CAPABILITIES(7) Linux Programmer's Manual CAPABILITIES(7)
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 unpriv-
ileged processes are subject to full permission checking based on the
process's credentials (usually: effective UID, effective GID, and sup-
plementary group list).
Starting with kernel 2.2, Linux divides the privileges traditionally
associated with superuser into distinct units, known as capabilities,
which can be independently enabled and disabled. Capabilities are a
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_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 exe-
cute permission checks;
* invoke open_by_handle_at(2);
* use the linkat(2) AT_EMPTY_PATH flag to create a link to a
file referred to by a file descriptor.
CAP_FOWNER
* Bypass permission checks on operations that normally require
the filesystem UID of the process to match the UID of the file
(e.g., chmod(2), utime(2)), excluding those operations covered
by CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH;
* set inode flags (see ioctl_iflags(2)) on arbitrary files;
* set Access Control Lists (ACLs) on arbitrary files;
* ignore directory sticky bit on file deletion;
* specify O_NOATIME for arbitrary files in open(2) and fcntl(2).
CAP_FSETID
* Don't clear set-user-ID and set-group-ID mode bits when a file
is modified;
* set the set-group-ID bit for a file whose GID does not match
the filesystem or any of the supplementary GIDs of the calling
process.
CAP_IPC_LOCK
Lock memory (mlock(2), mlockall(2), mmap(2), shmctl(2)).
CAP_IPC_OWNER
Bypass permission checks for operations on System V IPC objects.
CAP_KILL
Bypass permission checks for sending signals (see kill(2)).
This includes use of the ioctl(2) KDSIGACCEPT operation.
CAP_LEASE (since Linux 2.4)
Establish leases on arbitrary files (see fcntl(2)).
CAP_LINUX_IMMUTABLE
Set the FS_APPEND_FL and FS_IMMUTABLE_FL inode flags (see
ioctl_iflags(2)).
CAP_MAC_ADMIN (since Linux 2.6.25)
Allow MAC configuration or state changes. Implemented for the
Smack Linux Security Module (LSM).
CAP_MAC_OVERRIDE (since Linux 2.6.25)
Override Mandatory Access Control (MAC). Implemented for the
Smack LSM.
CAP_MKNOD (since Linux 2.4)
Create special files using mknod(2).
CAP_NET_ADMIN
Perform various network-related operations:
* interface configuration;
* administration of IP firewall, masquerading, and accounting;
* modify routing tables;
* bind to any address for transparent proxying;
* set type-of-service (TOS)
* clear driver statistics;
* set promiscuous mode;
* enabling multicasting;
* use setsockopt(2) to set the following socket options:
SO_DEBUG, SO_MARK, SO_PRIORITY (for a priority outside the
range 0 to 6), SO_RCVBUFFORCE, and SO_SNDBUFFORCE.
CAP_NET_BIND_SERVICE
Bind a socket to Internet domain privileged ports (port numbers
less than 1024).
CAP_NET_BROADCAST
(Unused) Make socket broadcasts, and listen to multicasts.
CAP_NET_RAW
* Use RAW and PACKET sockets;
* bind to any address for transparent proxying.
CAP_SETGID
* Make arbitrary manipulations of process GIDs and supplementary
GID list;
* forge GID when passing socket credentials via UNIX domain
sockets;
* write a group ID mapping in a user namespace (see user_names-
paces(7)).
CAP_SETFCAP (since Linux 2.6.24)
Set arbitrary capabilities on a file.
CAP_SETPCAP
If file capabilities are supported (i.e., since Linux 2.6.24):
add any capability from the calling thread's bounding set to its
inheritable set; drop capabilities from the bounding set (via
prctl(2) PR_CAPBSET_DROP); make changes to the securebits flags.
If file capabilities are not supported (i.e., kernels before
Linux 2.6.24): grant or remove any capability in the caller's
permitted capability set to or from any other process. (This
property of CAP_SETPCAP is not available when the kernel is con-
figured to support file capabilities, since CAP_SETPCAP has
entirely different semantics for such kernels.)
CAP_SETUID
* Make arbitrary manipulations of process UIDs (setuid(2),
setreuid(2), setresuid(2), setfsuid(2));
* forge UID when passing socket credentials via UNIX domain
sockets;
* write a user ID mapping in a user namespace (see user_names-
paces(7)).
CAP_SYS_ADMIN
Note: this capability is overloaded; see Notes to kernel devel-
opers, below.
* Perform a range of system administration operations including:
quotactl(2), mount(2), umount(2), swapon(2), swapoff(2),
sethostname(2), and setdomainname(2);
* perform privileged syslog(2) operations (since Linux 2.6.37,
CAP_SYSLOG should be used to permit such operations);
* perform VM86_REQUEST_IRQ vm86(2) command;
* perform IPC_SET and IPC_RMID operations on arbitrary System V
IPC objects;
* override RLIMIT_NPROC resource limit;
* perform operations on trusted and security Extended Attributes
(see xattr(7));
* use lookup_dcookie(2);
* use ioprio_set(2) to assign IOPRIO_CLASS_RT and (before Linux
2.6.25) IOPRIO_CLASS_IDLE I/O scheduling classes;
* forge PID when passing socket credentials via UNIX domain
sockets;
* exceed /proc/sys/fs/file-max, the system-wide limit on the
number of open files, in system calls that open files (e.g.,
accept(2), execve(2), open(2), pipe(2));
* employ CLONE_* flags that create new namespaces with clone(2)
and unshare(2) (but, since Linux 3.8, creating user namespaces
does not require any capability);
* call perf_event_open(2);
* access privileged perf event information;
* call setns(2) (requires CAP_SYS_ADMIN in the target names-
pace);
* call fanotify_init(2);
* call bpf(2);
* perform privileged KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2)
operations;
* use ptrace(2) PTRACE_SECCOMP_GET_FILTER to dump a tracees sec-
comp filters;
* perform madvise(2) MADV_HWPOISON operation;
* employ the TIOCSTI ioctl(2) to insert characters into the
input queue of a terminal other than the caller's controlling
terminal;
* employ the obsolete nfsservctl(2) system call;
* employ the obsolete bdflush(2) system call;
* perform various privileged block-device ioctl(2) operations;
* perform various privileged filesystem ioctl(2) operations;
* perform privileged ioctl(2) operations on the /dev/random
device (see random(4));
* install a seccomp(2) filter without first having to set the
no_new_privs thread attribute;
* modify allow/deny rules for device control groups;
* employ the ptrace(2) PTRACE_SECCOMP_GET_FILTER operation to
dump tracee's seccomp filters;
* employ the ptrace(2) PTRACE_SETOPTIONS operation to suspend
the tracee's seccomp protections (i.e., the PTRACE_O_SUS-
PEND_SECCOMP flag).
* perform administrative operations on many device drivers.
CAP_SYS_BOOT
Use reboot(2) and kexec_load(2).
CAP_SYS_CHROOT
Use chroot(2).
CAP_SYS_MODULE
* Load and unload kernel modules (see init_module(2) and
delete_module(2));
* in kernels before 2.6.25: drop capabilities from the system-
wide capability bounding set.
CAP_SYS_NICE
* Raise process nice value (nice(2), setpriority(2)) and change
the nice value for arbitrary processes;
* set real-time scheduling policies for calling process, and set
scheduling policies and priorities for arbitrary processes
(sched_setscheduler(2), sched_setparam(2), shed_setattr(2));
* set CPU affinity for arbitrary processes (sched_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
devices.
CAP_SYS_RESOURCE
* Use reserved space on ext2 filesystems;
* make ioctl(2) calls controlling ext3 journaling;
* override disk quota limits;
* increase resource limits (see setrlimit(2));
* override RLIMIT_NPROC resource limit;
* override maximum number of consoles on console allocation;
* override maximum number of keymaps;
* allow more than 64hz interrupts from the real-time clock;
* raise msg_qbytes limit for a System V message queue above the
limit in /proc/sys/kernel/msgmnb (see msgop(2) and msgctl(2));
* allow the RLIMIT_NOFILE resource limit on the number of "in-
flight" file descriptors to be bypassed when passing file
descriptors to another process via a UNIX domain socket (see
unix(7));
* override the /proc/sys/fs/pipe-size-max limit when setting the
capacity of a pipe using the F_SETPIPE_SZ fcntl(2) command.
* use F_SETPIPE_SZ to increase the capacity of a pipe above the
limit specified by /proc/sys/fs/pipe-max-size;
* override /proc/sys/fs/mqueue/queues_max limit when creating
POSIX message queues (see mq_overview(7));
* employ the prctl(2) PR_SET_MM operation;
* set /proc/[pid]/oom_score_adj to a value lower than the value
last set by a process with CAP_SYS_RESOURCE.
CAP_SYS_TIME
Set system clock (settimeofday(2), stime(2), adjtimex(2)); set
real-time (hardware) clock.
CAP_SYS_TTY_CONFIG
Use vhangup(2); employ various privileged ioctl(2) operations on
virtual terminals.
CAP_SYSLOG (since Linux 2.6.37)
* Perform privileged syslog(2) operations. See syslog(2) for
information on which operations require privilege.
* View kernel addresses exposed via /proc and other interfaces
when /proc/sys/kernel/kptr_restrict has the value 1. (See the
discussion of the kptr_restrict in proc(5).)
CAP_WAKE_ALARM (since Linux 3.0)
Trigger something that will wake up the system (set CLOCK_REAL-
TIME_ALARM and CLOCK_BOOTTIME_ALARM timers).
Past and current implementation
A full implementation of capabilities requires that:
1. For all privileged operations, the kernel must check whether the
thread has the required capability in its effective set.
2. The kernel must provide system calls allowing a thread's capability
sets to be changed and retrieved.
3. The filesystem must support attaching capabilities to an executable
file, so that a process gains those capabilities when the file is
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 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 use along with the new feature. If the
new feature is useless without these other features, you should use
the same capability as the other features.
* Don't choose CAP_SYS_ADMIN if you can possibly avoid it! A vast
proportion of existing capability checks are associated with this
capability (see the partial list above). It can plausibly be called
"the new root", since on the one hand, it confers a wide range of
powers, and on the other hand, its broad scope means that this is
the capability that is required by many privileged programs. Don't
make the problem worse. The only new features that should be 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 three capability sets containing zero or more of the
above capabilities:
Permitted:
This is a limiting superset for the effective capabilities that
the thread may assume. It is also a limiting superset for the
capabilities that may be added to the inheritable set by a
thread that does not have the CAP_SETPCAP capability in its
effective set.
If a thread drops a capability from its permitted set, it can
never reacquire that capability (unless it execve(2)s either a
set-user-ID-root program, or a program whose associated file
capabilities grant that capability).
Inheritable:
This is a set of capabilities preserved across an execve(2).
Inheritable capabilities remain inheritable when executing any
program, and inheritable capabilities are added to the permitted
set when executing a program that has the corresponding bits set
in the file inheritable set.
Because inheritable capabilities are not generally preserved
across execve(2) when running as a non-root user, applications
that wish to run helper programs with elevated capabilities
should consider using ambient capabilities, described below.
Effective:
This is the set of capabilities used by the kernel to perform
permission checks for the thread.
Ambient (since Linux 4.3):
This is a set of capabilities that are preserved across an
execve(2) of a program that is not privileged. The ambient
capability set obeys the invariant that no capability can ever
be ambient if it is not both permitted and inheritable.
The ambient capability set can be directly modified using
prctl(2). Ambient capabilities are automatically lowered if
either of the corresponding permitted or inheritable capabili-
ties 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.
A child created via fork(2) inherits copies of its parent's capability
sets. See below for a discussion of the treatment of capabilities dur-
ing 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 ker-
nel; this can be used to determine the highest bit that may be set in a
capability set.
File capabilities
Since kernel 2.6.24, the kernel supports associating capability sets
with an executable file using setcap(8). The file capability sets are
stored in an extended attribute (see setxattr(2) and xattr(7)) named
security.capability. Writing to this extended attribute requires the
CAP_SETFCAP capability. The file capability sets, in conjunction with
the capability sets of the thread, determine the capabilities of a
thread after an execve(2).
The three file capability sets are:
Permitted (formerly known as forced):
These capabilities are automatically permitted to the thread,
regardless of the thread's inheritable capabilities.
Inheritable (formerly known as allowed):
This set is ANDed with the thread's inheritable set to determine
which inheritable capabilities are enabled in the permitted set
of the thread after the execve(2).
Effective:
This is not a set, but rather just a single bit. If this bit is
set, then during an execve(2) all of the new permitted capabili-
ties for the thread are also raised in the effective set. If
this bit is not set, then after an execve(2), none of the new
permitted capabilities is in the new effective set.
Enabling the file effective capability bit implies that any file
permitted or inheritable capability that causes a thread to
acquire the corresponding permitted capability during an
execve(2) (see the transformation rules described below) will
also acquire that capability in its effective set. Therefore,
when assigning capabilities to a file (setcap(8),
cap_set_file(3), cap_set_fd(3)), if we specify the effective
flag as being enabled for any capability, then the effective
flag must also be specified as enabled for all other capabili-
ties for which the corresponding permitted or inheritable flags
is enabled.
Transformation of capabilities during execve()
During an execve(2), the kernel calculates the new capabilities of the
process using the following algorithm:
P'(ambient) = (file is privileged) ? 0 : P(ambient)
P'(permitted) = (P(inheritable) & F(inheritable)) |
(F(permitted) & cap_bset) | P'(ambient)
P'(effective) = F(effective) ? P'(permitted) : P'(ambient)
P'(inheritable) = P(inheritable) [i.e., unchanged]
where:
P denotes the value of a thread capability set before the
execve(2)
P' denotes the value of a thread capability set after the
execve(2)
F denotes a file capability set
cap_bset is the value of the capability bounding set (described
below).
A privileged file is one that has capabilities or has the set-user-ID
or set-group-ID bit set.
Note: the capability transitions described above may not be performed
(i.e., file capabilities may be ignored) for the same reasons that the
set-user-ID and set-group-ID bits are ignored; see execve(2).
Note: according to the rules above, if a process with nonzero user IDs
performs an execve(2) then any capabilities that are present in its
permitted and effective sets will be cleared. For the treatment of
capabilities when a process with a user ID of zero performs an
execve(2), see below under Capabilities and execution of programs by
root.
Safety checking for capability-dumb binaries
A capability-dumb binary is an application that has been marked to have
file capabilities, but has not been converted to use the libcap(3) API
to manipulate its capabilities. (In other words, this is a traditional
set-user-ID-root program that has been switched to use file 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
occur is that the capability bounding set masked out some of the capa-
bilities 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 provide an all-powerful root using capability sets, during
an execve(2):
1. If a set-user-ID-root program is being executed, or the real or
effective user ID of the process is 0 (root) then the file inherita-
ble and permitted sets are defined to be all ones (i.e., all capa-
bilities enabled).
2. If a set-user-ID-root program is being executed, or the effective
user ID of the process is 0 (root) then the file effective bit is
defined to be one (enabled).
The upshot of the above rules, combined with the capabilities transfor-
mations described above, is as follows:
* When a process execve(2)s a set-user-ID-root program, or when a
process with an effective UID of 0 execve(2)s a program, it gains
all capabilities in its permitted and effective capability sets,
except those masked out by the capability bounding set.
* When a process with a real UID of 0 execve(2)s a program, it gains
all capabilities in its permitted capability set, except those
masked out by the capability bounding set.
The above steps yield semantics that are the same as those provided by
traditional UNIX systems.
Set-user-ID-root programs that have file capabilities
Executing a program that is both set-user-ID root and has file capabil-
ities will cause the process to gain 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 capa-
bilities). 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 exe-
cutes 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
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 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!), mod-
ify the definition of CAP_INIT_EFF_SET in include/linux/capability.h
and rebuild the kernel.
The system-wide capability bounding set feature was added to Linux
starting with kernel version 2.2.11.
Capability bounding set from Linux 2.6.25 onward
From Linux 2.6.25, the capability bounding set is a per-thread
attribute. (There is no longer a system-wide capability bounding set.)
The bounding set is inherited at fork(2) from the thread's parent, and
is preserved across an execve(2).
A thread may remove capabilities from its capability bounding set using
the prctl(2) PR_CAPBSET_DROP operation, provided it has the CAP_SETPCAP
capability. Once a capability has been dropped from the bounding set,
it cannot be restored to that set. A thread can 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. In kernels before Linux
2.6.33, file capabilities were an optional feature configurable via the
CONFIG_SECURITY_FILE_CAPABILITIES option. Since Linux 2.6.33, the con-
figuration option has been removed and file capabilities are always
part of the kernel. When file capabilities are compiled into the ker-
nel, the init process (the ancestor of all processes) begins with a
full bounding set. If file capabilities are not compiled into the ker-
nel, then init begins with a full bounding set minus CAP_SETPCAP,
because this capability has a different meaning when there are no file
capabilities.
Removing a capability from the bounding set does not remove it from the
thread's inheritable set. However it does prevent the capability from
being added back into the thread's inheritable set in the future.
Effect of user ID changes on capabilities
To preserve the traditional semantics for transitions between 0 and
nonzero user IDs, the kernel makes the following changes to a thread's
capability sets on changes to the thread's real, effective, saved set,
and filesystem user IDs (using setuid(2), setresuid(2), or similar):
1. If one or more of the real, effective or saved set user IDs was pre-
viously 0, and as a result of the UID changes all of these IDs have
a nonzero value, then all capabilities are cleared from the permit-
ted, effective, and ambient capability sets.
2. If the effective user ID is changed from 0 to nonzero, then all
capabilities are cleared from the effective set.
3. If the effective user ID is changed from nonzero to 0, then the per-
mitted set is copied to the effective set.
4. If the filesystem user ID is changed from 0 to nonzero (see setf-
suid(2)), then the following capabilities are cleared from the
effective set: CAP_CHOWN, CAP_DAC_OVERRIDE, CAP_DAC_READ_SEARCH,
CAP_FOWNER, CAP_FSETID, CAP_LINUX_IMMUTABLE (since Linux 2.6.30),
CAP_MAC_OVERRIDE, and CAP_MKNOD (since Linux 2.6.30). If the
filesystem UID is changed from nonzero to 0, then any of these 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 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 pack-
age, is preferred for this purpose. The following rules govern changes
to the thread capability sets:
1. If the caller does not have the CAP_SETPCAP capability, the new
inheritable set must be a subset of the combination of the existing
inheritable and permitted sets.
2. (Since Linux 2.6.25) The new inheritable set must be a subset of the
combination of the existing inheritable set and the capability
bounding set.
3. The new permitted set must be a subset of the existing permitted set
(i.e., it is not possible to acquire permitted capabilities that the
thread does not currently have).
4. The new effective set must be a subset of the new permitted set.
The securebits flags: establishing a capabilities-only environment
Starting with kernel 2.6.26, and with a kernel in which file 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 and effective sets when it
switches all of its UIDs to nonzero values. If this flag is not
set, then such a UID switch causes the thread to lose all capa-
bilities in those sets. This flag is always cleared on an
execve(2).
The setting of the SECBIT_KEEP_CAPS flag is ignored if the
SECBIT_NO_SETUID_FIXUP flag is set. (The latter flag provides a
superset of the effect of the former flag.)
This flag provides the same functionality as the older prctl(2)
PR_SET_KEEPCAPS operation.
SECBIT_NO_SETUID_FIXUP
Setting this flag stops the kernel from adjusting the process's
permitted, effective, and ambient capability sets when the
thread's effective and filesystem UIDs are switched between zero
and nonzero values. (See the subsection Effect of user ID
changes on capabilities.)
SECBIT_NOROOT
If this bit is set, then the kernel does not grant 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 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.
The securebits flags are inherited by child processes. During an
execve(2), all of the flags are preserved, except SECBIT_KEEP_CAPS
which is always cleared.
An application can use the following call to lock itself, and all of
its descendants, into an environment where the only way of gaining
capabilities is by executing a program with associated file capabili-
ties:
prctl(PR_SET_SECUREBITS,
/* SECBIT_KEEP_CAPS off */
SECBIT_KEEP_CAPS_LOCKED |
SECBIT_NO_SETUID_FIXUP |
SECBIT_NO_SETUID_FIXUP_LOCKED |
SECBIT_NOROOT |
SECBIT_NOROOT_LOCKED);
/* Setting/locking SECBIT_NO_CAP_AMBIENT_RAISE
is not required */
Interaction with user namespaces
For a discussion of the interaction of capabilities and user names-
paces, see user_namespaces(7).
CONFORMING TO
No standards govern capabilities, but the Linux capability implementa-
tion is based on the withdrawn POSIX.1e draft standard; see
<http://wt.tuxomania.net/publications/posix.1e/>;.
NOTES
From kernel 2.5.27 to kernel 2.6.26, capabilities were an optional ker-
nel component, and could be enabled/disabled via the CONFIG_SECU-
RITY_CAPABILITIES kernel configuration option.
The /proc/[pid]/task/TID/status file can be used to view the capability
sets of a thread. The /proc/[pid]/status file shows the capability
sets of a process's main thread. Before Linux 3.8, nonexistent capa-
bilities were shown as being enabled (1) in these sets. Since Linux
3.8, all nonexistent capabilities (above CAP_LAST_CAP) are shown as
disabled (0).
The libcap package provides a suite of routines for setting and getting
capabilities that is more comfortable and less likely to change than
the interface provided by capset(2) and capget(2). This package also
provides the setcap(8) and getcap(8) programs. It can be found at
<http://www.kernel.org/pub/linux/libs/security/linux-privs>;.
Before kernel 2.6.24, and from kernel 2.6.24 to kernel 2.6.32 if file
capabilities are not enabled, a thread with the CAP_SETPCAP 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 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 pro-
cesses 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), netcap(8), pscap(8), setcap(8)
include/linux/capability.h in the Linux kernel source tree
COLOPHON
This page is part of release 4.15 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/.
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