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CentOS 7.0 - man page for capabilities (centos section 7)

CAPABILITIES(7) 		    Linux Programmer's Manual			  CAPABILITIES(7)

       capabilities - overview of Linux capabilities

       For  the purpose of performing permission checks, traditional UNIX implementations distin-
       guish 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 (usu-
       ally: effective UID, effective GID, and supplementary group list).

       Starting with kernel 2.2, Linux divides the privileges traditionally associated with supe-
       ruser  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_WRITE (since Linux 2.6.11)
	      Write records to kernel auditing log.

       CAP_BLOCK_SUSPEND (since Linux 3.5)
	      Employ  features	 that	can   block   system   suspend	 (epoll(7)   EPOLLWAKEUP,

	      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
	      * Invoke open_by_handle_at(2).

	      * Bypass	permission checks on operations that normally require the file system 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 file system 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.

       CAP_LEASE (since Linux 2.4)
	      Establish leases on arbitrary files (see fcntl(2)).

	      Set the FS_APPEND_FL and FS_IMMUTABLE_FL i-node flags (see chattr(1)).

       CAP_MAC_ADMIN (since Linux 2.6.25)
	      Override Mandatory Access Control (MAC).	Implemented for the Smack Linux  Security
	      Module (LSM).

       CAP_MAC_OVERRIDE (since Linux 2.6.25)
	      Allow MAC configuration or state changes.  Implemented for the Smack LSM.

       CAP_MKNOD (since Linux 2.4)
	      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_PRI-
		ORITY (for a priority outside the range 0 to 6), SO_RCVBUFFORCE,  and  SO_SNDBUF-

	      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.

       CAP_SETFCAP (since Linux 2.6.24)
	      Set file capabilities.

	      If file capabilities are not supported: grant or remove any capability in the call-
	      er'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 capabil-
	      ities, 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 sock-

	      * 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 termi-
		nal 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 file-system 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  ker-
	      nels before 2.6.25: drop capabilities from the system-wide capability bounding set.

	      * 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  poli-
		cies  and  priorities  for arbitrary processes (sched_setscheduler(2), sched_set-
	      * set CPU affinity for arbitrary processes (sched_setaffinity(2));
	      * set I/O scheduling class and priority for arbitrary processes (ioprio_set(2));
	      * apply migrate_pages(2) to arbitrary processes and allow processes to be  migrated
		to arbitrary nodes;
	      * apply move_pages(2) to arbitrary processes;
	      * use the MPOL_MF_MOVE_ALL flag with mbind(2) and move_pages(2).

	      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
	      * 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 file systems;
	      * 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
	      * 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

	      Set system clock (settimeofday(2), stime(2), adjtimex(2)); set real-time (hardware)

	      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 opera-
	  tions require privilege.

       *  View kernel addresses exposed  via  /proc  and  other  interfaces  when  /proc/sys/ker-
	  nel/kptr_restrict  has  the  value  1.   (See  the  discussion  of the kptr_restrict in

       CAP_WAKE_ALARM (since Linux 3.0)
	  Trigger  something  that  will  wake	up  the  system  (set  CLOCK_REALTIME_ALARM   and

   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  file  system  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.

   Thread capability sets
       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  mecha-
	      nism  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.

   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)) 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).

	      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 capa-
	      bilities	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.

   Transformation of capabilities during execve()
       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]


	   P	     denotes the value of a thread capability set before the execve(2)

	   P'	     denotes the value of a capability set after the execve(2)

	   F	     denotes a file capability set

	   cap_bset  is the value of the capability bounding set (described below).

   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 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  permit-
       ted 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.

   Capability bounding set
       The capability bounding set is a security mechanism that can be used to limit the capabil-
       ities  that  can be gained during an execve(2).	The bounding set is used in the following

       * During an execve(2), the capability bounding set is ANDed with the file permitted  capa-
	 bility set, and the result of this operation is assigned to the thread's permitted capa-
	 bility set.  The capability bounding set thus places a limit on the permitted	capabili-
	 ties 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 capabil-
       ity.  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 opera-

       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.

   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 file system user IDs (using setuid(2), setre-
       suid(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 file system user ID is changed from 0 to nonzero (see setfsuid(2)) then the fol-
	  lowing  capabilities	are  cleared from the effective set: CAP_CHOWN, CAP_DAC_OVERRIDE,
	  CAP_MAC_OVERRIDE,  and  CAP_MKNOD  (since  Linux  2.2.30).   If  the file system UID is
	  changed from nonzero to 0, then any of these capabilities that are enabled in the  per-
	  mitted 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 per-
       mitted 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.

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

       1. If the caller does not have the CAP_SETPCAP capability, the new inheritable set must be
	  a subset of the combination of the 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 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 capa-
	      bilities 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 file system UIDs are switched between zero and nonzero val-
	      ues.  (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

       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 correspond-
       ing    "base"	flag.	   The	   locked     flags	are:	 SECBIT_KEEP_CAPS_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

       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:


       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

       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/ker-
	 nel/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),  creden-
       tials(7), pthreads(7), getcap(8), setcap(8)

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

       This  page  is  part of release 3.53 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at

Linux					    2013-07-21				  CAPABILITIES(7)

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