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

       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  non-zero).
       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_WRITE (since Linux 2.6.11)
	      Write records to kernel auditing log.

	      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  exe‐
	      cute permission checks.

	      * 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
	      * set extended file  attributes  (see  chattr(1))	 on  arbitrary
	      * 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

       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 (e.g., setting privi‐
	      leged  socket options, enabling multicasting, interface configu‐
	      ration, modifying routing tables).

	      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.

	      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 caller's permitted capability set to  or  from
	      any  other process.  (This property of CAP_SETPCAP is not avail‐
	      able when the kernel is configured to support file capabilities,
	      since CAP_SETPCAP has entirely different semantics for such ker‐

	      If file capabilities are supported: add any capability from  the
	      calling thread's bounding set to its inheritable set; drop capa‐
	      bilities 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	 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_NEWNS flag with clone(2) and unshare(2);
	      * perform KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2) operations.

	      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.

	      * 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));
	      * set  CPU  affinity  for	 arbitrary  processes (sched_setaffin‐
	      * set I/O scheduling class and priority for arbitrary  processes
	      * 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).

	      Use acct(2).

	      Trace arbitrary processes using ptrace(2)

	      Perform I/O port	operations  (iopl(2)  and  ioperm(2));	access

	      * 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;
	      * raise  msg_qbytes limit for a System V message queue above the
		limit in /proc/sys/kernel/msgmnb (see msgop(2) and msgctl(2)).

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

	      Use vhangup(2).

   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

       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 re-acquire 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 dur‐
       ing execve(2).

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

   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.capa‐
       bility.	 Writing  to  this extended attribute requires the CAP_SETFCAP
       capability.  The file capability sets, in conjunction with the capabil‐
       ity sets of the thread, determine the capabilities of a thread after an

       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 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'(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

	   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

   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 transfor‐
       mations	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 pro‐
       vided by traditional Unix systems.

   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 the capability is not in
	 the  bounding set, then a thread can't add one of its permitted capa‐
	 bilities to its inheritable set and thereby have that capability pre‐
	 served	 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

       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!),  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 onwards

       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‐

       Removing capabilities from the bounding set is only supported  if  file
       capabilities  are  compiled into the kernel (CONFIG_SECURITY_FILE_CAPA‐
       BILITIES).  In that case, the init process (the ancestor	 of  all  pro‐
       cesses)	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
       non-zero 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), 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 non-zero value, then all capabilities are cleared from the permit‐
	  ted and effective capability sets.

       2. If the effective user ID is changed from 0  to  non-zero,  then  all
	  capabilities are cleared from the effective set.

       3. If  the  effective  user  ID is changed from non-zero to 0, then the
	  permitted set is copied to the effective set.

       4. If the file system user ID is changed from 0 to non-zero (see	 setf‐
	  suid(2)) then the following capabilities are cleared from the effec‐
	  CAP_FOWNER,  CAP_FSETID,  and	 CAP_MAC_OVERRIDE.  If the file system
	  UID is changed from non-zero to 0, then any  of  these  capabilities
	  that	are  enabled in the permitted set are enabled in the effective

       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 non-zero 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 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  kernel  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:

	      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
	      non-zero 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 function‐
	      ality 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 non-zero 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  pre‐
       venting	further	 changes to the corresponding "base" flag.  The locked

       The  securebits	flags can be modified and retrieved using the prctl(2)
       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  SECURE_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‐

		   1 << SECURE_NOROOT |

       No standards govern capabilities, but the Linux capability  implementa‐
       tion   is   based   on  the  withdrawn  POSIX.1e	 draft	standard;  see

       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.

       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/kernel/cap-bound, always masks	out  this  capability,
	 and  this  can not be changed without modifying the kernel source and

       * 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.

       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),	 credentials(7),  pthreads(7), getcap(8), set‐

       include/linux/capability.h in the kernel source

       This page is part of release 3.22 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2008-11-27		       CAPABILITIES(7)

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