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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  unprivileged
       processes  are  subject  to  full  permission  checking based on the process's credentials
       (usually: effective UID, effective GID, and supplementary group list).

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

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

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

       CAP_AUDIT_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)   EPOLLWAKEUP,
              /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  execute  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_namespaces(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  available  when  the  kernel  is  configured  to support file
              capabilities, since CAP_SETPCAP has entirely different semantics for such kernels.)

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

       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_namespaces(7)).

       CAP_SYS_ADMIN
              Note: this capability is overloaded; see Notes to kernel developers, 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 namespace);
              * 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 seccomp 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_SUSPEND_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_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).

       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_REALTIME_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 capability, 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 capabilities.  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  capabilities 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 associated 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 capabilities 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 capabilities are added to the permitted set and  assigned  to
              the effective 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 during 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 kernel; 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.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).

       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 capabilities for the thread are also raised
              in the effective set.  If this bit is not set, then after an execve(2), none of the
              new permitted capabilities is in the new effective set.

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

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

           P'(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 capabilities, 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 recognizes 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 capabilities 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 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,  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 transformations 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 capabilities 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 capabilities).  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  executes  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 inheritable set using capset(2).   This  means
         that if a capability is not in the bounding set, then a thread can't add this capability
         to its inheritable set, even if it was in its permitted capabilities, and thereby cannot
         have  this  capability preserved in its permitted set when it execve(2)s a file that has
         the capability in its inheritable set.

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

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

       Capability bounding set prior to Linux 2.6.25

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

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

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

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

       Capability bounding set from Linux 2.6.25 onward

       From Linux 2.6.25, the capability bounding set is a per-thread attribute.   (There  is  no
       longer a system-wide capability bounding set.)

       The bounding set is inherited at fork(2) from the thread's parent, and is preserved across
       an execve(2).

       A thread may remove capabilities from its  capability  bounding  set  using  the  prctl(2)
       PR_CAPBSET_DROP  operation, provided it has the CAP_SETPCAP capability.  Once a capability
       has been dropped from the bounding set, it cannot be restored to that set.  A  thread  can
       determine  if  a  capability  is  in  its  bounding set using the prctl(2) PR_CAPBSET_READ
       operation.

       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 configuration option has been removed and file capabilities are always
       part of the kernel.  When file capabilities are compiled into the kernel, the init process
       (the ancestor of all processes) begins with a full bounding set.  If file capabilities are
       not compiled into the kernel, then init begins with a full bounding set minus CAP_SETPCAP,
       because this capability has a different meaning when there are no file capabilities.

       Removing a capability from the bounding set does not remove it from the thread's inherited
       set.  However it does prevent the capability from  being  added  back  into  the  thread's
       inherited set in the future.

   Effect of user ID changes on capabilities
       To  preserve the traditional semantics for transitions between 0 and nonzero user IDs, the
       kernel makes the following changes to  a  thread's  capability  sets  on  changes  to  the
       thread's   real,   effective,  saved  set,  and  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 previously 0, and as  a
          result  of the UID changes all of these IDs have a nonzero value, then all capabilities
          are cleared from the permitted and effective capability sets.

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

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

       4. If the filesystem user ID is changed from 0 to  nonzero  (see  setfsuid(2)),  then  the
          following capabilities are cleared from the effective set: CAP_CHOWN, CAP_DAC_OVERRIDE,
          CAP_DAC_READ_SEARCH, CAP_FOWNER, CAP_FSETID, CAP_LINUX_IMMUTABLE (since Linux  2.6.30),
          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 capabilities that are enabled in the permitted set
          are enabled in the effective set.

       If  a  thread  that  has  a  0  value for one or more of its user IDs wants to prevent its
       permitted capability set being cleared when it resets all  of  its  user  IDs  to  nonzero
       values,  it can do so using the prctl(2) PR_SET_KEEPCAPS operation 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 package, is preferred for this purpose.  The following rules govern changes  to
       the thread capability sets:

       1. If the caller does not have the CAP_SETPCAP capability, the new inheritable set must be
          a subset of the combination of the existing inheritable and permitted sets.

       2. (Since 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  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:

       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 functionality
              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  preventing  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 capabilities:

           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  namespaces,  see
       user_namespaces(7).

CONFORMING TO

       No standards govern capabilities, but the Linux capability implementation 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 kernel component, and
       could  be  enabled/disabled  via  the  CONFIG_SECURITY_CAPABILITIES  kernel  configuration
       option.

       The  /proc/[pid]/task/TID/status file can be used to view the capability sets of a thread.
       The /proc/[pid]/status file shows the capability sets of a process's main thread.   Before
       Linux  3.8, nonexistent capabilities 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_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.

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),   credentials(7),   pthreads(7),   user_namespaces(7),   filecap(8),
       getcap(8), netcap(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/.