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

       capabilities - overview of Linux capabilities

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

       Starting with kernel 2.2, Linux provides an (as yet incomplete)  system
       of  capabilities,  which divide the privileges traditionally associated
       with superuser into distinct units(1,7) that can  be  independently  enabled
       and disabled.

   Capabilities List
       As at Linux 2.6.6, the following capabilities are implemented:

              Allow arbitrary changes to file(1,n) UIDs and GIDs (see chown(1,2)(2)).

              Bypass  file(1,n) read(2,n,1 builtins), write(1,2), and execute permission checks.  (DAC =
              "discretionary access(2,5) control".)

              Bypass file(1,n) read(2,n,1 builtins) permission checks and directory read(2,n,1 builtins)  and  exe-
              cute permission checks.

              Bypass permission checks on operations that normally require the
              file(1,n) system UID of the process to match  the  UID  of  the  file(1,n)
              (e.g.,  chmod(1,2)(2),  utime(2)), excluding those operations covered
              by the CAP_DAC_OVERRIDE and  CAP_DAC_READ_SEARCH;  set(7,n,1 builtins)  extended
              file(1,n)  attributes  (see chattr(1)) on arbitrary files; set(7,n,1 builtins) Access
              Control Lists (ACLs) on arbitrary files; ignore directory sticky
              bit on file(1,n) deletion.

              Don't  clear(1,3x,3x clrtobot)  set-user-ID  and  set-group-ID bits when a file(1,n) is
              modified; permit setting of the  set-group-ID  bit  for  a  file(1,n)
              whose  GID  does not match the file(1,n) system or any of the supple-
              mentary GIDs of the calling process.

              Permit memory  locking  (mlock(2),  mlockall(2),  mmap(2),  shm-

              Bypass permission checks for operations on System V IPC objects.

              Bypass permission checks  for  sending  signals  (see  kill(1,2,1 builtins)(2)).
              This includes use of the KDSIGACCEPT ioctl.

              (Linux  2.4  onwards)   Allow  file(1,n)  leases to be established on
              arbitrary files (see fcntl(2)).

              Allow  setting  of  the  EXT2_APPEND_FL  and   EXT2_IMMUTABLE_FL
              extended file(1,n) attributes (see chattr(1)).

              (Linux  2.4  onwards)  Allow  creation  of  special  files using

              Allow various network-related operations (e.g.,  setting  privi-
              leged  socket(2,7,n) options, enabling multicasting, interface configu-
              ration, modifying routing tables).

              Allow binding to Internet domain  reserved  socket(2,7,n)  ports  (port
              numbers less(1,3) than 1024).

              (Unused)  Allow socket(2,7,n) broadcasting, and listening multicasts.

              Permit use of RAW and PACKET sockets.

              Allow  arbitrary manipulations of process GIDs and supplementary
              GID list; allow forged GID when passing socket(2,7,n)  credentials  via
              Unix domain sockets.

              Grant  or  remove any capability in(1,8) the caller's permitted capa-
              bility set(7,n,1 builtins) to or from any other process.

              Allow  arbitrary  manipulations  of  process  UIDs   (setuid(2),
              setreuid(2),  setresuid(2),  setfsuid(2)); allow forged UID when
              passing socket(2,7,n) credentials via Unix domain sockets.

              Permit a range of system  administration  operations  including:
              quotactl(2),   mount(2,8)(2),   umount(2),   swapon(2,8)(2),   swapoff(2),
              sethostname(2), setdomainname(2), IPC_SET  and  IPC_RMID  opera-
              tions  on  arbitrary System V IPC objects; perform operations on
              trusted and security Extended Attributes  (see  attr(1,5)(5));  allow
              forged    UID    when   passing   socket(2,7,n)   credentials;   exceed
              /proc(5,n)/sys/fs/file-max limit in(1,8)  system  calls  that  open(2,3,n)  files
              (e.g., accept(2,8)(2), execve(2), open(2,3,n)(2), pipe(2,8)(2))

              Permit calls to reboot(2).

              Permit calls to chroot(1,2)(2).

              Allow  loading  and unloading of kernel modules; allow modifica-
              tions  to  capability  bounding  set(7,n,1 builtins)  (see  init_module(2)   and

              Allow  raising  process nice(1,2) value (nice(1,2)(2), setpriority(2)) and
              changing of the nice(1,2) value for arbitrary processes;  allow  set-
              ting  of  real-time scheduling policies for calling process, and
              setting scheduling policies and priorities  for  arbitrary  pro-
              cesses   (sched_setscheduler(2),   sched_setparam(2));  set(7,n,1 builtins)  CPU
              affinity for arbitrary processes (sched_setaffinity()).

              Permit calls to acct(2,5)(2).

              Allow arbitrary processes to be traced using ptrace(2)

              Permit I/O port operations (iopl(2) and ioperm(2)); access(2,5)

              Permit: use of reserved space on  ext2  file(1,n)  systems;  ioctl(2)
              calls controlling ext3 journaling; disk quota(1,8) limits to be over-
              ridden; resource limits  to  be  increased  (see  setrlimit(2));
              RLIMIT_NPROC  resource  limit to be overridden; msg_qbytes limit
              for  a  message  queue(1,3)  to  be  raised  above   the   limit   in(1,8)
              /proc(5,n)/sys/kernel/msgmnb (see msgop(2) and msgctl(2).

              Allow  modification  of system clock(3,n) (settimeofday(2), stime(2),
              adjtimex(2)); allow modification of real-time (hardware) clock(3,n)

              Permit calls to vhangup(2).

   Process Capabilities
       Each process has three capability sets containing zero or more  of  the
       above capabilities:

              the capabilities used by the kernel to perform permission checks
              for the process.

              the capabilities that the process may assume (i.e.,  a  limiting
              superset  for the effective and inheritable sets).  If a process
              drops a capability from its permitted  set(7,n,1 builtins),  it  can  never  re-
              acquire  that  capability  (unless  it execs a set-UID-root pro-

              the capabilities preserved across an execve(2).

       In the current implementation, a process is granted all  permitted  and
       effective  capabilities  (subject  to  the  operation of the capability
       bounding set(7,n,1 builtins) described below) when it execs a set-UID-root program,  or
       if(3,n) a process with a real UID of zero execs a new program.

       A  child created via fork(2) inherits copies of its parent's capability

       Using capset(2), a process may manipulate its own capability sets,  or,
       if(3,n) it has the CAP_SETPCAP capability, those of another process.

   Capability bounding set(7,n,1 builtins)
       When  a  program is execed, the permitted and  effective capabities are
       ANDed with the current value of the so-called capability bounding  set(7,n,1 builtins),
       defined  in(1,8) the file(1,n) /proc(5,n)/sys/kernel/cap-bound.  This parameter can be
       used to place a system-wide limit on the capabilities  granted  to  all
       subsequently  executed programs.  (Confusingly, this bit mask parameter
       is expressed as a signed decimal number in(1,8) /proc(5,n)/sys/kernel/cap-bound.)

       Only  the  init  process  may  set(7,n,1 builtins) bits in(1,8) the capability bounding set(7,n,1 builtins);
       other than that, the superuser may only clear(1,3x,3x clrtobot) bits in(1,8) this set.

       On a standard system the capability bounding set(7,n,1 builtins) always masks  out  the
       CAP_SETPCAP capability.  To remove this restriction, modify the defini-
       tion of CAP_INIT_EFF_SET in(1,8) include/linux/capability.h and rebuild  the

   Current and Future Implementation
       A full implementation of capabilities requires:

       1.  that  for  all  privileged operations, the kernel check whether the
           process has the required capability in(1,8) its effective set.

       2.  that the kernel provide system calls allowing a process's  capabil-
           ity sets to be changed and retrieved.

       3.  file(1,n)  system  support  for  attaching capabilities to an executable
           file(1,n), so that a process gains those capabilities when the  file(1,n)  is

       As at Linux 2.6.6, only the first two of these requirements are met.

       Eventually,  it  should  be possible to associate three capability sets
       with an executable file(1,n), which, in(1,8) conjunction with the capability sets
       of  the  process, will determine the capabilities of a process after an
       exec(3,n,1 builtins):

              this set(7,n,1 builtins) is ANDed with the process's inherited set(7,n,1 builtins) to  determine
              which  inherited capabilities are permitted to the process after
              the exec.

              the capabilities automatically permitted to the process, regard-
              less(1,3) of the process's inherited capabilities.

              those  capabilities  in(1,8) the process's new permitted set(7,n,1 builtins) are also
              to be set(7,n,1 builtins) in(1,8) the new effective set.   (F(effective)  would  nor-
              mally be either all zeroes or all ones.)

       In the meantime, since the current implementation does not support file(1,n)
       capability sets, during an exec:

       1.  All three file(1,n) capability sets are initially assumed to be cleared.

       2.  If  a  set-UID-root program is being execed, or the real user ID of
           the process is 0 (root) then the file(1,n) allowed and forced  sets  are
           defined to be all ones (i.e., all capabilities set(7,n,1 builtins)).

       3.  If  a  set-UID-root program is being executed, then the file(1,n) effec-
           tive set(7,n,1 builtins) is defined to be all ones.

       During an exec(3,n,1 builtins), the kernel  calculates  the  new  capabilities  of  the
       process using the following algorithm:

           P'(permitted) = (P(inherited) & F(allowed)) | (F(forced) & cap_bset)

           P'(effective) = P'(permitted) & F(effective)

           P'(inherited) = P(inherited)    [i.e., unchanged]


       P         denotes the value of a process capability set(7,n,1 builtins) before the exec(3,n,1 builtins)

       P'        denotes the value of a capability set(7,n,1 builtins) after the exec(3,n,1 builtins)

       F         denotes a file(1,n) capability set(7,n,1 builtins)

       cap_bset  is the value of the capability bounding set.

       The libcap package provides a suite of routines for setting and getting
       process capabilities that is more comfortable and less(1,3) likely to change
       than the interface provided by capset(2) and capget(2).

       No standards govern capabilities, but the Linux capability  implementa-
       tion is based on the withdrawn POSIX 1003.1e draft standard.

       There  is  as  yet  no  file(1,n) system support allowing capabilities to be
       associated with executable files.

       capget(2), prctl(2)

Linux 2.6.6                       2004-05-27                   CAPABILITIES(7)

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