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



NAME
       capabilities - overview of Linux capabilities

DESCRIPTION
       For  the	 purpose  of  performing  permission  checks, traditional UNIX
       implementations distinguish two	categories  of	processes:  privileged
       processes  (whose  effective  user ID is 0, referred to as superuser or
       root), and unprivileged processes (whose	 effective  UID	 is  nonzero).
       Privileged processes bypass all kernel permission checks, while 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_READ (since Linux 3.16)
	      Allow reading the audit log via a multicast netlink socket.

       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);
	      * use the linkat(2) AT_EMPTY_PATH flag to create	a  link	 to  a
		file referred to by a file descriptor.

       CAP_FOWNER
	      * Bypass	permission  checks on operations that normally require
		the filesystem UID of the process to match the UID of the file
		(e.g., chmod(2), utime(2)), excluding those operations covered
		by CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH;
	      * set inode flags (see ioctl_iflags(2)) 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 mode bits when a file
		is modified;
	      * set  the  set-group-ID bit for a file whose GID does not match
		the filesystem or any of the supplementary GIDs of the calling
		process.

       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  inode  flags  (see
	      ioctl_iflags(2)).

       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_DEBUG, 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 domain
		sockets;
	      * write a group ID mapping in a user namespace (see  user_names-
		paces(7)).

       CAP_SETFCAP (since Linux 2.6.24)
	      Set file capabilities.

       CAP_SETPCAP
	      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-
	      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),
		setreuid(2), setresuid(2), setfsuid(2));
	      * forge UID when passing	socket	credentials  via  UNIX	domain
		sockets;
	      * write  a  user ID mapping in a user namespace (see user_names-
		paces(7)).

       CAP_SYS_ADMIN
	      Note: this capability is overloaded; see Notes to kernel	devel-
	      opers, below.

	      * 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;
	      * override RLIMIT_NPROC resource limit;
	      * perform operations on trusted and security Extended Attributes
		(see xattr(7));
	      * 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  PID  when  passing  socket  credentials via UNIX domain
		sockets;
	      * 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) (but, since Linux 3.8, creating user namespaces
		does not require any capability);
	      * call perf_event_open(2);
	      * access privileged perf event information;
	      * call setns(2) (requires CAP_SYS_ADMIN  in  the	target	names-
		pace);
	      * call fanotify_init(2);
	      * call bpf(2);
	      * perform	 privileged  KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2)
		operations;
	      * use ptrace(2) PTRACE_SECCOMP_GET_FILTER to dump a tracees sec-
		comp filters;
	      * perform madvise(2) MADV_HWPOISON operation;
	      * employ	the  TIOCSTI  ioctl(2)	to  insert characters into the
		input queue of a terminal other than the caller's  controlling
		terminal;
	      * employ the obsolete nfsservctl(2) system call;
	      * employ the obsolete bdflush(2) system call;
	      * perform various privileged block-device ioctl(2) operations;
	      * perform various privileged filesystem ioctl(2) operations;
	      * perform	 privileged  ioctl(2)  operations  on  the /dev/random
		device (see random(4));
	      * install a seccomp(2) filter without first having  to  set  the
		no_new_privs thread attribute;
	      * modify allow/deny rules for device control groups;
	      * employ	the  ptrace(2)	PTRACE_SECCOMP_GET_FILTER operation to
		dump tracee's seccomp filters;
	      * employ the ptrace(2) PTRACE_SETOPTIONS	operation  to  suspend
		the  tracee's  seccomp	protections  (i.e.,  the PTRACE_O_SUS-
		PEND_SECCOMP flag).
	      * 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), shed_setattr(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_robust_list(2) to arbitrary processes;
	      * transfer data to or from the  memory  of  arbitrary  processes
		using process_vm_readv(2) and process_vm_writev(2);
	      * 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
		devices.

       CAP_SYS_RESOURCE
	      * Use reserved space on ext2 filesystems;
	      * make ioctl(2) calls controlling ext3 journaling;
	      * override disk quota limits;
	      * increase resource limits (see setrlimit(2));
	      * override RLIMIT_NPROC resource limit;
	      * override maximum number of consoles on console allocation;
	      * override maximum number of keymaps;
	      * allow more than 64hz interrupts from the real-time clock;
	      * raise msg_qbytes limit for a System V message queue above  the
		limit in /proc/sys/kernel/msgmnb (see msgop(2) and msgctl(2));
	      * allow  the  RLIMIT_NOFILE resource limit on the number of "in-
		flight" file descriptors to  be	 bypassed  when	 passing  file
		descriptors  to	 another process via a UNIX domain socket (see
		unix(7));
	      * 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 the 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
		information on which operations require privilege.
	      * View  kernel  addresses exposed via /proc and other interfaces
		when /proc/sys/kernel/kptr_restrict has the value 1.  (See the
		discussion of the kptr_restrict in proc(5).)

       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 filesystem must support attaching capabilities to an  executable
	  file,	 so  that  a process gains those capabilities when the file is
	  executed.

       Before kernel 2.6.24, only the first two of these requirements are met;
       since kernel 2.6.24, all three requirements are met.

   Notes to kernel developers
       When  adding a new kernel feature that should be governed by a capabil-
       ity, consider the following points.

       *  The goal of capabilities is  divide  the  power  of  superuser  into
	  pieces,  such that if a program that has one or more capabilities is
	  compromised, its power to do damage to the system would be less than
	  the same program running with root privilege.

       *  You have the choice of either creating a new capability for your new
	  feature, or associating the feature with one of the  existing	 capa-
	  bilities.   In order to keep the set of capabilities to a manageable
	  size, the latter option is preferable, unless there  are  compelling
	  reasons  to  take  the  former  option.   (There is also a technical
	  limit: the size of capability sets is currently limited to 64 bits.)

       *  To determine which existing capability might best be associated with
	  your	new feature, review the list of capabilities above in order to
	  find a "silo" into which your new feature best fits.	 One  approach
	  to  take is to determine if there are other features requiring capa-
	  bilities that will always be use along with the new feature.	If the
	  new  feature is useless without these other features, you should use
	  the same capability as the other features.

       *  Don't choose CAP_SYS_ADMIN if you can possibly  avoid	 it!   A  vast
	  proportion  of  existing  capability checks are associated with this
	  capability (see the partial list above).  It can plausibly be called
	  "the	new  root",  since on the one hand, it confers a wide range of
	  powers, and on the other hand, its broad scope means	that  this  is
	  the  capability that is required by many privileged programs.	 Don't
	  make the problem worse.  The only new features that should be	 asso-
	  ciated  with CAP_SYS_ADMIN are ones that closely match existing uses
	  in that silo.

       *  If you have determined that it really is necessary to create	a  new
	  capability for your feature, don't make or name it as a "single-use"
	  capability.  Thus, for example, the addition of the highly  specific
	  CAP_PACCT was probably a mistake.  Instead, try to identify and name
	  your new capability as a  broader  silo  into	 which	other  related
	  future use cases might fit.

   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
	      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).

       Inheritable:
	      This  is	a  set	of capabilities preserved across an execve(2).
	      Inheritable capabilities remain inheritable when	executing  any
	      program, and inheritable capabilities are added to the permitted
	      set when executing a program that has the corresponding bits set
	      in the file inheritable set.

	      Because  inheritable  capabilities  are  not generally preserved
	      across execve(2) when running as a non-root  user,  applications
	      that  wish  to  run  helper  programs with elevated capabilities
	      should consider using ambient capabilities, described below.

       Effective:
	      This is the set of capabilities used by the  kernel  to  perform
	      permission checks for the thread.

       Ambient (since Linux 4.3):
	      This  is	a  set	of  capabilities  that are preserved across an
	      execve(2) of a program that  is  not  privileged.	  The  ambient
	      capability  set  obeys the invariant that no capability can ever
	      be ambient if it is not both permitted and inheritable.

	      The ambient  capability  set  can	 be  directly  modified	 using
	      prctl(2).	  Ambient  capabilities	 are  automatically lowered if
	      either of the corresponding permitted or	inheritable  capabili-
	      ties is lowered.

	      Executing a program that changes UID or GID due to the set-user-
	      ID or set-group-ID bits or executing a program that has any file
	      capabilities  set will clear the ambient set.  Ambient capabili-
	      ties are added to the permitted set and assigned to  the	effec-
	      tive set when execve(2) is called.

       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
       numerical 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
	      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'(ambient)	   = (file is privileged) ? 0 : P(ambient)

	   P'(permitted)   = (P(inheritable) & F(inheritable)) |
			     (F(permitted) & cap_bset) | P'(ambient)

	   P'(effective)   = F(effective) ? P'(permitted) : P'(ambient)

	   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 thread capability  set  after  the
		     execve(2)

	   F	     denotes a file capability set

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

       A privileged file is one that has capabilities or has  the  set-user-ID
       or set-group-ID bit set.

       Note:  the  capability  transitions  described  above may not performed
       (i.e., file capabilities may be ignored) for the same reasons that  the
       set-user-ID and set-group-ID bits are ignored; see execve(2).

   Safety checking for capability-dumb binaries
       A capability-dumb binary is an application that has been marked to have
       file capabilities, but has not been converted to use the libcap(3)  API
       to manipulate its capabilities.	(In other words, this is a traditional
       set-user-ID-root program that has been switched to use  file  capabili-
       ties, but whose code has not been modified to understand capabilities.)
       For such applications, the effective capability bit is set on the file,
       so  that	 the  file permitted capabilities are automatically enabled in
       the process effective set when executing the file.  The	kernel	recog-
       nizes  a file which has the effective capability bit set as capability-
       dumb for the purpose of the check described here.

       When executing a capability-dumb	 binary,  the  kernel  checks  if  the
       process	obtained all permitted capabilities that were specified in the
       file permitted set,  after  the	capability  transformations  described
       above  have  been  performed.   (The  typical reason why this might not
       occur is that the capability bounding set masked out some of the	 capa-
       bilities in the file permitted set.)  If the process did not obtain the
       full set of file permitted capabilities, then execve(2) fails with  the
       error  EPERM.   This  prevents possible security risks that could arise
       when a capability-dumb application is executed with less privilege that
       it  needs.   Note that, by definition, the application could not itself
       recognize this problem, since it does not employ the libcap(3) API.

   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 a capability is  not  in
	 the  bounding	set,  then  a  thread can't add this capability to its
	 inheritable set, even if it was in its	 permitted  capabilities,  and
	 thereby  cannot  have	this capability preserved in its permitted set
	 when it execve(2)s a file that has the capability in its  inheritable
	 set.

       Note  that  the bounding set masks the file permitted capabilities, but
       not the inherited capabilities.	If a thread maintains a capability  in
       its  inherited  set  that is not in its bounding set, then it can still
       gain that capability in its permitted set by executing a file that  has
       the capability in its inherited set.

       Depending  on the kernel version, the capability bounding set is either
       a system-wide attribute, or a per-process attribute.

       Capability bounding set prior to Linux 2.6.25

       In kernels before 2.6.25, the capability bounding set is a  system-wide
       attribute  that affects all threads on the system.  The bounding set is
       accessible via the file /proc/sys/kernel/cap-bound.  (Confusingly, this
       bit  mask  parameter  is	 expressed  as	a  signed  decimal  number  in
       /proc/sys/kernel/cap-bound.)

       Only the init process may set capabilities in the  capability  bounding
       set;  other than that, the superuser (more precisely: programs with the
       CAP_SYS_MODULE capability) may only clear capabilities from this set.

       On a standard system the capability bounding set always masks  out  the
       CAP_SETPCAP  capability.	 To remove this restriction (dangerous!), 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
       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-
       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,
       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 filesystem user IDs (using setuid(2), setresuid(2), or similar):

       1. If one or more of the real, effective or saved set user IDs was 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
	  capabilities 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  filesystem  user ID is changed from 0 to nonzero (see setf-
	  suid(2)), then the  following	 capabilities  are  cleared  from  the
	  effective  set:  CAP_CHOWN,  CAP_DAC_OVERRIDE,  CAP_DAC_READ_SEARCH,
	  CAP_FOWNER, CAP_FSETID, CAP_LINUX_IMMUTABLE  (since  Linux  2.6.30),
	  CAP_MAC_OVERRIDE,  and  CAP_MKNOD  (since  Linux  2.6.30).   If  the
	  filesystem UID is changed from nonzero to 0, then any of these capa-
	  bilities  that  are  enabled in the permitted set are enabled in the
	  effective set.

       If a thread that has a 0 value for one or more of its user IDs wants to
       prevent	its  permitted capability set being cleared when it resets all
       of its user IDs to nonzero values, it can  do  so  using	 the  prctl(2)
       PR_SET_KEEPCAPS	operation  or  the  SECBIT_KEEP_CAPS  securebits  flag
       described below.

   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 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  thread's	 effective  and	 filesystem  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.)

       SECBIT_NO_CAP_AMBIENT_RAISE
	      Setting this flag disallows raising ambient capabilities via the
	      prctl(2) PR_CAP_AMBIENT_RAISE operation.

       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,
       SECBIT_NOROOT_LOCKED, and SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED.

       The  securebits	flags can be modified and retrieved using the prctl(2)
       PR_SET_SECUREBITS and PR_GET_SECUREBITS	operations.   The  CAP_SETPCAP
       capability is required to modify the flags.

       The  securebits	flags  are  inherited  by  child processes.  During an
       execve(2), all of the  flags  are  preserved,  except  SECBIT_KEEP_CAPS
       which is always cleared.

       An  application	can  use the following call to lock itself, and all of
       its descendants, into an environment where  the	only  way  of  gaining
       capabilities  is	 by executing a program with associated file capabili-
       ties:

	   prctl(PR_SET_SECUREBITS,
		/* SECBIT_KEEP_CAPS off */
		   SECBIT_KEEP_CAPS_LOCKED |
		   SECBIT_NO_SETUID_FIXUP |
		   SECBIT_NO_SETUID_FIXUP_LOCKED |
		   SECBIT_NOROOT |
		   SECBIT_NOROOT_LOCKED);
		   /* Setting/locking SECURE_NO_CAP_AMBIENT_RAISE
		      is not required */

   Interaction with user namespaces
       For a discussion of the interaction of  capabilities  and  user	names-
       paces, see user_namespaces(7).

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
       From kernel 2.5.27 to kernel 2.6.26, capabilities were an optional ker-
       nel component, and  could  be  enabled/disabled	via  the  CONFIG_SECU-
       RITY_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 capa-
       bilities were shown as being enabled (1) in these  sets.	  Since	 Linux
       3.8,  all  nonexistent  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 from kernel 2.6.24 to kernel 2.6.32 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_SETP-
       CAP 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
       capsh(1),     setpriv(1),    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),  proc(5),  creden-
       tials(7), pthreads(7), user_namespaces(7), getcap(8), setcap(8)

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

COLOPHON
       This page is part of release 4.10 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2016-12-12		       CAPABILITIES(7)