Capabilities(7) Miscellaneous Information Manual Capabilities(7)
NAME
capabilities - overview of Linux capabilities
DESCRIPTION
For the purpose of performing permission checks, traditional UNIX im-
plementations distinguish two categories of processes: privileged pro-
cesses (whose effective user ID is 0, referred to as superuser or
root), and unprivileged processes (whose effective UID is nonzero).
Privileged processes bypass all kernel permission checks, while unpriv-
ileged processes are subject to full permission checking based on the
process's credentials (usually: effective UID, effective GID, and sup-
plementary group list).
Starting with Linux 2.2, Linux divides the privileges traditionally as-
sociated 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) EPOLL-
WAKEUP, /proc/sys/wake_lock).
CAP_BPF (since Linux 5.8)
Employ privileged BPF operations; see bpf(2) and bpf-helpers(7).
This capability was added in Linux 5.8 to separate out BPF func-
tionality from the overloaded CAP_SYS_ADMIN capability.
CAP_CHECKPOINT_RESTORE (since Linux 5.9)
• Update /proc/sys/kernel/ns_last_pid (see pid_namespaces(7));
• employ the set_tid feature of clone3(2);
• read the contents of the symbolic links in
/proc/pid/map_files for other processes.
This capability was added in Linux 5.9 to separate out check-
point/restore functionality from the overloaded CAP_SYS_ADMIN
capability.
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 ex-
ecute 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;
• modify user extended attributes on sticky directory owned by
any user;
• specify O_NOATIME for arbitrary files in open(2) and fc-
ntl(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 call-
ing process.
CAP_IPC_LOCK
• Lock memory (mlock(2), mlockall(2), mmap(2), shmctl(2));
• Allocate memory using huge pages (memfd_create(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_DE-
BUG, 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_PERFMON (since Linux 5.8)
Employ various performance-monitoring mechanisms, including:
• call perf_event_open(2);
• employ various BPF operations that have performance implica-
tions.
This capability was added in Linux 5.8 to separate out perfor-
mance monitoring functionality from the overloaded CAP_SYS_ADMIN
capability. See also the kernel source file Documentation/ad-
min-guide/perf-security.rst.
CAP_SETGID
• Make arbitrary manipulations of process GIDs and supplemen-
tary GID list;
• forge GID when passing socket credentials via UNIX domain
sockets;
• write a group ID mapping in a user namespace (see user_name-
spaces(7)).
CAP_SETFCAP (since Linux 2.6.24)
Set arbitrary capabilities on a file.
Since Linux 5.12, this capability is also needed to map user ID
0 in a new user namespace; see user_namespaces(7) for details.
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., before Linux
2.6.24): grant or remove any capability in the caller's permit-
ted 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 dif-
ferent semantics for such kernels.)
CAP_SETUID
• Make arbitrary manipulations of process UIDs (setuid(2), se-
treuid(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_name-
spaces(7)).
CAP_SYS_ADMIN
Note: this capability is overloaded; see Notes to kernel devel-
opers below.
• Perform a range of system administration operations includ-
ing: 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;
• access the same checkpoint/restore functionality that is gov-
erned by CAP_CHECKPOINT_RESTORE (but the latter, weaker capa-
bility is preferred for accessing that functionality).
• perform the same BPF operations as are governed by CAP_BPF
(but the latter, weaker capability is preferred for accessing
that functionality).
• employ the same performance monitoring mechanisms as are gov-
erned by CAP_PERFMON (but the latter, weaker capability is
preferred for accessing that functionality).
• 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 at-
tributes (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 name-
spaces does not require any capability);
• access privileged perf event information;
• call setns(2) (requires CAP_SYS_ADMIN in the target name-
space);
• call fanotify_init(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 in-
put 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 de-
vice (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_SUS-
PEND_SECCOMP flag);
• perform administrative operations on many device drivers;
• modify autogroup nice values by writing to /proc/pid/auto-
group (see sched(7)).
CAP_SYS_BOOT
Use reboot(2) and kexec_load(2).
CAP_SYS_CHROOT
• Use chroot(2);
• change mount namespaces using setns(2).
CAP_SYS_MODULE
• Load and unload kernel modules (see init_module(2) and
delete_module(2));
• before Linux 2.6.25: drop capabilities from the system-wide
capability bounding set.
CAP_SYS_NICE
• Lower the 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 pro-
cesses (sched_setscheduler(2), sched_setparam(2), sched_se-
tattr(2));
• set CPU affinity for arbitrary processes (sched_setaffin-
ity(2));
• set I/O scheduling class and priority for arbitrary processes
(ioprio_set(2));
• apply migrate_pages(2) to arbitrary processes and allow pro-
cesses 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
• 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 de-
vices.
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 ms-
gctl(2));
• allow the RLIMIT_NOFILE resource limit on the number of "in-
flight" file descriptors to be bypassed when passing file de-
scriptors 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) com-
mand;
• 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,
/proc/sys/fs/mqueue/msg_max, and /proc/sys/fs/mqueue/msg-
size_max limits 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_REAL-
TIME_ALARM and CLOCK_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'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 Linux 2.6.24, only the first two of these requirements are met;
since Linux 2.6.24, all three requirements are met.
Notes to kernel developers
When adding a new kernel feature that should be governed by a capabil-
ity, 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 capa-
bilities. 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 capa-
bilities 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 asso-
ciated 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 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 ef-
fective 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 ca-
pabilities 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 at-
tribute shared by all threads on the system.
For more details, see Capability bounding set below.
Ambient (since Linux 4.3)
This is a set of capabilities that are preserved across an ex-
ecve(2) of a program that is not privileged. The ambient capa-
bility 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 ei-
ther 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 capabili-
ties are added to the permitted set and assigned to the effec-
tive set when execve(2) is called. If ambient capabilities
cause a process's permitted and effective capabilities to in-
crease during an execve(2), this does not trigger the secure-ex-
ecution mode described in ld.so(8).
A child created via fork(2) inherits copies of its parent's capability
sets. For details on how execve(2) affects capabilities, see Transfor-
mation of capabilities during execve() below.
Using capset(2), a thread may manipulate its own capability sets; see
Programmatically adjusting capability sets below.
Since Linux 3.2, the file /proc/sys/kernel/cap_last_cap exposes the nu-
merical value of the highest capability supported by the running ker-
nel; this can be used to determine the highest bit that may be set in a
capability set.
File capabilities
Since Linux 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 capabili-
ties for the thread are also raised in the effective set. If
this bit is not set, then after an execve(2), none of the new
permitted capabilities is in the new effective set.
Enabling the file effective capability bit implies that any file
permitted or inheritable capability that causes a thread to ac-
quire the corresponding permitted capability during an execve(2)
(see Transformation of capabilities during execve() below) will
also acquire that capability in its effective set. Therefore,
when assigning capabilities to a file (setcap(8),
cap_set_file(3), cap_set_fd(3)), if we specify the effective
flag as being enabled for any capability, then the effective
flag must also be specified as enabled for all other capabili-
ties for which the corresponding permitted or inheritable flag
is enabled.
File capability extended attribute versioning
To allow extensibility, the kernel supports a scheme to encode a ver-
sion number inside the security.capability extended attribute that is
used to implement file capabilities. These version numbers are inter-
nal to the implementation, and not directly visible to user-space ap-
plications. To date, the following versions are supported:
VFS_CAP_REVISION_1
This was the original file capability implementation, which sup-
ported 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 capabili-
ties grew beyond 32. The kernel transparently continues to sup-
port the execution of files that have 32-bit version 1 capabil-
ity 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 name-
space 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 attribute.
Since Linux 4.14, the version of the security.capability extended at-
tribute 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_REVI-
SION_3) attribute if both of the following are true:
• The thread writing the attribute resides in a noninitial user name-
space. (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, mean-
ing 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 map-
pings 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 or modifying a security.capability extended at-
tribute 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 at-
tribute 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. Corre-
spondingly, 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 name-
space), 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 au-
tomatic 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 secu-
rity.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) & 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]
where:
P() denotes the value of a thread capability set before the ex-
ecve(2)
P'() denotes the value of a thread capability set after the ex-
ecve(2)
F() denotes a file capability set
Note the following details relating to the above capability transforma-
tion 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 cal-
culate 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 capabili-
ties 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 ca-
pabilities are similarly ignored if the kernel was booted with the
no_file_caps option.
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 ca-
pabilities when a process with a user ID of zero performs an execve(2),
see Capabilities and execution of programs by root below.
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 capabili-
ties, 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 recog-
nizes 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 oc-
cur is that the capability bounding set masked out some of the capabil-
ities 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 than
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 spe-
cial 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., switch-
ing the effective user ID to 0 (root) because a set-user-ID-root pro-
gram was executed—the kernel calculates the file capability sets as
follows:
(1) 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 in
Set-user-ID-root programs that have file capabilities below.)
(2) 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 capa-
bilities 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 calcu-
lation 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.
Set-user-ID-root programs that have file capabilities
There is one exception to the behavior described in Capabilities and
execution of programs by root above. 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 capa-
bilities, 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's permitted capability set. The capability
bounding set thus places a limit on the permitted capabilities that
may be granted by an executable file.
• (Since Linux 2.6.25) The capability bounding set acts as a limiting
superset for the capabilities that a thread can add to its inherita-
ble set using capset(2). This means that if a capability is not in
the bounding set, then a thread can't add this capability to its in-
heritable 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 attri-
bute. (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(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 capa-
bility is in its bounding set using the prctl(2) PR_CAPBSET_READ opera-
tion.
Removing capabilities from the bounding set is supported only if file
capabilities are compiled into the kernel. Before Linux 2.6.33, file
capabilities were an optional feature configurable via the CONFIG_SECU-
RITY_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 capa-
bility 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
Before Linux 2.6.25, the capability bounding set is a system-wide at-
tribute 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!), mod-
ify the definition of CAP_INIT_EFF_SET in include/linux/capability.h
and rebuild the kernel.
The system-wide capability bounding set feature was added to Linux
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's
capability sets on changes to the thread'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 ca-
pabilities are cleared from the effective set.
• If the effective user ID is changed from nonzero to 0, then the per-
mitted set is copied to the effective set.
• If the filesystem user ID is changed from 0 to nonzero (see setf-
suid(2)), then the following capabilities are cleared from the ef-
fective set: CAP_CHOWN, CAP_DAC_OVERRIDE, CAP_DAC_READ_SEARCH,
CAP_FOWNER, CAP_FSETID, CAP_LINUX_IMMUTABLE (since Linux 2.6.30),
CAP_MAC_OVERRIDE, and CAP_MKNOD (since Linux 2.6.30). If the
filesystem UID is changed from nonzero to 0, then any of these capa-
bilities that are enabled in the permitted set are enabled in the
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 inheri-
table 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 in-
heritable 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 Linux 2.6.26, and with a kernel in which file capabili-
ties are enabled, Linux implements a set of per-thread securebits flags
that can be used to disable special handling of capabilities for UID 0
(root). These flags are as follows:
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 capabili-
ties. 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 effec-
tive 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 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 Effect of user ID changes on capabili-
ties above.
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 Capa-
bilities and execution of programs by root above.)
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 pre-
venting 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 ex-
ecve(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 ca-
pabilities 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's permitted and ef-
fective sets when executed by any process inside that namespace or any
descendant user namespace.
The rules about the transformation of the process's capabilities during
the execve(2) are exactly as described in Transformation of capabili-
ties during execve() and Capabilities and execution of programs by root
above, 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 exe-
cutes a binary with such capabilities, it gains the associated capabil-
ities (within its user namespace) as per the rules described in Trans-
formation of capabilities during execve() above.
Because version 2 file capabilities confer capabilities to the execut-
ing 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 ca-
pability in the user namespace where the filesystem was mounted (nor-
mally the initial user namespace). This limitation renders file capa-
bilities useless for certain use cases. For example, in user-names-
paced containers, it can be desirable to be able to create a binary
that confers capabilities only to processes executed inside that con-
tainer, 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 de-
scribed in File capability extended attribute versioning above. 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 bi-
nary 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 at-
tribute, 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).
STANDARDS
No standards govern capabilities, but the Linux capability implementa-
tion 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 Linux 2.5.27 to Linux 2.6.26, capabilities were an optional kernel
component, and could be enabled/disabled via the CONFIG_SECURITY_CAPA-
BILITIES 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
⟨https://git.kernel.org/pub/scm/libs/libcap/libcap.git/refs/⟩.
Before Linux 2.6.24, and from Linux 2.6.24 to Linux 2.6.32 if file ca-
pabilities 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_SETP-
CAP 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 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_SECU-
RITY_FILE_CAPABILITIES option is disabled), then init starts out
with the CAP_SETPCAP capability removed from its per-process bound-
ing 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), creden-
tials(7), pthreads(7), user_namespaces(7), captest(8), filecap(8), get-
cap(8), getpcaps(8), netcap(8), pscap(8), setcap(8)
include/linux/capability.h in the Linux kernel source tree
Linux man-pages 6.03 2023-02-05 Capabilities(7)
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