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CAPABILITIES(7) 	   Linux Programmers 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
       processs  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).

	      Dont 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 reserved  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 callers 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 threads 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), 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;
	      * perform keyctl(2) KEYCTL_CHOWN and KEYCTL_SETPERM operations;
	      * 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) (without this  capabil
		ity these system calls will fail with the error ENFILE if this
		limit is encountered);
	      * 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 threads 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 parents  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 threads inheritable capabilities.

       Inheritable (formerly known as allowed):
	      This set is ANDed with the threads 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  transormation  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 threads 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  the  capability
	 is  not  in the bounding set, then a thread cant add one of its per
	 mitted capabilities to its inheritable  set  and  thereby  have  that
	 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

       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 threads 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
       threads inherited set.  However it does prevent	the  capability  from
       being added back into the threads 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 threads
       capability  sets on changes to the threads 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
	  permitted 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  threadss  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 processs 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 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2008-07-15		       CAPABILITIES(7)

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