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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

              * 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).

              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 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).

              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

              * 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 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);
              * call setns(2);
              * perform KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2) operations;
              * perform madvise(2) MADV_HWPOISON operation.

              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

              * 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),
              * set CPU affinity for arbitrary processes (sched_setaffinity(2));
              * set I/O scheduling class and priority for arbitrary processes (ioprio_set(2));
              * 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); apply get_robust_list(2) to arbitrary

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

              * 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)).
              * use  F_SETPIPE_SZ to increase the capacity of a pipe above the limit specified by

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

              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:

              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 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.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 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 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 /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!),  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 onward

       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.   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 configuration option has been removed and file capabilities are always
       part of the kernel.  When file capabilities are compiled into the kernel, 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_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

       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) PR_SET_SECUREBITS
       and PR_GET_SECUREBITS operations.  The CAP_SETPCAP capability is required  to  modify  the

       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 capabilities:

                   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 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 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), setcap(8)

       include/linux/capability.h in the kernel source


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       project, and information about reporting bugs, can be found at