Section (7) capabilities
Name
capabilities — overview of Linux capabilities
DESCRIPTION
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_zsingle_quotesz_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
andCAP_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 user; -
specify
O_NOATIME
for arbitrary files in open(2) and fcntl(2).
-
CAP_FSETID
-
-
Don_zsingle_quotesz_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
andFS_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
, andSO_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_zsingle_quotesz_s bounding set to its inheritable set; drop capabilities from the bounding set (via prctl(2)
PR_CAPBSET_DROP
); make changes to thesecurebits
flags.If file capabilities are not supported (i.e., kernels before Linux 2.6.24): grant or remove any capability in the caller_zsingle_quotesz_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, sinceCAP_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), 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
andIPC_RMID
operations on arbitrary System V IPC objects; -
override
RLIMIT_NPROC
resource limit; -
perform operations on
trusted
andsecurity
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 thetarget
namespace); -
call fanotify_init(2);
-
call bpf(2);
-
perform privileged
KEYCTL_CHOWN
andKEYCTL_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_zsingle_quotesz_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_zsingle_quotesz_s seccomp filters; -
employ the ptrace(2)
PTRACE_SETOPTIONS
operation to suspend the tracee_zsingle_quotesz_s seccomp protections (i.e., thePTRACE_O_SUSPEND_SECCOMP
flag); -
perform administrative operations on many device drivers.
-
Modify autogroup nice values by writing to
/proc/[pid]/autogroup
(see sched(7)).
-
CAP_SYS_BOOT
-
Use reboot(2) and kexec_load(2).
CAP_SYS_CHROOT
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), sched_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_readv(2) and process_vm_writev(2);
-
inspect processes using kcmp(2).
-
CAP_SYS_RAWIO
-
-
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 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 theF_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 withCAP_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)CAP_WAKE_ALARM
(since Linux 3.0)-
Trigger something that will wake up the system (set
CLOCK_REALTIME_ALARM
andCLOCK_BOOTTIME_ALARM
timers).
Past and current implementation
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_zsingle_quotesz_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.
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 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_zsingle_quotesz_t
chooseCAP_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_zsingle_quotesz_t make the problem worse. The only new features that should be associated withCAP_SYS_ADMIN
are ones thatclosely
match existing uses in that silo. -
If you have determined that it really is necessary to create a new capability for your feature, don_zsingle_quotesz_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 the following 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.
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_zsingle_quotesz_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_zsingle_quotesz_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 asforced
):-
These capabilities are automatically permitted to the thread, regardless of the thread_zsingle_quotesz_s inheritable capabilities.
Inheritable
(formerly known asallowed
):-
This set is ANDed with the thread_zsingle_quotesz_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 extended attribute 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_zsingle_quotesz_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
attribute.
Since Linux 4.14, the version of the security.capability
extended attribute that is attached to a file depends on
the circumstances in which the 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 user 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 theCAP_SETFCAP
capability in its own user namespace; and (b) the UID and GID of the file inode have mappings in the writer_zsingle_quotesz_s user namespace.
When a VFS_CAP_REVISION_3
security.capability
extended attribute is created, the root user ID of the
creating thread_zsingle_quotesz_s user namespace is saved in the extended
attribute.
By contrast, creating or modifying 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 the creation of a version 2 (VFS_CAP_REVISION_2
) attribute.
Note that the creation of a version 3 security.capability
extended attribute is automatic. That is to say, when a
user-space application writes (setxattr(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 (getxattr(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) and getcap(1)) in order for those
tools to be used to create and retrieve version 3
security.capability
attributes.
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_zsingle_quotesz_(ambient) = (file is privileged) ? 0 : P(ambient) P_zsingle_quotesz_(permitted) = (P(inheritable) & F(inheritable)) | (F(permitted) & P(bounding)) | P_zsingle_quotesz_(ambient) P_zsingle_quotesz_(effective) = F(effective) ? P_zsingle_quotesz_(permitted) : P_zsingle_quotesz_(ambient) P_zsingle_quotesz_(inheritable) = P(inheritable) [i.e., unchanged] P_zsingle_quotesz_(bounding) = P(bounding) [i.e., unchanged]
where:
Note the following details relating to the above capability transformation rules:
-
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 for
P(bounding)
.
![]() |
Note |
---|---|
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(2). File
capabilities are similarly ignored if the kernel
was booted with the |
![]() |
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 understand capabilities.) For such applications, the effective capability bit is set on the file, so that the file permitted capabilities are automatically enabled in the process effective set when executing the file. The kernel recognizes a file which has the effective capability bit set as capability-dumb for the purpose of the check described here.
When executing a capability-dumb binary, the kernel
checks if the process obtained all permitted capabilities
that were specified in the file permitted set, after the
capability transformations described above have been
performed. (The typical reason why this might not
occur is that the
capability bounding set masked out some of the capabilities
in the file permitted set.) If the process did not obtain
the full set of file permitted capabilities, then execve(2) fails with the
error EPERM. This prevents
possible security risks that could arise when a
capability-dumb application is executed with less privilege
that it needs. Note that, by definition, the application
could not itself recognize this problem, since it does not
employ the libcap(3) API.
Capabilities and execution of programs by root
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_zsingle_quotesz_s capability sets are then used as described above to calculate the transformation of the process_zsingle_quotesz_s capabilities during execve(2).
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_zsingle_quotesz_s new permitted capabilities simplifies to:
P_zsingle_quotesz_(permitted) = P(inheritable) | P(bounding) P_zsingle_quotesz_(effective) = P_zsingle_quotesz_(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_zsingle_quotesz_(permitted), the P_zsingle_quotesz_(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.
Set-user-ID-root programs that have file capabilities
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 not
0 (root) and (c) the
effective user ID of the process is
0 (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.
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_zsingle_quotesz_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_zsingle_quotesz_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_zsingle_quotesz_s parent, and is preserved across an execve(2).
A thread may remove capabilities from its capability
bounding set using the prctl(2) PR_CAPBSET_DROP
operation, provided it
has the CAP_SETPCAP
capability. Once a capability has been dropped from the
bounding set, it cannot be restored to that set. A thread
can determine if a capability is in its bounding set using
the prctl(2) PR_CAPBSET_READ
operation.
Removing capabilities from the bounding set is supported
only if file capabilities are compiled into the kernel. In
kernels before Linux 2.6.33, file capabilities were an
optional feature configurable via the CONFIG_SECURITY_FILE_CAPABILITIES
option.
Since Linux 2.6.33, the configuration option has been
removed and file capabilities are always part of the
kernel. When file capabilities are compiled into the
kernel, the init
process (the ancestor of all processes) begins with a full
bounding set. If file capabilities are not compiled into
the kernel, then init
begins with a full
bounding set minus CAP_SETPCAP
, because this capability has
a different meaning when there are no file
capabilities.
Removing a capability from the bounding set does not remove it from the thread_zsingle_quotesz_s inheritable set. However it does prevent the capability from being added back into the thread_zsingle_quotesz_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 file
/proc/sys/kernel/cap-bound
.
(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 CAP_SYS_MODULE
capability) may only clear
capabilities from this set.
On a standard system the capability bounding set always
masks out the CAP_SETPCAP
capability. To remove this restriction (dangerous!), modify
the definition of CAP_INIT_EFF_SET
in include/linux/capability.h
and rebuild
the kernel.
The system-wide capability bounding set feature was added to Linux starting with kernel version 2.2.11.
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_zsingle_quotesz_s capability sets on changes to the thread_zsingle_quotesz_s real, effective, saved set, and filesystem user IDs (using setuid(2), setresuid(2), or similar):
-
If one or more of the real, effective or saved set user IDs was previously 0, and as a result of the UID changes all of these IDs have a nonzero value, then all capabilities are cleared from the permitted, effective, and ambient capability sets.
-
If the effective user ID is changed from 0 to nonzero, then all capabilities are cleared from the effective set.
-
If the effective user ID is changed from nonzero to 0, then the permitted set is copied to the effective set.
-
If the filesystem user ID is changed from 0 to nonzero (see setfsuid(2)), then the following capabilities are cleared from the effective set:
CAP_CHOWN
,CAP_DAC_OVERRIDE
,CAP_DAC_READ_SEARCH
,CAP_FOWNER
,CAP_FSETID
,CAP_LINUX_IMMUTABLE
(since Linux 2.6.30),CAP_MAC_OVERRIDE
, andCAP_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 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(3), both
provided in the libcap
package, is
preferred for this purpose. The following rules govern
changes to the thread capability sets:
-
If the caller does not have the
CAP_SETPCAP
capability, the new inheritable set must be a subset of the combination of the existing inheritable and permitted sets. -
(Since Linux 2.6.25) The new inheritable set must be a subset of the combination of the existing inheritable set and the capability bounding set.
-
The new permitted set must be a subset of the existing permitted set (i.e., it is not possible to acquire permitted capabilities that the thread does not currently have).
-
The new effective set must be a subset of the new permitted set.
The securebits flags: establishing a capabilities-only environment
Starting with kernel 2.6.26, and with a kernel in which
file capabilities are enabled, Linux implements a set of
per-thread securebits
flags that can
be used to disable special handling of capabilities for UID
0 (root
). These
flags are as follows:
SECBIT_KEEP_CAPS
-
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 an execve(2).
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 cleared.The setting of the
SECBIT_KEEP_CAPS
flag is ignored if theSECBIT_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_zsingle_quotesz_s permitted, effective, and ambient capability sets when the thread_zsingle_quotesz_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 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.)
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. Setting any of the locked flags is irreversible,
and has the effect of preventing further changes to the
corresponding base flag. The locked flags are:
SECBIT_KEEP_CAPS_LOCKED
,
SECBIT_NO_SETUID_FIXUP_LOCKED
,
SECBIT_NOROOT_LOCKED
, and
SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED
.
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. Note that the SECBIT_*
constants are
available only after including the <
linux/securebits.h
>
header file.
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 capabilities:
prctl(PR_SET_SECUREBITS, /* SECBIT_KEEP_CAPS off */ SECBIT_KEEP_CAPS_LOCKED | SECBIT_NO_SETUID_FIXUP | SECBIT_NO_SETUID_FIXUP_LOCKED | SECBIT_NOROOT | SECBIT_NOROOT_LOCKED); /* Setting/locking SECBIT_NO_CAP_AMBIENT_RAISE is not required */
Per-user-namespace set-user-ID-root programs
A set-user-ID program whose UID matches the UID that created a user namespace will confer capabilities in the process_zsingle_quotesz_s permitted and effective sets when executed by any process inside that namespace or any descendant user namespace.
The rules about the transformation of the process_zsingle_quotesz_s
capabilities during the execve(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.
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 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., VFS_CAP_REVISION_3
) security.capability
extended attributes. 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 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 a descendant of such
a namespace.
Interaction with user namespaces
For further information on the interaction of capabilities and user namespaces, see user_namespaces(7).
CONFORMING TO
No standards govern capabilities, but the Linux capability implementation is based on the withdrawn POSIX.1e draft standard; see https://archive.org/details/posix_1003.1e-990310
NOTES
When attempting to strace(1) binaries that have
capabilities (or set-user-ID-root binaries), you may find the
−u <username>
option
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 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_zsingle_quotesz_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
https://git.kernel.org/pub/scm/libs/libcap/libcap.git/refs/
Before kernel 2.6.24, and from kernel 2.6.24 to kernel
2.6.32 if file capabilities are not enabled, a thread with
the CAP_SETPCAP
capability can
manipulate the capabilities of threads other than itself.
However, this is only theoretically possible, since no thread
ever has CAP_SETPCAP
in either
of these cases:
-
In the pre-2.6.25 implementation the system-wide capability bounding set,
/proc/sys/kernel/cap-bound
, always masks out this capability, and this can not be changed without modifying the kernel source and rebuilding. -
If file capabilities are disabled in the current implementation, then
init
starts out with this capability removed from its per-process bounding set, and that bounding set is inherited by all other processes created on the system.
SEE ALSO
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
This page is part of release 5.04 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/.
Copyright (c) 2002 by Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END 6 Aug 2002 - Initial Creation Modified 2003-05-23, Michael Kerrisk, <mtk.manpagesgmail.com> Modified 2004-05-27, Michael Kerrisk, <mtk.manpagesgmail.com> 2004-12-08, mtk Added O_NOATIME for CAP_FOWNER 2005-08-16, mtk, Added CAP_AUDIT_CONTROL and CAP_AUDIT_WRITE 2008-07-15, Serge Hallyn <serueus.bbm.com> Document file capabilities, per-process capability bounding set, changed semantics for CAP_SETPCAP, and other changes in 2.6.2[45]. Add CAP_MAC_ADMIN, CAP_MAC_OVERRIDE, CAP_SETFCAP. 2008-07-15, mtk Add text describing circumstances in which CAP_SETPCAP (theoretically) permits a thread to change the capability sets of another thread. Add section describing rules for programmatically adjusting thread capability sets. Describe rationale for capability bounding set. Document securebits flags. Add text noting that if we set the effective flag for one file capability, then we must also set the effective flag for all other capabilities where the permitted or inheritable bit is set. 2011-09-07, mtk/Serge hallyn: Add CAP_SYSLOG |