NAME top
capabilities - overview of Linux capabilities
DESCRIPTION top
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.
Capabilities list
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)
EPOLLWAKEUP, /proc/sys/wake_lock).
CAP_CHOWN
Make arbitrary changes to file UIDs and GIDs (see chown(2)).
CAP_DAC_OVERRIDE
Bypass file read, write, and execute permission checks. (DAC
is an abbreviation of "discretionary access control".)
CAP_DAC_READ_SEARCH
* Bypass file read permission checks and directory read and
execute permission checks;
* 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.
CAP_FOWNER
* 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;
* specify O_NOATIME for arbitrary files in open(2) and
fcntl(2).
CAP_FSETID
* 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.
CAP_IPC_LOCK
Lock memory (mlock(2), mlockall(2), mmap(2), shmctl(2)).
CAP_IPC_OWNER
Bypass permission checks for operations on System V IPC
objects.
CAP_KILL
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)).
CAP_LINUX_IMMUTABLE
Set the FS_APPEND_FL and FS_IMMUTABLE_FL inode flags (see
ioctl_iflags(2)).
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 LSM.
CAP_MKNOD (since Linux 2.4)
Create special files using mknod(2).
CAP_NET_ADMIN
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.
CAP_NET_BIND_SERVICE
Bind a socket to Internet domain privileged ports (port
numbers less than 1024).
CAP_NET_BROADCAST
(Unused) Make socket broadcasts, and listen to multicasts.
CAP_NET_RAW
* Use RAW and PACKET sockets;
* bind to any address for transparent proxying.
CAP_SETGID
* 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)).
CAP_SETFCAP (since Linux 2.6.24)
Set arbitrary capabilities on a file.
CAP_SETPCAP
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.)
CAP_SETUID
* 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
user_namespaces(7)).
CAP_SYS_ADMIN
Note: this capability is overloaded; see Notes to kernel
developers, below.
* Perform a range of system administration operations
including: quotactl(2), mount(2), umount(2), swapon(2),
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
namespace);
* call fanotify_init(2);
* call bpf(2);
* perform privileged 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 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;
* 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
PTRACE_O_SUSPEND_SECCOMP flag);
* perform administrative operations on many device drivers.
CAP_SYS_BOOT
Use reboot(2) and kexec_load(2).
CAP_SYS_CHROOT
Use chroot(2).
CAP_SYS_MODULE
* 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.
CAP_SYS_NICE
* 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 (sched_setscheduler(2), sched_setparam(2),
shed_setattr(2));
* 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;
* use the MPOL_MF_MOVE_ALL flag with mbind(2) and
move_pages(2).
CAP_SYS_PACCT
Use acct(2).
CAP_SYS_PTRACE
* 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_writev(2);
* inspect processes using kcmp(2).
CAP_SYS_RAWIO
* 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 msr(4));
* update /proc/sys/vm/mmap_min_addr;
* create memory mappings at addresses below the value
specified by /proc/sys/vm/mmap_min_addr;
* 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
devices.
CAP_SYS_RESOURCE
* 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));
* 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));
* 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 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.
CAP_SYS_TIME
Set system clock (settimeofday(2), stime(2), adjtimex(2)); set
real-time (hardware) clock.
CAP_SYS_TTY_CONFIG
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).
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 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.
Notes to kernel developers
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.
Thread capability sets
Each thread has three capability sets containing zero or more of the
above capabilities:
Permitted:
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).
Inheritable:
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.
Effective:
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.
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 execve(2).
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.
File capabilities
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 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).
Effective:
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.
File capability mask versioning
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:
VFS_CAP_REVISION_1
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 capability mask that could be
attached to a file was a VFS_CAP_REVISION_2 mask. Since Linux 4.14,
the version of the capability mask that is attached to a file depends
on the circumstances in which the security.capability extended
attribute was created.
Starting with Linux 4.14, a security.capability extended attribute is
automatically created as (or converted to) a version 3
(VFS_CAP_REVISION_3) attribute if both of the following are true:
(1) The thread writing the attribute resides in a noninitial
namespace. (More precisely: the thread resides in a user
namespace other than the one from which the underlying filesystem
was mounted.)
(2) 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 extended attribute is
created, the root user ID of the creating thread's user namespace is
saved in the extended attribute.
By contrast, creating a security.capability extended 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 a version 2
(VFS_CAP_REVISION_2) attribute.
Note that a file can have either a version 2 or a version 3
security.capability extended attribute associated with it, but not
both: creation or modification of the security.capability extended
attribute will automatically modify the version according to the
circumstances in which the extended attribute is created or modified.
Transformation of capabilities during execve()
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]
where:
P denotes the value of a thread capability set before the
execve(2)
P' denotes the value of a thread capability set after the
execve(2)
F denotes a file capability set
cap_bset 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.
Safety checking for capability-dumb binaries
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 under‐
stand 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 capabil‐
ity 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 secu‐
rity risks that could arise when a capability-dumb application is
executed with less privilege that it needs. Note that, by defini‐
tion, the application could not itself recognize this problem, since
it does not employ the libcap(3) API.
Capabilities and execution of programs by root
In order to provide an all-powerful root using capability sets, dur‐
ing an execve(2):
1. 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 inheri‐
table 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, 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 trans‐
formations 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.
Set-user-ID-root programs that have file capabilities
Executing a program that is both set-user-ID root and has file capa‐
bilities will cause the process to gain just the capabilities granted
by the program (i.e., not all capabilities, as would occur when exe‐
cuting 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.
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 inheri‐
table 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 inherita‐
ble 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. (Confus‐
ingly, this bit mask parameter is expressed as a signed decimal num‐
ber 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 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/capabil‐
ity.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.)
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 deter‐
mine 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_SETP‐
CAP, 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.
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
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.
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
permitted set is copied to the effective set.
4. 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.
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 exist‐
ing inheritable and permitted sets.
2. (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.
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 capabil‐
ities 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:
SECBIT_KEEP_CAPS
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.
SECBIT_NO_SETUID_FIXUP
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.)
SECBIT_NOROOT
If this bit is set, then the kernel does not grant capabili‐
ties 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.)
SECBIT_NO_CAP_AMBIENT_RAISE
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. Set‐
ting 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.
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‐
ties:
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 */
Interaction with user namespaces
For a discussion of the interaction of capabilities and user names‐
paces, see user_namespaces(7).
Namespaced file capabilities
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 exe‐
cuting 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., VFS_CAP_REVISION_3) security.capability extended attributes.
Such an attribute is automatically created when a process that
resides in a noninitial user namespace associates (setxattr(2)) file
capabilities with a file whose user ID matches the user ID of the
creator of the namespace. In this case, the kernel records not just
the capability masks in the extended attribute, but also the names‐
pace root user ID. For further details, see File capability mask
versioning, above.
As with a binary that has VFS_CAP_REVISION_2 file capabilities, a
binary with VFS_CAP_REVISION_3 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
descendant of such a namespace.
CONFORMING TO top
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/〉.
NOTES top
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.
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. 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).
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
〈http://www.kernel.org/pub/linux/libs/security/linux-privs〉.
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 capabil‐
ity 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 bound‐
ing set, /proc/sys/kernel/cap-bound, always masks out this capabil‐
ity, 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.
SEE ALSO top
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
COLOPHON top
This page is part of release 4.16 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
