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capabilities

overview of Linux capabilities

For the purpose of performing permission checks, traditional UNIX implementations distinguish two categories of processes: privileged processes (whose effective user ID is 0, referred to as superuser or root), and unprivileged processes (whose effective UID is nonzero). Privileged processes bypass all kernel permission checks, while unprivileged processes are subject to full permission checking based on the process's credentials (usually: effective UID, effective GID, and supplementary group list).

Starting with kernel 2.2, Linux divides the privileges traditionally associated with superuser into distinct units, known as capabilities, which can be independently enabled and disabled. Capabilities are a per-thread attribute.

The following list shows the capabilities implemented on Linux, and the operations or behaviors that each capability permits:

Enable and disable kernel auditing; change auditing filter rules; retrieve auditing status and filtering rules.
Write records to kernel auditing log.
Employ features that can block system suspend (epoll(7) EPOLLWAKEUP, /proc/sys/wake_lock).
Make arbitrary changes to file UIDs and GIDs (see chown(2)).
Bypass file read, write, and execute permission checks. (DAC is an abbreviation of "discretionary access control".)
  • Bypass file read permission checks and directory read and execute permission checks;
  • Invoke open_by_handle_at(2).

  • 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 extended file attributes (see chattr(1)) 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).
Don't clear set-user-ID and set-group-ID permission 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.
Lock memory (mlock(2), mlockall(2), mmap(2), shmctl(2)).
Bypass permission checks for operations on System V IPC objects.
Bypass permission checks for sending signals (see kill(2)). This includes use of the ioctl(2) KDSIGACCEPT operation.
Establish leases on arbitrary files (see fcntl(2)).
Set the FS_APPEND_FL and FS_IMMUTABLE_FL i-node flags (see chattr(1)).
Override Mandatory Access Control (MAC). Implemented for the Smack Linux Security Module (LSM).
Allow MAC configuration or state changes. Implemented for the Smack LSM.
Create special files using mknod(2).
Perform various network-related operations:
  • interface configuration;
  • administration of IP firewall, masquerading, and accounting;
  • modify routing tables;
  • bind to any address for transparent proxying;
  • set type-of-service (TOS)
  • clear driver statistics;
  • set promiscuous mode;
  • enabling multicasting;
  • use setsockopt(2) to set the following socket options: SO_DEBUG, SO_MARK, SO_PRIORITY (for a priority outside the range 0 to 6), SO_RCVBUFFORCE, and SO_SNDBUFFORCE.
Bind a socket to Internet domain privileged ports (port numbers less than 1024).
(Unused) Make socket broadcasts, and listen to multicasts.
  • use RAW and PACKET sockets;
  • bind to any address for transparent proxying.
Make arbitrary manipulations of process GIDs and supplementary GID list; forge GID when passing socket credentials via UNIX domain sockets.
Set file capabilities.
If file capabilities are not supported: grant or remove any capability in the caller's permitted capability set to or from any other process. (This property of CAP_SETPCAP is not available when the kernel is configured to support file capabilities, since CAP_SETPCAP has entirely different semantics for such kernels.)

If file capabilities are supported: 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.

Make arbitrary manipulations of process UIDs (setuid(2), setreuid(2), setresuid(2), setfsuid(2)); make forged UID when passing socket credentials via UNIX domain sockets.
  • 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;
  • perform operations on trusted and security Extended Attributes (see attr(5));
  • 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 UID when passing socket credentials;
  • 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);
  • call perf_event_open(2);
  • access privileged perf event information;
  • call setns(2);
  • call fanotify_init(2);
  • perform KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2) operations;
  • perform madvise(2) MADV_HWPOISON operation;
  • employ the TIOCSTI ioctl(2) to insert characters into the input queue of a terminal other than the caller's controlling terminal.
  • employ the obsolete nfsservctl(2) system call;
  • employ the obsolete bdflush(2) system call;
  • perform various privileged block-device ioctl(2) operations;
  • perform various privileged filesystem ioctl(2) operations;
  • perform administrative operations on many device drivers.
Use reboot(2) and kexec_load(2).
Use chroot(2).
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.
Use acct(2).
Trace arbitrary processes using ptrace(2); apply get_robust_list(2) to arbitrary processes; inspect processes using kcmp(2).
  • Perform I/O port operations (iopl(2) and ioperm(2));
  • access /proc/kcore;
  • employ the FIBMAP ioctl(2) operation;
  • open devices for accessing x86 model-specific registers (MSRs, see 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.
  • 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));
  • 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 prctl(2) PR_SET_MM operation;
  • set /proc/PID/oom_score_adj to a value lower than the value last set by a process with CAP_SYS_RESOURCE.
Set system clock (settimeofday(2), stime(2), adjtimex(2)); set real-time (hardware) clock.
Use vhangup(2); employ various privileged ioctl(2) operations on virtual terminals.
  • 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).)
Trigger something that will wake up the system (set CLOCK_REALTIME_ALARM and CLOCK_BOOTTIME_ALARM timers).

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.

Each thread has three capability sets containing zero or more of the above capabilities:

This is a limiting superset for the effective capabilities that the thread may assume. It is also a limiting superset for the capabilities that may be added to the inheritable set by a thread that does not have the CAP_SETPCAP capability in its effective set.

If a thread drops a capability from its permitted set, it can never reacquire that capability (unless it execve(2)s either a set-user-ID-root program, or a program whose associated file capabilities grant that capability).

This is a set of capabilities preserved across an execve(2). It provides a mechanism for a process to assign capabilities to the permitted set of the new program during an execve(2).
This is the set of capabilities used by the kernel to perform permission checks for the thread.

A child created via fork(2) inherits copies of its parent's capability sets. See below for a discussion of the treatment of capabilities during execve(2).

Using capset(2), a thread may manipulate its own capability sets (see below).

Since Linux 3.2, the file /proc/sys/kernel/cap_last_cap exposes the numerical value of the highest capability supported by the running kernel; this can be used to determine the highest bit that may be set in a capability set.

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)) 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:

These capabilities are automatically permitted to the thread, regardless of the thread's inheritable capabilities.
This set is ANDed with the thread's inheritable set to determine which inheritable capabilities are enabled in the permitted set of the thread after the execve(2).
This is not a set, but rather just a single bit. If this bit is set, then during an execve(2) all of the new permitted capabilities for the thread are also raised in the effective set. If this bit is not set, then after an execve(2), none of the new permitted capabilities is in the new effective set.

Enabling the file effective capability bit implies that any file permitted or inheritable capability that causes a thread to acquire the corresponding permitted capability during an execve(2) (see the transformation rules described below) will also acquire that capability in its effective set. Therefore, when assigning capabilities to a file (setcap(8), cap_set_file(3), cap_set_fd(3)), if we specify the effective flag as being enabled for any capability, then the effective flag must also be specified as enabled for all other capabilities for which the corresponding permitted or inheritable flags is enabled.

During an execve(2), the kernel calculates the new capabilities of the process using the following algorithm: 

P'(permitted) = (P(inheritable) & F(inheritable)) |
                (F(permitted) & cap_bset)
P'(effective) = F(effective) ? P'(permitted) : 0
P'(inheritable) = P(inheritable)    [i.e., unchanged]

where:

denotes the value of a thread capability set before the execve(2)
denotes the value of a capability set after the execve(2)
denotes a file capability set
is the value of the capability bounding set (described below).

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 user ID of the process is 0 (root) then the file inheritable and permitted sets are defined to be all ones (i.e., all capabilities enabled).
2.
If a set-user-ID-root program is being executed, then the file effective bit is defined to be one (enabled).

The upshot of the above rules, combined with the capabilities transformations described above, is that 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. This provides semantics that are the same as those provided by traditional UNIX systems.

The capability bounding set is a security mechanism that can be used to limit the capabilities that can be gained during an execve(2). The bounding set is used in the following ways:

  • During an execve(2), the capability bounding set is ANDed with the file permitted capability set, and the result of this operation is assigned to the thread's permitted capability set. The capability bounding set thus places a limit on the permitted capabilities that may be granted by an executable file.
  • (Since Linux 2.6.25) The capability bounding set acts as a limiting superset for the capabilities that a thread can add to its inheritable set using capset(2). This means that if a capability is not in the bounding set, then a thread can't add this capability to its inheritable set, even if it was in its permitted capabilities, and thereby cannot have this capability preserved in its permitted set when it execve(2)s a file that has the capability in its inheritable set.

Note that the bounding set masks the file permitted capabilities, but not the inherited capabilities. If a thread maintains a capability in its inherited 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 inherited 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: programs 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.

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 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's inherited set. However it does prevent the capability from being added back into the thread's inherited set in the future.

To preserve the traditional semantics for transitions between 0 and nonzero user IDs, the kernel makes the following changes to a thread's capability sets on changes to the thread's real, effective, saved set, and filesystem user IDs (using setuid(2), setresuid(2), or similar):

1.
If one or more of the real, effective or saved set user IDs was previously 0, and as a result of the UID changes all of these IDs have a nonzero value, then all capabilities are cleared from the permitted and effective capability sets.
2.
If the effective user ID is changed from 0 to nonzero, then all capabilities are cleared from the effective set.
3.
If the effective user ID is changed from nonzero to 0, then the permitted set is copied to the effective set.
4.
If the filesystem user ID is changed from 0 to nonzero (see setfsuid(2)) then the following capabilities are cleared from the effective set: CAP_CHOWN, CAP_DAC_OVERRIDE, CAP_DAC_READ_SEARCH, CAP_FOWNER, CAP_FSETID, CAP_LINUX_IMMUTABLE (since Linux 2.2.30), CAP_MAC_OVERRIDE, and CAP_MKNOD (since Linux 2.2.30). 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 prctl(2) PR_SET_KEEPCAPS operation.

A thread can retrieve and change its capability sets using the capget(2) and capset(2) system calls. However, the use of cap_get_proc(3) and cap_set_proc(3), both provided in the libcap package, is preferred for this purpose. The following rules govern changes to the thread capability sets:

1.
If the caller does not have the CAP_SETPCAP capability, the new inheritable set must be a subset of the combination of the 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.

Starting with kernel 2.6.26, and with a kernel in which file capabilities are enabled, Linux implements a set of per-thread securebits flags that can be used to disable special handling of capabilities for UID 0 (root). These flags are as follows:

Setting this flag allows a thread that has one or more 0 UIDs to retain its capabilities when it switches all of its UIDs to a nonzero value. If this flag is not set, then such a UID switch causes the thread to lose all capabilities. This flag is always cleared on an execve(2). (This flag provides the same functionality as the older prctl(2) PR_SET_KEEPCAPS operation.)
Setting this flag stops the kernel from adjusting capability sets when the threads's effective and filesystem UIDs are switched between zero and nonzero values. (See the subsection Effect of User ID Changes on Capabilities.)
If this bit is set, then the kernel does not grant capabilities when a set-user-ID-root program is executed, or when a process with an effective or real UID of 0 calls execve(2). (See the subsection Capabilities and execution of programs by root.)

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, and SECBIT_NOROOT_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 capabilities: 

prctl(PR_SET_SECUREBITS,
        SECBIT_KEEP_CAPS_LOCKED |
        SECBIT_NO_SETUID_FIXUP |
        SECBIT_NO_SETUID_FIXUP_LOCKED |
        SECBIT_NOROOT |
        SECBIT_NOROOT_LOCKED);

No standards govern capabilities, but the Linux capability implementation is based on the withdrawn POSIX.1e draft standard; see http://wt.tuxomania.net/publications/posix.1e/.

Since kernel 2.5.27, capabilities are an optional kernel component, and can be enabled/disabled via the CONFIG_SECURITY_CAPABILITIES kernel configuration option.

The /proc/PID/task/TID/status file can be used to view the capability sets of a thread. The /proc/PID/status file shows the capability sets of a process's main thread. Before Linux 3.8, nonexistent capabilities were shown as being enabled (1) in these sets. Since Linux 3.8, all non-existent 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 since kernel 2.6.24 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.

capget(2), 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), credentials(7), pthreads(7), getcap(8), setcap(8)

include/linux/capability.h in the Linux kernel source tree

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