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_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 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  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_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_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.

       CAP_SETFCAP (since Linux 2.6.24)
              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));  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
                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 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.

              * 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;  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));
              * 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

          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.

   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).  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
       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

       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'(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
       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.

       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 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
          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.

   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-
          (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  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.)

       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
       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_LOCKED |
                   SECBIT_NO_SETUID_FIXUP |
                   SECBIT_NO_SETUID_FIXUP_LOCKED |

       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 3.54 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)
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