = F(effective) ? P'(permitted) : P'(ambient)
P'(inheritable) = P(inheritable) [i.e., unchanged]
P'(bounding) = P(bounding) [i.e., unchanged]
- denotes the value of a thread capability set before the
- denotes the value of a thread capability set after the
- denotes a file capability set
Note the following details relating to the above capability transformation
- The ambient capability set is present only since Linux 4.3. When
determining the transformation of the ambient set during execve(2),
a privileged file is one that has capabilities or has the set-user-ID or
set-group-ID bit set.
- Prior to Linux 2.6.25, the bounding set was a system-wide attribute shared
by all threads. That system-wide value was employed to calculate the new
permitted set during execve(2) in the same manner as shown above
: during the capability transitions described above, file
capabilities may be ignored (treated as empty) for the same reasons that the
set-user-ID and set-group-ID bits are ignored; see execve
capabilities are similarly ignored if the kernel was booted with the
: 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
(2), see below under Capabilities and execution of programs by
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
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,
(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
In order to mirror traditional UNIX semantics, the kernel performs special
treatment of file capabilities when a process with UID 0 (root) executes a
program and when a set-user-ID-root program is executed.
After having performed any changes to the process effective ID that were
triggered by the set-user-ID mode bit of the binary—e.g., switching the
effective user ID to 0 (root) because a set-user-ID-root program was
executed—the kernel calculates the file capability sets as follows:
- If the real or effective user ID of the process is 0 (root), then the file
inheritable and permitted sets are ignored; instead they are notionally
considered to be all ones (i.e., all capabilities enabled). (There is one
exception to this behavior, described below in Set-user-ID-root
programs that have file capabilities.)
- If the effective user ID of the process is 0 (root) or the file effective
bit is in fact enabled, then the file effective bit is notionally defined
to be one (enabled).
These notional values for the file's capability sets are then used as described
above to calculate the transformation of the process's capabilities during
Thus, when a process with nonzero UIDs execve
(2)s a set-user-ID-root
program that does not have capabilities attached, or when a process whose real
and effective UIDs are zero execve
(2)s a program, the calculation of
the process's new permitted capabilities simplifies to:
P'(permitted) = P(inheritable) | P(bounding)
P'(effective) = P'(permitted)
Consequently, the process gains all capabilities in its permitted and effective
capability sets, except those masked out by the capability bounding set. (In
the calculation of P'(permitted), the P'(ambient) term can be simplified away
because it is by definition a proper subset of P(inheritable).)
The special treatments of user ID 0 (root) described in this subsection can be
disabled using the securebits mechanism described below.
There is one exception to the behavior described under Capabilities and
execution of programs by root
. If (a) the binary that is being executed
has capabilities attached and (b) the real user ID of the process is
0 (root) and (c) the effective user ID of the process is
(root), then the file capability bits are honored (i.e., they are not
notionally considered to be all ones). The usual way in which this situation
can arise is when executing a set-UID-root program that also has file
capabilities. When such a program is executed, the process gains 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 from Linux 2.6.25 onward
From Linux 2.6.25, the capability bounding set
is a per-thread attribute.
(The system-wide capability bounding set described below no longer exists.)
The bounding set is inherited at fork
(2) from the thread's parent, and is
preserved across an execve
A thread may remove capabilities from its capability bounding set using the
operation, provided it has the
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
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
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
process (the ancestor of all processes) begins with a full
bounding set. If file capabilities are not compiled into the kernel, then
begins with a full bounding set minus CAP_SETPCAP
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.
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
. (Confusingly, this bit mask parameter
is expressed as a signed decimal number in /proc/sys/kernel/cap-bound
Only the init
process may set capabilities in the capability bounding
set; other than that, the superuser (more precisely: a process with the
capability) may only clear capabilities from this set.
On a standard system the capability bounding set always masks out the
capability. To remove this restriction (dangerous!), modify
the definition of CAP_INIT_EFF_SET
and rebuild the kernel.
The system-wide capability bounding set feature was added to Linux starting with
kernel version 2.2.11.
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
(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
flag described below.
A thread can retrieve and change its permitted, effective, and inheritable
capability sets using the capget
(2) and capset
(2) system calls.
However, the use of cap_get_proc
(3) and cap_set_proc
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
- 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 set 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 permitted capabilities. This flag is always cleared on
- Note that even with the SECBIT_KEEP_CAPS flag set, the effective
capabilities of a thread are cleared when it switches its effective UID to
a nonzero value. However, if the thread has set this flag and its
effective UID is already nonzero, and the thread subsequently switches all
other UIDs to nonzero values, then the effective capabilities will not be
- 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)
- 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
flags can be modified and retrieved using the
operations. The CAP_SETPCAP
capability is required to modify the flags.
Note that the SECBIT_*
constants are available only after including the
flags are inherited by child processes. During an
(2), all of the flags are preserved, except
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:
/* SECBIT_KEEP_CAPS off */
/* Setting/locking SECBIT_NO_CAP_AMBIENT_RAISE
is not required */
A set-user-ID program whose UID matches the UID that created a user namespace
will confer capabilities in the process's permitted and effective sets when
executed by any process inside that namespace or any descendant user
The rules about the transformation of the process's capabilities during the
(2) are exactly as described in the subsections Transformation
of capabilities during execve()
and Capabilities and execution of
programs by root
, with the difference that, in the latter subsection,
"root" is the UID of the creator of the user namespace.
Traditional (i.e., version 2) file capabilities associate only a set of
capability masks with a binary executable file. When a process executes a
binary with such capabilities, it gains the associated capabilities (within
its user namespace) as per the rules described above in "Transformation
of capabilities during execve()".
Because version 2 file capabilities confer capabilities to the executing process
regardless of which user namespace it resides in, only privileged processes
are permitted to associate capabilities with a file. Here,
"privileged" means a process that has the CAP_SETFCAP
capability in the user namespace where the filesystem was mounted (normally
the initial user namespace). This limitation renders file capabilities useless
for certain use cases. For example, in user-namespaced containers, it can be
desirable to be able to create a binary that confers capabilities only to
processes executed inside that container, but not to processes that are
executed outside the container.
Linux 4.14 added so-called namespaced file capabilities to support such use
cases. Namespaced file capabilities are recorded as version 3 (i.e.,
Such an attribute is automatically created in the circumstances described
above under "File capability extended attribute versioning". When a
version 3 security.capability
extended attribute is created, the kernel
records not just the capability masks in the extended attribute, but also the
namespace root user ID.
As with a binary that has VFS_CAP_REVISION_2
file capabilities, a binary
file capabilities confers capabilities to a
process during execve
(). However, capabilities are conferred only if
the binary is executed by a process that resides in a user namespace whose UID
0 maps to the root user ID that is saved in the extended attribute, or when
executed by a process that resides in a descendant of such a namespace.
For further information on the interaction of capabilities and user namespaces,
No standards govern capabilities, but the Linux capability implementation is
based on the withdrawn POSIX.1e draft standard; see
When attempting to strace
(1) binaries that have capabilities (or
set-user-ID-root binaries), you may find the -u <username>
useful. Something like:
$ sudo strace -o trace.log -u ceci ./myprivprog
From kernel 2.5.27 to kernel 2.6.26, capabilities were an optional kernel
component, and could be enabled/disabled via the
kernel configuration option.
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).
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
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
can manipulate the capabilities of threads other than itself. However, this is
only theoretically possible, since no thread ever has CAP_SETPCAP
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 the CAP_SETPCAP
capability, and this can not be changed without modifying the kernel
source and rebuilding the kernel.
- If file capabilities are disabled (i.e., the kernel
CONFIG_SECURITY_FILE_CAPABILITIES option is disabled), then
init starts out with the CAP_SETPCAP capability removed from
its per-process bounding set, and that bounding set is inherited by all
other processes created on the system.
in the Linux kernel source tree