capabilities(7)



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

NAME
       capabilities - overview of Linux capabilities

DESCRIPTION
       For  the  purpose  of  performing  permission  checks, traditional UNIX
       implementations distinguish two  categories  of  processes:  privileged
       processes  (whose  effective  user ID is 0, referred to as superuser or
       root), and unprivileged processes (whose  effective  UID  is  nonzero).
       Privileged processes bypass all kernel permission checks, while unpriv-
       ileged processes are subject to full permission checking based  on  the
       process's  credentials (usually: effective UID, effective GID, and sup-
       plementary group list).

       Starting with kernel 2.2, Linux divides  the  privileges  traditionally
       associated  with  superuser into distinct units, known as capabilities,
       which can be independently enabled and disabled.   Capabilities  are  a
       per-thread attribute.

   Capabilities list
       The following list shows the capabilities implemented on Linux, and the
       operations or behaviors that each capability permits:

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

       CAP_AUDIT_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).

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

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

       CAP_DAC_READ_SEARCH
              * Bypass file read permission checks and directory read and exe-
                cute permission checks;
              * 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.

       CAP_FOWNER
              * 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
                by CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH;
              * 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).

       CAP_FSETID
              * 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
                process.

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

       CAP_IPC_OWNER
              Bypass permission checks for operations on System V IPC objects.

       CAP_KILL
              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)).

       CAP_LINUX_IMMUTABLE
              Set  the  FS_APPEND_FL  and  FS_IMMUTABLE_FL  inode  flags  (see
              ioctl_iflags(2)).

       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).

       CAP_NET_ADMIN
              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.

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

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

       CAP_NET_RAW
              * Use RAW and PACKET sockets;
              * bind to any address for transparent proxying.

       CAP_SETGID
              * Make arbitrary manipulations of process GIDs and supplementary
                GID list;
              * forge  GID  when  passing  socket  credentials via UNIX domain
                sockets;
              * write a group ID mapping in a user namespace (see  user_names-
                paces(7)).

       CAP_SETFCAP (since Linux 2.6.24)
              Set file capabilities.

       CAP_SETPCAP
              If  file  capabilities  are  not  supported: grant or remove any
              capability in the caller's permitted capability set to  or  from
              any  other process.  (This property of CAP_SETPCAP is not avail-
              able when the kernel is configured to support file capabilities,
              since CAP_SETPCAP has entirely different semantics for such ker-
              nels.)

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

       CAP_SETUID
              * Make  arbitrary  manipulations  of  process  UIDs  (setuid(2),
                setreuid(2), setresuid(2), setfsuid(2));
              * forge UID when passing  socket  credentials  via  UNIX  domain
                sockets;
              * write  a  user ID mapping in a user namespace (see user_names-
                paces(7)).

       CAP_SYS_ADMIN
              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
                sockets;
              * 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-
                pace);
              * call fanotify_init(2);
              * call bpf(2);
              * perform  privileged  KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2)
                operations;
              * use ptrace(2) PTRACE_SECCOMP_GET_FILTER to dump a tracees sec-
                comp filters;
              * 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
                terminal;
              * 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.

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

       CAP_SYS_CHROOT
              Use chroot(2).

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

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

       CAP_SYS_PACCT
              Use acct(2).

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

       CAP_SYS_RAWIO
              * 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
                devices.

       CAP_SYS_RESOURCE
              * 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
                unix(7));
              * 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.

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

       CAP_SYS_TTY_CONFIG
              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
          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.

   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_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:

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

              If a thread drops a capability from its permitted  set,  it  can
              never  reacquire  that capability (unless it execve(2)s either a
              set-user-ID-root program, or a  program  whose  associated  file
              capabilities grant that capability).

       Inheritable:
              This  is  a  set  of capabilities preserved across an execve(2).
              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.

       Effective:
              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
       below).

       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)) named security.capa-
       bility.  Writing to this extended attribute  requires  the  CAP_SETFCAP
       capability.  The file capability sets, in conjunction with the capabil-
       ity 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).

       Effective:
              This is not a set, but rather just a single bit.  If this bit is
              set, then during an execve(2) all of the new permitted capabili-
              ties for the thread are also raised in the  effective  set.   If
              this  bit  is  not set, then after an execve(2), none of the new
              permitted capabilities is in the new effective set.

              Enabling the file effective capability bit implies that any file
              permitted  or  inheritable  capability  that  causes a thread to
              acquire  the  corresponding  permitted  capability   during   an
              execve(2)  (see  the  transformation rules described below) will
              also acquire that capability in its effective  set.   Therefore,
              when    assigning    capabilities    to   a   file   (setcap(8),
              cap_set_file(3), cap_set_fd(3)), if  we  specify  the  effective
              flag  as  being  enabled  for any capability, then the effective
              flag must also be specified as enabled for all  other  capabili-
              ties  for which the corresponding permitted or inheritable flags
              is enabled.

   Transformation of capabilities during execve()
       During an execve(2), the kernel calculates the new capabilities of  the
       process using the following algorithm:

           P'(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]

       where:

           P         denotes  the  value of a thread capability set before the
                     execve(2)

           P'        denotes the value of a thread capability  set  after  the
                     execve(2)

           F         denotes a file capability set

           cap_bset  is  the  value  of the capability bounding set (described
                     below).

       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
       root.

   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
         set.

       Note that the bounding set masks the file permitted  capabilities,  but
       not  the inherited capabilities.  If a thread maintains a capability in
       its inherited set that is not in its bounding set, then  it  can  still
       gain  that capability in its permitted set by executing a file that has
       the capability in its inherited set.

       Depending on the kernel version, the capability bounding set is  either
       a system-wide attribute, or a per-process attribute.

       Capability bounding set prior to Linux 2.6.25

       In  kernels before 2.6.25, the capability bounding set is a system-wide
       attribute that affects all threads on the system.  The bounding set  is
       accessible via the file /proc/sys/kernel/cap-bound.  (Confusingly, this
       bit  mask  parameter  is  expressed  as  a  signed  decimal  number  in
       /proc/sys/kernel/cap-bound.)

       Only  the  init process may set capabilities in the capability bounding
       set; other than that, the superuser (more precisely: programs with  the
       CAP_SYS_MODULE capability) may only clear capabilities from this set.

       On  a  standard system the capability bounding set always masks out the
       CAP_SETPCAP capability.  To remove this restriction (dangerous!),  mod-
       ify  the  definition  of CAP_INIT_EFF_SET in include/linux/capability.h
       and rebuild the kernel.

       The system-wide capability bounding set  feature  was  added  to  Linux
       starting with kernel version 2.2.11.

       Capability bounding set from Linux 2.6.25 onward

       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-
       tion.

       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
       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 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 and effective capability sets.

       2. If the effective user ID is changed from  0  to  nonzero,  then  all
          capabilities are cleared from the effective set.

       3. If the effective user ID is changed from nonzero to 0, then the per-
          mitted set is copied to the effective set.

       4. If the 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 prctl(2)
       PR_SET_KEEPCAPS  operation  or  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:

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

       SECBIT_NO_SETUID_FIXUP
              Setting  this  flag  stops  the kernel from adjusting capability
              sets  when  the  thread's  effective  and  filesystem  UIDs  are
              switched  between  zero and nonzero values.  (See the subsection
              Effect of user ID changes on capabilities.)

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

       SECBIT_NO_CAP_AMBIENT_RAISE
              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
       flags   are:   SECBIT_KEEP_CAPS_LOCKED,  SECBIT_NO_SETUID_FIXUP_LOCKED,
       SECBIT_NOROOT_LOCKED, and SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED.

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

       The securebits flags are  inherited  by  child  processes.   During  an
       execve(2),  all  of  the  flags  are preserved, except SECBIT_KEEP_CAPS
       which is always cleared.

       An application can use the following call to lock itself,  and  all  of
       its  descendants,  into  an  environment  where the only way of gaining
       capabilities is by executing a program with associated  file  capabili-
       ties:

           prctl(PR_SET_SECUREBITS,
                /* SECBIT_KEEP_CAPS off */
                   SECBIT_KEEP_CAPS_LOCKED |
                   SECBIT_NO_SETUID_FIXUP |
                   SECBIT_NO_SETUID_FIXUP_LOCKED |
                   SECBIT_NOROOT |
                   SECBIT_NOROOT_LOCKED);
                   /* Setting/locking SECURE_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).

CONFORMING TO
       No standards govern capabilities, but the Linux capability  implementa-
       tion   is   based   on  the  withdrawn  POSIX.1e  draft  standard;  see
       <http://wt.tuxomania.net/publications/posix.1e/>.

NOTES
       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
       <http://www.kernel.org/pub/linux/libs/security/linux-privs>.

       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
         rebuilding.

       * If file capabilities are disabled in the current implementation, then
         init starts out with this capability  removed  from  its  per-process
         bounding  set,  and  that bounding set is inherited by all other pro-
         cesses created on the system.

SEE ALSO
       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), filecap(8), getcap(8), net-
       cap(8), pscap(8), setcap(8)

       include/linux/capability.h in the Linux kernel source tree

COLOPHON
       This page is part of release 4.13 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                   CAPABILITIES(7)

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