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       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  nonzero).
       Privileged   processes  bypass  all  kernel  permission  checks,  while
       unprivileged processes are subject to full permission checking based on
       the  process’s  credentials (usually: effective UID, effective GID, and
       supplementary 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.

              Make arbitrary changes to file UIDs and GIDs (see chown(2)).

              Bypass file read, write, and execute permission checks.  (DAC is
              an abbreviation of "discretionary access control".)

              Bypass file  read  permission  checks  and  directory  read  and
              execute permission checks.

              * 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
              * 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).

              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.

              Lock memory (mlock(2), mlockall(2), mmap(2), shmctl(2)).

              Bypass permission checks for operations on System V IPC objects.

              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)).

              Set  the  FS_APPEND_FL  and  FS_IMMUTABLE_FL  i-node  flags (see

       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).

              Perform  various  network-related  operations   (e.g.,   setting
              privileged  socket  options,  enabling  multicasting,  interface
              configuration, modifying routing tables).

              Bind a socket to Internet domain privileged ports (port  numbers
              less than 1024).

              (Unused)  Make socket broadcasts, and listen to multicasts.

              Use RAW and PACKET sockets.

              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.

              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
              available  when  the  kernel  is  configured  to  support   file
              capabilities, since CAP_SETPCAP has entirely different semantics
              for such kernels.)

              If file capabilities are supported: add any capability from  the
              calling  thread’s  bounding  set  to  its  inheritable set; drop
              capabilities   from   the    bounding    set    (via    prctl(2)
              PR_CAPBSET_DROP); make changes to the securebits flags.

              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.

              * Perform a range of system administration operations including:
                quotactl(2),  mount(2),  umount(2),   swapon(2),   swapoff(2),
                sethostname(2), and setdomainname(2);
              * 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_NEWNS flag with clone(2) and unshare(2);
              * perform KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2)  operations.

              Use reboot(2) and kexec_load(2).

              Use chroot(2).

              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.

              * 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
              * set  I/O scheduling class and priority for arbitrary processes
              * apply  migrate_pages(2)  to  arbitrary  processes  and   allow
                processes 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).

              Use acct(2).

              Trace arbitrary processes using ptrace(2)

              Perform I/O port  operations  (iopl(2)  and  ioperm(2));  access

              * 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)).

              Set  system  clock (settimeofday(2), stime(2), adjtimex(2)); set
              real-time (hardware) clock.

              Use vhangup(2).

   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

       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:

              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).

              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).

              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
       during execve(2).

       Using capset(2), a thread may manipulate its own capability  sets  (see

   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.capability.  Writing to this extended attribute  requires  the
       CAP_SETFCAP  capability.  The file capability sets, in conjunction with
       the capability 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).

              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
              capabilities 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
              capabilities   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]


           P         denotes  the  value of a thread capability set before the

           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

   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
       transformations  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
       provided 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
         inheritable 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

       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!),
       modify 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
       capability  is  in  its bounding set using the prctl(2) PR_CAPBSET_READ

       Removing capabilities from the bounding set is only supported  if  file
       capabilities       are       compiled       into       the       kernel
       (CONFIG_SECURITY_FILE_CAPABILITIES).  In that case,  the  init  process
       (the  ancestor  of  all processes) begins with a full bounding set.  If
       file capabilities are not compiled into the kernel,  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
          previously 0, and as a result of the UID changes all  of  these  IDs
          have  a  nonzero  value,  then all capabilities are cleared from the
          permitted 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
          permitted set is copied to the effective set.

       4. If the file system user  ID  is  changed  from  0  to  nonzero  (see
          setfsuid(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 2.2.30),
          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
          capabilities 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
       package,  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.

   The "securebits" flags: establishing a capabilities-only environment
       Starting  with  kernel  2.6.26,  and  with  a  kernel  in  which   file
       capabilities   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:

              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
              functionality as the older prctl(2) PR_SET_KEEPCAPS  operation.)

              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.)

              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
       preventing  further  changes  to  the  corresponding  "base" flag.  The
       locked          flags           are:           SECBIT_KEEP_CAPS_LOCKED,

       The  securebits  flags can be modified and retrieved using the prctl(2)
       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

                   SECBIT_KEEP_CAPS_LOCKED |
                   SECBIT_NO_SETUID_FIXUP |
                   SECBIT_NO_SETUID_FIXUP_LOCKED |
                   SECBIT_NOROOT |


       No  standards   govern   capabilities,   but   the   Linux   capability
       implementation  is  based on the withdrawn POSIX.1e draft standard; see


       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.

       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

       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

       * 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
         processes created on the system.


       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),

       include/linux/capability.h in the kernel source


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