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_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
              * 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_NEWNS flag with clone(2) and unshare(2);
              * call setns(2);
              * perform KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2) operations;
              * perform madvise(2) MADV_HWPOISON operation.

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

       CAP_SYS_CHROOT
              Use chroot(2).

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

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

       CAP_SYS_PACCT
              Use acct(2).

       CAP_SYS_PTRACE
              Trace     arbitrary    processes    using    ptrace(2);    apply
              get_robust_list(2) to arbitrary processes.

       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;

       CAP_SYS_TTY_CONFIG
              Use vhangup(2).

       CAP_SYSLOG (since Linux 2.6.37)
              Perform privileged  syslog(2)  operations.   See  syslog(2)  for
              information on which operations require privilege.

   Past and Current Implementation
       A full implementation of capabilities requires that:

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

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

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

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

   Thread Capability Sets
       Each thread has three capability sets containing zero or  more  of  the
       above capabilities:

       Permitted:
              This  is a limiting superset for the effective capabilities that
              the thread may assume.  It is also a limiting superset  for  the
              capabilities  that  may  be  added  to  the inheritable set by a
              thread that does not have  the  CAP_SETPCAP  capability  in  its
              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).
       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
       an execve(2):
       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
       tion.

       Removing  capabilities  from the bounding set is only supported 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 file system user IDs (using setuid(2), setresuid(2), or similar):

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

       2. If  the  effective  user  ID  is changed from 0 to nonzero, then all
          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  file system user ID is changed from 0 to nonzero (see setf-
          suid(2)) then the following capabilities are cleared from the effec-
          tive    set:   CAP_CHOWN,   CAP_DAC_OVERRIDE,   CAP_DAC_READ_SEARCH,
          CAP_FOWNER, CAP_FSETID, CAP_LINUX_IMMUTABLE  (since  Linux  2.2.30),
          CAP_MAC_OVERRIDE,  and  CAP_MKNOD (since Linux 2.2.30).  If the file
          system UID is changed from nonzero to 0, then any of these capabili-
          ties that are enabled in the permitted set are enabled in the effec-
          tive set.

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

   Programmatically adjusting capability sets
       A  thread  can  retrieve  and  change  its  capability  sets  using the
       capget(2)  and  capset(2)  system   calls.    However,   the   use   of
       cap_get_proc(3)  and cap_set_proc(3), both provided in the libcap pack-
       age, is preferred for this purpose.  The following rules govern changes
          thread does not currently have).

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

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

       SECBIT_KEEP_CAPS
              Setting this flag allows a thread that has one or more 0 UIDs to
              retain  its  capabilities  when it switches all of its UIDs to a
              nonzero value.  If this flag is not set, then such a UID  switch
              causes the thread to lose all capabilities.  This flag is always
              cleared on an execve(2).  (This flag provides the same function-
              ality as the older prctl(2) PR_SET_KEEPCAPS operation.)

       SECBIT_NO_SETUID_FIXUP
              Setting  this  flag  stops  the kernel from adjusting capability
              sets when the threads's  effective  and  file  system  UIDs  are
              switched  between  zero and nonzero values.  (See the subsection
              Effect of User ID Changes on Capabilities.)

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

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

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

       The  securebits  flags  are  inherited  by  child processes.  During an
       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 |
                   SECBIT_NOROOT |

       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.

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

       include/linux/capability.h in the kernel source

COLOPHON
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       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.



Linux                             2011-10-04                   CAPABILITIES(7)
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