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  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.

       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.

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

       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.

       CAP_SETGID
              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.

       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
              * 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.

       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)

       CAP_SYS_RAWIO
              Perform I/O port  operations  (iopl(2)  and  ioperm(2));  access
              /proc/kcore.

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

          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
              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).

       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).

   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:
              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]

       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
          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
         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 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-
       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-
          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 non-zero to 0, then any of these capabil-
          ities  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 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.
              cleared on an execve(2).  (This flag provides the same function-
              ality as the older prctl(2) PR_SET_KEEPCAPS operation.)

       SECURE_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 non-zero values.  (See the subsection
              Effect of User ID Changes on Capabilities.)

       SECURE_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: SECURE_KEEP_CAPS_LOCKED, SECURE_NO_SETUID_FIXUP_LOCKED,  and
       SECURE_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
       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-
       ties:

           prctl(PR_SET_SECUREBITS,
                   1 << SECURE_KEEP_CAPS_LOCKED |
                   1 << SECURE_NO_SETUID_FIXUP |
                   1 << SECURE_NO_SETUID_FIXUP_LOCKED |
                   1 << SECURE_NOROOT |
                   1 << SECURE_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.xpilot.org/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.

       * 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),  credentials(7),  pthreads(7), getcap(8), set-
       cap(8)

       include/linux/capability.h in the kernel source

COLOPHON
       This page is part of release 3.23 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                             2009-08-03                   CAPABILITIES(7)
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