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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  unprivi-
       leged  processes  (whose effective UID is non-zero).  Privileged processes bypass all kernel permission checks,
       while unprivileged processes are subject to full permission checking based on the process's  credentials  (usu-
       ally: 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

   Capabilities List
       The following list shows the capabilities implemented on Linux, and the operations or behaviors that each capa-
       bility permits:

       CAP_AUDIT_CONTROL (since Linux 2.6.11)
              Enable and disable kernel auditing; change auditing filter rules; retrieve auditing status and filtering

       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
              * 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

              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)  KDSIGAC-
              CEPT 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 multicast-
              ing, 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 capa-
              bility 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

              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
              * 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 (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)

              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)).

              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 executed.

       Before kernel 2.6.24, only the first two of these requirements are met; since kernel 2.6.24, all three require-
       ments 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 re-acquire 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  conjunc-
       tion 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

       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.  There-
              fore, 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

   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  pro-
       gram,  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  sys-

   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 the capability is not in the
         bounding set, then a thread can't add one of its permitted capabilities to its inheritable  set  and  thereby
         have  that 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-pro-
       cess 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 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  can-
       not  be restored to that set.  A thread can determine if a capability is in its bounding set using the prctl(2)
       PR_CAPBSET_READ operation.

       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 capabil-

       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 non-zero 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 non-zero value, then all capabilities are cleared  from  the  permitted  and
          effective capability sets.

       2. If the effective user ID is changed from 0 to non-zero, then all capabilities are cleared from the effective

       3. If the effective user ID is changed from non-zero to 0, then the permitted set is copied  to  the  effective

       4. If  the  file system user ID is changed from 0 to non-zero (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,  and  CAP_MAC_OVERRIDE.  If the file system UID is changed from non-zero 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  non-zero 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 pur-
       pose.  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 com-
          bination 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 inherita-
          ble 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 non-zero 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  non-zero 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  exe-
              cuted,  or when a process with an effective or real UID of 0 calls execve(2).  (See the subsection Capa-
              bilities and execution of programs by root.)

       Each of the above "base" flags has a companion "locked" flag.  Setting any  of  the  "locked"  flags  is  irre-
       versible,  and has the effect of preventing further changes to the corresponding "base" flag.  The locked flags

       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 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 capabilities:

                   1 << SECURE_KEEP_CAPS_LOCKED |
                   1 << SECURE_NO_SETUID_FIXUP |
                   1 << SECURE_NO_SETUID_FIXUP_LOCKED |
                   1 << SECURE_NOROOT |
                   1 << SECURE_NOROOT_LOCKED);

       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  CON-
       FIG_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_SETP-
       CAP  capability  can manipulate the capabilities of threads other than itself.  However, this is only theoreti-
       cally 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

       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at

Linux                             2008-11-27                   CAPABILITIES(7)