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

NAME
       capabilities - overview of Linux	capabilities

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

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

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

       CAP_AUDIT_CONTROL (since	Linux 2.6.11)
	      Enable and  disable  kernel  auditing;  change  auditing	filter
	      rules; retrieve auditing status and filtering rules.

       CAP_AUDIT_WRITE (since Linux 2.6.11)
	      Write records to kernel auditing log.

       CAP_BLOCK_SUSPEND (since	Linux 3.5)
	      Employ  features	that can block system suspend (epoll(7)	EPOLL-
	      WAKEUP, /proc/sys/wake_lock).

       CAP_CHOWN
	      Make arbitrary changes to	file UIDs and GIDs (see	chown(2)).

       CAP_DAC_OVERRIDE
	      Bypass file read,	write, and execute permission checks.  (DAC is
	      an abbreviation of "discretionary	access control".)

       CAP_DAC_READ_SEARCH
	      *	Bypass file read permission checks and directory read and exe-
		cute permission	checks;
	      *	Invoke open_by_handle_at(2).

       CAP_FOWNER
	      *	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).

       CAP_FSETID
	      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.

       CAP_IPC_LOCK
	      Lock memory (mlock(2), mlockall(2), mmap(2), shmctl(2)).

       CAP_IPC_OWNER
	      Bypass permission	checks for operations on System	V IPC objects.

       CAP_KILL
	      Bypass  permission  checks  for  sending	signals	(see kill(2)).
	      This includes use	of the ioctl(2)	KDSIGACCEPT operation.

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

       CAP_LINUX_IMMUTABLE
	      Set the  FS_APPEND_FL  and  FS_IMMUTABLE_FL  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).

       CAP_NET_ADMIN
	      Perform various network-related operations:
	      *	interface configuration;
	      *	administration of IP firewall, masquerading, and accounting;
	      *	modify routing tables;
	      *	bind to	any address for	transparent proxying;
	      *	set type-of-service (TOS)
	      *	clear driver statistics;
	      *	set promiscuous	mode;
	      *	enabling multicasting;
	      *	use  setsockopt(2) to set the following	socket options:	SO_DE-
		BUG, SO_MARK, SO_PRIORITY (for a priority outside the range  0
		to 6), SO_RCVBUFFORCE, and SO_SNDBUFFORCE.

       CAP_NET_BIND_SERVICE
	      Bind  a socket to	Internet domain	privileged ports (port numbers
	      less than	1024).

       CAP_NET_BROADCAST
	      (Unused)	Make socket broadcasts,	and listen to multicasts.

       CAP_NET_RAW
	      *	use RAW	and PACKET sockets;
	      *	bind to	any address for	transparent proxying.

       CAP_SETGID
	      Make arbitrary manipulations of process GIDs  and	 supplementary
	      GID list;	forge GID when passing socket credentials via UNIX do-
	      main sockets.

       CAP_SETFCAP (since Linux	2.6.24)
	      Set file capabilities.

       CAP_SETPCAP
	      If file capabilities are not supported: grant or remove any  ca-
	      pability 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 ker-
	      nels.)

	      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.

       CAP_SETUID
	      Make arbitrary manipulations of  process	UIDs  (setuid(2),  se-
	      treuid(2),  setresuid(2),	 setfsuid(2));	make  forged  UID when
	      passing socket credentials via UNIX domain sockets.

       CAP_SYS_ADMIN
	      *	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 in-
		put 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 file-system ioctl(2)	operations;
	      *	perform	administrative operations on many device drivers.

       CAP_SYS_BOOT
	      Use reboot(2) and	kexec_load(2).

       CAP_SYS_CHROOT
	      Use chroot(2).

       CAP_SYS_MODULE
	      Load   and   unload   kernel  modules  (see  init_module(2)  and
	      delete_module(2)); in kernels before 2.6.25:  drop  capabilities
	      from the system-wide capability bounding set.

       CAP_SYS_NICE
	      *	Raise  process nice value (nice(2), setpriority(2)) and	change
		the nice value for arbitrary processes;
	      *	set real-time scheduling policies for calling process, and set
		scheduling  policies  and  priorities  for arbitrary processes
		(sched_setscheduler(2),	sched_setparam(2));
	      *	set CPU	 affinity  for	arbitrary  processes  (sched_setaffin-
		ity(2));
	      *	set  I/O scheduling class and priority for arbitrary processes
		(ioprio_set(2));
	      *	apply migrate_pages(2) to arbitrary processes and  allow  pro-
		cesses to be migrated to arbitrary nodes;
	      *	apply move_pages(2) to arbitrary processes;
	      *	use the	MPOL_MF_MOVE_ALL flag with mbind(2) and	move_pages(2).

       CAP_SYS_PACCT
	      Use acct(2).

       CAP_SYS_PTRACE
	      Trace   arbitrary	  processes  using  ptrace(2);	apply  get_ro-
	      bust_list(2) to arbitrary	 processes;  inspect  processes	 using
	      kcmp(2).

       CAP_SYS_RAWIO
	      *	Perform	I/O port operations (iopl(2) and ioperm(2));
	      *	access /proc/kcore;
	      *	employ the FIBMAP ioctl(2) operation;
	      *	open devices for accessing x86 model-specific registers	(MSRs,
		see msr(4))
	      *	update /proc/sys/vm/mmap_min_addr;
	      *	create memory mappings at addresses below the value  specified
		by /proc/sys/vm/mmap_min_addr;
	      *	map files in /proc/bus/pci;
	      *	open /dev/mem and /dev/kmem;
	      *	perform	various	SCSI device commands;
	      *	perform	certain	operations on hpsa(4) and cciss(4) devices;
	      *	perform	 a  range  of  device-specific operations on other de-
		vices.

       CAP_SYS_RESOURCE
	      *	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 /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.

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

       CAP_SYS_TTY_CONFIG
	      Use vhangup(2); employ various privileged	ioctl(2) operations on
	      virtual terminals.

       CAP_SYSLOG (since Linux 2.6.37)

       *  Perform privileged syslog(2) operations.  See	syslog(2) for informa-
	  tion on which	operations require privilege.

       *  View kernel addresses	exposed	via /proc and  other  interfaces  when
	  /proc/sys/kernel/kptr_restrict has the value 1.  (See	the discussion
	  of the kptr_restrict in proc(5).)

       CAP_WAKE_ALARM (since Linux 3.0)
	  Trigger something that will wake  up	the  system  (set  CLOCK_REAL-
	  TIME_ALARM and CLOCK_BOOTTIME_ALARM timers).

   Past	and current implementation
       A full implementation of	capabilities requires that:

       1. For  all  privileged	operations,  the kernel	must check whether the
	  thread has the required capability in	its effective set.

       2. The kernel must provide system calls allowing	a thread's  capability
	  sets to be changed and retrieved.

       3. The 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:

       Permitted:
	      This  is a limiting superset for the effective capabilities that
	      the thread may assume.  It is also a limiting superset  for  the
	      capabilities  that  may  be  added  to  the inheritable set by a
	      thread that does not have	the CAP_SETPCAP	capability in its  ef-
	      fective 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 ca-
	      pabilities grant that capability).

       Inheritable:
	      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).

       Effective:
	      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
       below).

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

   File	capabilities
       Since  kernel  2.6.24,  the kernel supports associating capability sets
       with an executable file using setcap(8).	 The file capability sets  are
       stored  in an extended attribute	(see setxattr(2)) 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
       execve(2).

       The three file capability sets are:

       Permitted (formerly known as forced):
	      These  capabilities  are	automatically permitted	to the thread,
	      regardless of the	thread's inheritable capabilities.

       Inheritable (formerly known as allowed):
	      This set is ANDed	with the thread's inheritable set to determine
	      which  inheritable capabilities are enabled in the permitted set
	      of the thread after the execve(2).

       Effective:
	      This is not a set, but rather just a single bit.	If this	bit is
	      set, then	during an execve(2) all	of the new permitted capabili-
	      ties for the thread are also raised in the  effective  set.   If
	      this  bit	 is  not set, then after an execve(2), none of the new
	      permitted	capabilities is	in the new effective set.

	      Enabling the file	effective capability bit implies that any file
	      permitted	 or inheritable	capability that	causes a thread	to ac-
	      quire 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  en-
	      abled  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]

       where:

	   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 ef-
	  fective 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  ex-
       ecve(2)s	 a program, it gains all capabilities in its permitted and ef-
       fective 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 a	capability is  not  in
	 the  bounding set, then a thread can't	add this capability to its in-
	 heritable 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!), mod-
       ify the definition of  CAP_INIT_EFF_SET	in  include/linux/capability.h
       and rebuild the kernel.

       The  system-wide	 capability  bounding  set  feature was	added to Linux
       starting	with kernel version 2.2.11.

       Capability bounding set from Linux 2.6.25 onward

       From Linux 2.6.25, the capability bounding set is a  per-thread	attri-
       bute.  (There is	no longer a system-wide	capability bounding set.)

       The  bounding set is inherited at fork(2) from the thread's parent, and
       is preserved across an execve(2).

       A thread	may remove capabilities	from its capability bounding set using
       the prctl(2) PR_CAPBSET_DROP operation, provided	it has the CAP_SETPCAP
       capability.  Once a capability has been dropped from the	bounding  set,
       it  cannot  be restored to that set.  A thread can determine if a capa-
       bility is in its	bounding set using the prctl(2)	PR_CAPBSET_READ	opera-
       tion.

       Removing	 capabilities  from the	bounding set is	supported only if file
       capabilities are	compiled into the kernel.   In	kernels	 before	 Linux
       2.6.33, file capabilities were an optional feature configurable via the
       CONFIG_SECURITY_FILE_CAPABILITIES option.  Since	Linux 2.6.33, the con-
       figuration  option  has	been  removed and file capabilities are	always
       part of the kernel.  When file capabilities are compiled	into the  ker-
       nel,  the  init	process	 (the ancestor of all processes) begins	with a
       full bounding set.  If file capabilities	are not	compiled into the ker-
       nel,  then  init	begins with a full bounding set	minus CAP_SETPCAP, be-
       cause 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), setresuid(2),	or similar):

       1. If one or more of the	real, effective	or saved set user IDs was pre-
	  viously  0, and as a result of the UID changes all of	these IDs have
	  a nonzero value, then	all capabilities are cleared from the  permit-
	  ted and effective capability sets.

       2. If  the effective user ID is changed from 0 to nonzero, then all ca-
	  pabilities are cleared from the effective set.

       3. If the effective user	ID is changed from nonzero to 0, then the per-
	  mitted set is	copied to the effective	set.

       4. If  the  file	system user ID is changed from 0 to nonzero (see setf-
	  suid(2)) then	the following capabilities are cleared from the	effec-
	  tive	  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 file
	  system UID is	changed	from nonzero to	0, then	any of these capabili-
	  ties that are	enabled	in the permitted set are enabled in the	effec-
	  tive 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.

   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 in-
	  heritable set	must be	a subset of the	combination  of	 the  existing
	  inheritable and permitted sets.

       2. (Since Linux 2.6.25) The new inheritable set must be a subset	of the
	  combination of the  existing	inheritable  set  and  the  capability
	  bounding set.

       3. The new permitted set	must be	a subset of the	existing permitted set
	  (i.e., it is not possible to acquire permitted capabilities that the
	  thread does not currently have).

       4. The new effective set	must be	a subset of the	new permitted set.

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

       SECBIT_KEEP_CAPS
	      Setting this flag	allows a thread	that has one or	more 0 UIDs to
	      retain  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 function-
	      ality as the older prctl(2) PR_SET_KEEPCAPS operation.)

       SECBIT_NO_SETUID_FIXUP
	      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 values.  (See	the subsection
	      Effect of	User ID	Changes	on Capabilities.)

       SECBIT_NOROOT
	      If this bit is set, then the kernel does not grant  capabilities
	      when  a  set-user-ID-root	program	is executed, or	when a process
	      with an effective	or real	UID of 0 calls	execve(2).   (See  the
	      subsection Capabilities and execution of programs	by root.)

       Each  of	the above "base" flags has a companion "locked"	flag.  Setting
       any of the "locked" flags is irreversible, and has the effect  of  pre-
       venting	further	 changes to the	corresponding "base" flag.  The	locked
       flags are: SECBIT_KEEP_CAPS_LOCKED, SECBIT_NO_SETUID_FIXUP_LOCKED,  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 ex-
       ecve(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  ca-
       pabilities 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);

CONFORMING TO
       No  standards govern capabilities, but the Linux	capability implementa-
       tion  is	 based	on  the	 withdrawn  POSIX.1e   draft   standard;   see
       <http://wt.tuxomania.net/publications/posix.1e/>.

NOTES
       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  pro-
	 cesses	created	on the system.

SEE ALSO
       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), get-
       cap(8), setcap(8)

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

COLOPHON
       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 http://www.kernel.org/doc/man-pages/.

Linux				  2013-07-21		       CAPABILITIES(7)

NAME | DESCRIPTION | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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