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


     capabilities - overview of Linux capabilities


     For  the  purpose  of  performing  permission  checks, traditional UNIX
     implementations distinguish two  categories  of  processes:  privileged
     processes  (whose  effective  user ID is 0, referred to as superuser or
     root), and unprivileged processes (whose  effective  UID  is  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_READ (since Linux 3.16)
            Allow reading the audit log via a multicast netlink socket.
     CAP_AUDIT_WRITE (since Linux 2.6.11)
            Write records to kernel auditing log.
     CAP_BLOCK_SUSPEND (since Linux 3.5)
            Employ  features  that can block system suspend (epoll(7) EPOLL-
            WAKEUP, /proc/sys/wake_lock).
            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 exe-
              cute permission checks;
            * invoke open_by_handle_at(2);
            * use the linkat(2) AT_EMPTY_PATH flag to create  a  link  to  a
              file referred to by a file descriptor.
            * Bypass  permission  checks on operations that normally require
              the filesystem UID of the process to match the UID of the file
              (e.g., chmod(2), utime(2)), excluding those operations covered
            * set inode flags (see ioctl_iflags(2)) 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 mode bits when a file
              is modified;
            * set the set-group-ID bit for a file whose GID does  not  match
              the filesystem 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) 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  inode  flags  (see
     CAP_MAC_ADMIN (since Linux 2.6.25)
            Allow  MAC  configuration or state changes.  Implemented for the
            Smack Linux Security Module (LSM).
     CAP_MAC_OVERRIDE (since Linux 2.6.25)
            Override Mandatory Access Control (MAC).   Implemented  for  the
            Smack LSM.
     CAP_MKNOD (since Linux 2.4)
            Create special files using mknod(2).
            Perform various network-related operations:
            * interface configuration;
            * administration of IP firewall, masquerading, and accounting;
            * modify routing tables;
            * bind to any address for transparent proxying;
            * set type-of-service (TOS)
            * clear driver statistics;
            * set promiscuous mode;
            * enabling multicasting;
            * use   setsockopt(2)  to  set  the  following  socket  options:
              SO_DEBUG, SO_MARK, SO_PRIORITY (for  a  priority  outside  the
              range 0 to 6), SO_RCVBUFFORCE, and SO_SNDBUFFORCE.
            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;
            * bind to any address for transparent proxying.
            * Make arbitrary manipulations of process GIDs and supplementary
              GID list;
            * forge  GID  when  passing  socket  credentials via UNIX domain
            * write a group ID mapping in a user namespace (see  user_names-
     CAP_SETFCAP (since Linux 2.6.24)
            Set arbitrary capabilities on a file.
            If  file  capabilities are supported (i.e., since Linux 2.6.24):
            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.
            If  file  capabilities  are  not supported (i.e., kernels before
            Linux 2.6.24): 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 con-
            figured  to  support  file  capabilities,  since CAP_SETPCAP has
            entirely different semantics for such kernels.)
            * Make  arbitrary  manipulations  of  process  UIDs  (setuid(2),
              setreuid(2), setresuid(2), setfsuid(2));
            * forge  UID  when  passing  socket  credentials via UNIX domain
            * write a user ID mapping in a user namespace  (see  user_names-
            Note:  this capability is overloaded; see Notes to kernel devel-
            opers, below.
  • 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 VM86_REQUEST_IRQ vm86(2) command;
            * perform  IPC_SET and IPC_RMID operations on arbitrary System V
              IPC objects;
            * override RLIMIT_NPROC resource limit;
            * perform operations on trusted and security Extended Attributes
              (see xattr(7));
            * 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 PID when passing  socket  credentials  via  UNIX  domain
            * 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_* flags that create new namespaces with clone(2)
              and unshare(2) (but, since Linux 3.8, creating user namespaces
              does not require any capability);
            * call perf_event_open(2);
            * access privileged perf event information;
            * call  setns(2)  (requires  CAP_SYS_ADMIN  in the target names-
            * call fanotify_init(2);
            * call bpf(2);
            * perform privileged KEYCTL_CHOWN and  KEYCTL_SETPERM  keyctl(2)
            * perform madvise(2) MADV_HWPOISON operation;
            * employ  the  TIOCSTI  ioctl(2)  to  insert characters into the
              input queue of a terminal other than the caller's  controlling
            * employ the obsolete nfsservctl(2) system call;
            * employ the obsolete bdflush(2) system call;
            * perform various privileged block-device ioctl(2) operations;
            * perform various privileged filesystem ioctl(2) operations;
            * perform  privileged  ioctl(2)  operations  on  the /dev/random
              device (see random(4));
            * install a seccomp(2) filter without first having  to  set  the
              no_new_privs thread attribute;
            * modify allow/deny rules for device control groups;
            * employ  the  ptrace(2)  PTRACE_SECCOMP_GET_FILTER operation to
              dump tracee's seccomp filters;
            * employ the ptrace(2) PTRACE_SETOPTIONS  operation  to  suspend
              the  tracee's  seccomp  protections  (i.e.,  the PTRACE_O_SUS-
              PEND_SECCOMP flag);
            * perform administrative operations on many device drivers.
            Use reboot(2) and kexec_load(2).
            Use chroot(2).
            * Load  and  unload  kernel  modules  (see  init_module(2)   and
            * 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), shed_setattr(2));
            * set  CPU  affinity  for  arbitrary  processes (sched_setaffin-
            * set I/O scheduling class and priority for arbitrary  processes
            * 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).
            Use acct(2).
            * Trace arbitrary processes using ptrace(2);
            * apply get_robust_list(2) to arbitrary processes;
            * transfer  data  to  or  from the memory of arbitrary processes
              using process_vm_readv(2) and process_vm_writev(2);
            * inspect processes using kcmp(2).
            * Perform I/O port operations (iopl(2) and ioperm(2));
            * access /proc/kcore;
            * employ the FIBMAP ioctl(2) operation;
            * open devices for accessing x86 model-specific registers (MSRs,
              see msr(4));
            * update /proc/sys/vm/mmap_min_addr;
            * create  memory mappings at addresses below the value specified
              by /proc/sys/vm/mmap_min_addr;
            * map files in /proc/bus/pci;
            * open /dev/mem and /dev/kmem;
            * perform various SCSI device commands;
            * perform certain operations on hpsa(4) and cciss(4) devices;
            * perform  a  range  of  device-specific  operations  on   other
            * Use reserved space on ext2 filesystems;
            * make ioctl(2) calls controlling ext3 journaling;
            * override disk quota limits;
            * increase resource limits (see setrlimit(2));
            * override RLIMIT_NPROC resource limit;
            * override maximum number of consoles on console allocation;
            * override maximum number of keymaps;
            * allow more than 64hz interrupts from the real-time clock;
            * raise  msg_qbytes limit for a System V message queue above the
              limit in /proc/sys/kernel/msgmnb (see msgop(2) and msgctl(2));
            * allow  the  RLIMIT_NOFILE resource limit on the number of "in-
              flight" file descriptors to  be  bypassed  when  passing  file
              descriptors  to  another process via a UNIX domain socket (see
            * override the /proc/sys/fs/pipe-size-max limit when setting the
              capacity of a pipe using the F_SETPIPE_SZ fcntl(2) command.
            * use  F_SETPIPE_SZ to increase the capacity of a pipe above the
              limit specified by /proc/sys/fs/pipe-max-size;
            * override /proc/sys/fs/mqueue/queues_max  limit  when  creating
              POSIX message queues (see mq_overview(7));
            * employ the prctl(2) PR_SET_MM operation;
            * set  /proc/[pid]/oom_score_adj to a value lower than the value
              last set by a process with CAP_SYS_RESOURCE.
            Set system clock (settimeofday(2), stime(2),  adjtimex(2));  set
            real-time (hardware) clock.
            Use vhangup(2); employ various privileged ioctl(2) operations on
            virtual terminals.
     CAP_SYSLOG (since Linux 2.6.37)
            * Perform privileged syslog(2) operations.   See  syslog(2)  for
              information on which operations require privilege.
            * View  kernel  addresses exposed via /proc and other interfaces
              when /proc/sys/kernel/kptr_restrict has the value 1.  (See the
              discussion of the kptr_restrict in proc(5).)
     CAP_WAKE_ALARM (since Linux 3.0)
            Trigger  something that will wake up the system (set CLOCK_REAL-
            TIME_ALARM and CLOCK_BOOTTIME_ALARM timers).
 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 filesystem 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.
 Notes to kernel developers
     When  adding a new kernel feature that should be governed by a capabil-
     ity, consider the following points.
  • The goal of capabilities is divide the power of superuser into

pieces, such that if a program that has one or more capabilities is

        compromised, its power to do damage to the system would be less than
        the same program running with root privilege.
  • You have the choice of either creating a new capability for your new

feature, or associating the feature with one of the existing capa-

        bilities.   In order to keep the set of capabilities to a manageable
        size, the latter option is preferable, unless there  are  compelling
        reasons  to  take  the  former  option.   (There is also a technical
        limit: the size of capability sets is currently limited to 64 bits.)
  • To determine which existing capability might best be associated with

your new feature, review the list of capabilities above in order to

        find  a  "silo" into which your new feature best fits.  One approach
        to take is to determine if there are other features requiring  capa-
        bilities that will always be use along with the new feature.  If the
        new feature is useless without these other features, you should  use
        the same capability as the other features.
  • Don't choose CAP_SYS_ADMIN if you can possibly avoid it! A vast

proportion of existing capability checks are associated with this

        capability (see the partial list above).  It can plausibly be called
        "the new root", since on the one hand, it confers a  wide  range  of
        powers,  and  on  the other hand, its broad scope means that this is
        the capability that is required by many privileged programs.   Don't
        make  the problem worse.  The only new features that should be asso-
        ciated with CAP_SYS_ADMIN are ones that closely match existing  uses
        in that silo.
  • If you have determined that it really is necessary to create a new

capability for your feature, don't make or name it as a "single-use"

        capability.   Thus, for example, the addition of the highly specific
        CAP_SYS_PACCT was probably a mistake.  Instead, try to identify  and
        name  your new capability as a broader silo into which other related
        future use cases might fit.
 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).
            Inheritable  capabilities  remain inheritable when executing any
            program, and inheritable capabilities are added to the permitted
            set when executing a program that has the corresponding bits set
            in the file inheritable set.
            Because inheritable capabilities  are  not  generally  preserved
            across  execve(2)  when running as a non-root user, applications
            that wish to run  helper  programs  with  elevated  capabilities
            should consider using ambient capabilities, described below.
            This  is  the  set of capabilities used by the kernel to perform
            permission checks for the thread.
     Ambient (since Linux 4.3):
            This is a set of  capabilities  that  are  preserved  across  an
            execve(2)  of  a  program  that  is not privileged.  The ambient
            capability set obeys the invariant that no capability  can  ever
            be ambient if it is not both permitted and inheritable.
            The  ambient  capability  set  can  be  directly  modified using
            prctl(2).  Ambient capabilities  are  automatically  lowered  if
            either  of  the corresponding permitted or inheritable capabili-
            ties is lowered.
            Executing a program that changes UID or GID due to the set-user-
            ID or set-group-ID bits or executing a program that has any file
            capabilities set will clear the ambient set.  Ambient  capabili-
            ties  are  added to the permitted set and assigned to the effec-
            tive set when execve(2) is called.
     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
     Since  Linux  3.2,  the  file /proc/sys/kernel/cap_last_cap exposes the
     numerical value of the highest capability supported by the running ker-
     nel; this can be used to determine the highest bit that may be set in a
     capability set.
 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)  and  xattr(7))  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 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.
 File capability mask versioning
     To  allow  extensibility, the kernel supports a scheme to encode a ver-
     sion number inside the security.capability extended attribute  that  is
     used  to implement file capabilities.  These version numbers are inter-
     nal to the implementation,  and  not  directly  visible  to  user-space
     applications.  To date, the following versions are supported:
            This was the original file capability implementation, which sup-
            ported 32-bit masks for file capabilities.
     VFS_CAP_REVISION_2 (since Linux 2.6.25)
            This version allows for file capability masks that are  64  bits
            in  size, and was necessary as the number of supported capabili-
            ties grew beyond 32.  The kernel transparently continues to sup-
            port  the execution of files that have 32-bit version 1 capabil-
            ity masks, but when adding capabilities to files  that  did  not
            previously  have  capabilities, or modifying the capabilities of
            existing files, it automatically uses the version 2  scheme  (or
            possibly the version 3 scheme, as described below).
     VFS_CAP_REVISION_3 (since Linux 4.14)
            Version  3  file capabilities are provided to support namespaced
            file capabilities (described below).
            As with version 2 file capabilities, version 3 capability  masks
            are  64  bits  in  size.   But  in addition, the root user ID of
            namespace  is  encoded  in  the   security.capability   extended
            attribute.   (A  namespace's root user ID is the value that user
            ID 0 inside that namespace maps to in the  initial  user  names-
            Version 3 file capabilities are designed to coexist with version
            2 capabilities; that is, on a modern Linux system, there may  be
            some files with version 2 capabilities while others have version
            3 capabilities.
     Before Linux 4.14, the only kind  of  capability  mask  that  could  be
     attached  to  a  file was a VFS_CAP_REVISION_2 mask.  Since Linux 4.14,
     the version of the capability mask that is attached to a  file  depends
     on   the   circumstances  in  which  the  security.capability  extended
     attribute was created.
     Starting with Linux 4.14, a security.capability extended  attribute  is
     automatically  created  as (or converted to) a version 3 (VFS_CAP_REVI-
     SION_3) attribute if both of the following are true:
     (1) The thread writing the attribute resides in a noninitial namespace.
         (More  precisely: the thread resides in a user namespace other than
         the one from which the underlying filesystem was mounted.)
     (2) The thread has the CAP_SETFCAP  capability  over  the  file  inode,
         meaning  that  (a) the thread has the CAP_SETFCAP capability in its
         own user namespace; and (b) the UID and GID of the file inode  have
         mappings in the writer's user namespace.
     When  a  VFS_CAP_REVISION_3  security.capability  extended attribute is
     created, the root user ID of the creating thread's  user  namespace  is
     saved in the extended attribute.
     By  contrast,  creating a security.capability extended attribute from a
     privileged (CAP_SETFCAP) thread that resides in the namespace where the
     underlying filesystem was mounted (this normally means the initial user
     namespace) automatically results in a  version  2  (VFS_CAP_REVISION_2)
     Note  that  a  file  can  have  either a version 2 or a version 3 secu-
     rity.capability extended attribute associated with it,  but  not  both:
     creation  or modification of the security.capability extended attribute
     will automatically modify the version according to the circumstances in
     which the extended attribute is created or modified.
 Transformation of capabilities during execve()
     During  an execve(2), the kernel calculates the new capabilities of the
     process using the following algorithm:
         P'(ambient)     = (file is privileged) ? 0 : P(ambient)
         P'(permitted)   = (P(inheritable) & F(inheritable)) |
                           (F(permitted) & cap_bset) | P'(ambient)
         P'(effective)   = F(effective) ? P'(permitted) : P'(ambient)
         P'(inheritable) = P(inheritable)    [i.e., unchanged]
         P         denotes the value of a thread capability set  before  the
         P'        denotes  the  value  of a thread capability set after the
         F         denotes a file capability set
         cap_bset  is the value of the capability  bounding  set  (described
     A  privileged  file is one that has capabilities or has the set-user-ID
     or set-group-ID bit set.
     Note: the capability transitions described above may not  be  performed
     (i.e.,  file capabilities may be ignored) for the same reasons that the
     set-user-ID and set-group-ID bits are ignored; see execve(2).
     Note: according to the rules above, if a process with nonzero user  IDs
     performs  an  execve(2)  then  any capabilities that are present in its
     permitted and effective sets will be cleared.   For  the  treatment  of
     capabilities  when  a  process  with  a  user  ID  of  zero performs an
     execve(2), see below under Capabilities and execution  of  programs  by
 Safety checking for capability-dumb binaries
     A capability-dumb binary is an application that has been marked to have
     file capabilities, but has not been converted to use the libcap(3)  API
     to manipulate its capabilities.  (In other words, this is a traditional
     set-user-ID-root program that has been switched to use  file  capabili-
     ties, but whose code has not been modified to understand capabilities.)
     For such applications, the effective capability bit is set on the file,
     so  that  the  file permitted capabilities are automatically enabled in
     the process effective set when executing the file.  The  kernel  recog-
     nizes  a file which has the effective capability bit set as capability-
     dumb for the purpose of the check described here.
     When executing a capability-dumb  binary,  the  kernel  checks  if  the
     process  obtained all permitted capabilities that were specified in the
     file permitted set,  after  the  capability  transformations  described
     above  have  been  performed.   (The  typical reason why this might not
     occur is that the capability bounding set masked out some of the  capa-
     bilities in the file permitted set.)  If the process did not obtain the
     full set of file permitted capabilities, then execve(2) fails with  the
     error  EPERM.   This  prevents possible security risks that could arise
     when a capability-dumb application is executed with less privilege that
     it  needs.   Note that, by definition, the application could not itself
     recognize this problem, since it does not employ the libcap(3) API.
 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 or
        effective user ID of the process is 0 (root) then the file inherita-
        ble  and  permitted sets are defined to be all ones (i.e., all capa-
        bilities enabled).
     2. If a set-user-ID-root program is being executed,  or  the  effective
        user  ID  of  the process is 0 (root) then the file effective bit is
        defined to be one (enabled).
     The upshot of the above rules, combined with the capabilities transfor-
     mations described above, is as follows:
  • 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.
  • When a process with a real UID of 0 execve(2)s a program, it gains

all capabilities in its permitted capability set, except those

        masked out by the capability bounding set.
     The above steps yield semantics that are the same as those provided  by
     traditional UNIX systems.
 Set-user-ID-root programs that have file capabilities
     Executing a program that is both set-user-ID root and has file capabil-
     ities will cause the process to gain just the capabilities  granted  by
     the  program (i.e., not all capabilities, as would occur when executing
     a set-user-ID-root program that does not have any associated file capa-
     bilities).  Note that one can assign empty capability sets to a program
     file, and thus it is possible to create a set-user-ID-root program that
     changes  the  effective  and saved set-user-ID of the process that exe-
     cutes the program to 0, but confers no capabilities to that process.
 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
     Note  that  the bounding set masks the file permitted capabilities, but
     not the inheritable capabilities.  If a thread maintains  a  capability
     in  its  inheritable  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 inheritable 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: a process 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
     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  capa-
     bility is in its bounding set using the prctl(2) PR_CAPBSET_READ opera-
     Removing capabilities from the bounding set is supported only  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
     Removing a capability from the bounding set does not remove it from the
     thread's  inheritable set.  However it does prevent the capability from
     being added back into the thread's inheritable 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 filesystem 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, effective, and ambient 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 filesystem user ID is changed from 0 to  nonzero  (see  setf-
        suid(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 2.6.30),
        CAP_MAC_OVERRIDE,  and  CAP_MKNOD  (since  Linux  2.6.30).   If  the
        filesystem UID is changed from nonzero to 0, then any of these capa-
        bilities 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
     SECBIT_KEEP_CAPS securebits flag described below.
 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
     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 Linux 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  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:
            Setting this flag allows a thread that has one or more 0 UIDs to
            retain capabilities in its permitted and effective sets when  it
            switches all of its UIDs to nonzero values.  If this flag is not
            set, then such a UID switch causes the thread to lose all  capa-
            bilities  in  those  sets.   This  flag  is always cleared on an
            The setting of the  SECBIT_KEEP_CAPS  flag  is  ignored  if  the
            SECBIT_NO_SETUID_FIXUP flag is set.  (The latter flag provides a
            superset of the effect of the former flag.)
            This flag provides the same functionality as the older  prctl(2)
            PR_SET_KEEPCAPS operation.
            Setting  this flag stops the kernel from adjusting the process's
            permitted, effective,  and  ambient  capability  sets  when  the
            thread's effective and filesystem 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.)
            Setting this flag disallows raising ambient capabilities via the
            prctl(2) PR_CAP_AMBIENT_RAISE operation.
     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
     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  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-
         prctl(PR_SET_SECUREBITS,      /* SECBIT_KEEP_CAPS off */
                 SECBIT_KEEP_CAPS_LOCKED |
                 SECBIT_NO_SETUID_FIXUP |
                 SECBIT_NOROOT |
                 /* Setting/locking SECBIT_NO_CAP_AMBIENT_RAISE
                    is not required */
 Interaction with user namespaces
     For a discussion of the interaction of  capabilities  and  user  names-
     paces, see user_namespaces(7).
 Namespaced file capabilities
     Traditional (i.e., version 2) file capabilities associate only a set of
     capability masks with a binary executable file.  When  a  process  exe-
     cutes a binary with such capabilities, it gains the associated capabil-
     ities (within its user namespace) as per the rules described  above  in
     "Transformation of capabilities during execve()".
     Because  version 2 file capabilities confer capabilities to the execut-
     ing process regardless of which user  namespace  it  resides  in,  only
     privileged  processes  are  permitted  to associate capabilities with a
     file.  Here, "privileged" means a  process  that  has  the  CAP_SETFCAP
     capability in the user namespace where the filesystem was mounted (nor-
     mally the initial user namespace).  This limitation renders file  capa-
     bilities  useless  for  certain use cases.  For example, in user-names-
     paced containers, it can be desirable to be able  to  create  a  binary
     that  confers  capabilities only to processes executed inside that con-
     tainer, but not to processes that are executed outside the container.
     Linux 4.14 added so-called namespaced file capabilities to support such
     use  cases.   Namespaced  file  capabilities  are recorded as version 3
     (i.e.,  VFS_CAP_REVISION_3)  security.capability  extended  attributes.
     Such  an attribute is automatically created when a process that resides
     in a noninitial user namespace associates (setxattr(2)) file  capabili-
     ties  with  a  file whose user ID matches the user ID of the creator of
     the namespace.  In this case, the kernel records not just the  capabil-
     ity  masks  in the extended attribute, but also the namespace root user
     ID.  For further details, see File capability mask versioning, above.
     As with a binary  that  has  VFS_CAP_REVISION_2  file  capabilities,  a
     binary  with  VFS_CAP_REVISION_3 file capabilities confers capabilities
     to a process during execve().  However, capabilities are conferred only
     if the binary is executed by a process that resides in a user namespace
     whose UID 0 maps to the root user ID that  is  saved  in  the  extended
     attribute,  or when executed by a process that resides in descendant of
     such a namespace.


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


     From kernel 2.5.27 to kernel 2.6.26, capabilities were an optional ker-
     nel component, and  could  be  enabled/disabled  via  the  CONFIG_SECU-
     RITY_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.  Before Linux 3.8, nonexistent capa-
     bilities were shown as being enabled (1) in these  sets.   Since  Linux
     3.8,  all  nonexistent  capabilities  (above CAP_LAST_CAP) are shown as
     disabled (0).
     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 from kernel 2.6.24 to kernel 2.6.32 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_SETP-
     CAP 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  pro-
       cesses created on the system.


     capsh(1),     setpriv(1),    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),  libcap(3),  proc(5),  creden-
     tials(7), pthreads(7), user_namespaces(7), captest(8), filecap(8), get-
     cap(8), netcap(8), pscap(8), setcap(8)
     include/linux/capability.h in the Linux kernel source tree


     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at

Linux 2018-02-02 CAPABILITIES(7)

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