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

       capabilities - overview of Linux capabilities

       For  the  purpose of performing permission checks, traditional UNIX im-
       plementations distinguish two categories of processes: privileged  pro-
       cesses  (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
                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;
              * modify user extended attributes on sticky directory  owned  by
                any user;
              * 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_DE-
                BUG,  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_name-

       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 en-
              tirely different semantics for such kernels.)

              * Make arbitrary manipulations of process UIDs  (setuid(2),  se-
                treuid(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_name-

              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),  pivot_root(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 name-
              * 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 in-
                put 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 de-
                vice (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.
              * Modify autogroup nice values by writing  to  /proc/[pid]/auto-
                group (see sched(7)).

              Use reboot(2) and kexec_load(2).

              * Use chroot(2);
              * change mount namespaces using setns(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), sched_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 us-
                ing 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  de-

              * 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 de-
                scriptors 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 used 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 the following 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  ef-
              fective 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 ca-
              pabilities 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.

       Bounding (per-thread since Linux 2.6.25)
              The capability bounding set is a mechanism that can be  used  to
              limit the capabilities that are gained during execve(2).

              Since  Linux  2.6.25,  this  is a per-thread capability set.  In
              older kernels, the capability bounding set was a system wide at-
              tribute shared by all threads on the system.

              For more details on the capability bounding set, see below.

       Ambient (since Linux 4.3)
              This  is  a set of capabilities that are preserved across an ex-
              ecve(2) of a program that is not privileged.  The ambient  capa-
              bility  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 ei-
              ther 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 capabili-
              ties are added to the permitted set and assigned to  the  effec-
              tive  set  when  execve(2)  is  called.  If ambient capabilities
              cause a process's permitted and effective  capabilities  to  in-
              crease during an execve(2), this does not trigger the secure-ex-
              ecution mode described in

       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 nu-
       merical 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  ac-
              quire 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 en-
              abled 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.

   File capability extended attribute 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 ap-
       plications.  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 name-
              space  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 namespace.)

              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 file capability extended  attribute
       that  could  be  attached to a file was a VFS_CAP_REVISION_2 attribute.
       Since Linux 4.14, the version of the security.capability  extended  at-
       tribute  that  is  attached  to  a file depends on the circumstances in
       which the 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 user name-
           space.   (More  precisely:  the  thread resides in a user namespace
           other than  the  one  from  which  the  underlying  filesystem  was

       (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 or modifying a security.capability  extended  at-
       tribute  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 the creation
       of a version 2 (VFS_CAP_REVISION_2) attribute.

       Note that the creation of a version 3 security.capability extended  at-
       tribute  is  automatic.   That is to say, when a user-space application
       writes (setxattr(2)) a security.capability attribute in the  version  2
       format,  the  kernel will automatically create a version 3 attribute if
       the attribute is created in the circumstances described above.   Corre-
       spondingly, when a version 3 security.capability attribute is retrieved
       (getxattr(2)) by a process that resides inside a  user  namespace  that
       was  created  by  the  root user ID (or a descendant of that user name-
       space), the returned attribute is (automatically) simplified to  appear
       as  a  version  2  attribute (i.e., the returned value is the size of a
       version 2 attribute and does not include the root user ID).  These  au-
       tomatic  translations  mean  that no changes are required to user-space
       tools (e.g., setcap(1) and getcap(1)) in order for those  tools  to  be
       used to create and retrieve version 3 security.capability attributes.

       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) & P(bounding)) | P'(ambient)

           P'(effective)   = F(effective) ? P'(permitted) : P'(ambient)

           P'(inheritable) = P(inheritable)    [i.e., unchanged]

           P'(bounding)    = P(bounding)       [i.e., unchanged]


           P()   denotes the value of a thread capability set before  the  ex-

           P'()  denotes  the  value  of a thread capability set after the ex-

           F()   denotes a file capability set

       Note the following details relating to the above capability transforma-
       tion rules:

       *  The  ambient  capability  set is present only since Linux 4.3.  When
          determining the transformation of the ambient set during  execve(2),
          a  privileged file is one that has capabilities or has the set-user-
          ID or set-group-ID bit set.

       *  Prior to Linux 2.6.25, the bounding set was a system-wide  attribute
          shared  by all threads.  That system-wide value was employed to cal-
          culate the new permitted set during execve(2) in the same manner  as
          shown above for P(bounding).

       Note: during the capability transitions described above, file capabili-
       ties may be ignored (treated as empty) for the same  reasons  that  the
       set-user-ID and set-group-ID bits are ignored; see execve(2).  File ca-
       pabilities are similarly ignored if the  kernel  was  booted  with  the
       no_file_caps option.

       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 ca-
       pabilities 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 oc-
       cur is that the capability bounding set masked out some of the capabil-
       ities  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 mirror traditional UNIX semantics, the kernel performs spe-
       cial  treatment  of  file capabilities when a process with UID 0 (root)
       executes a program and when a set-user-ID-root program is executed.

       After having performed any changes to the  process  effective  ID  that
       were triggered by the set-user-ID mode bit of the binary--e.g., switch-
       ing the effective user ID to 0 (root) because a  set-user-ID-root  pro-
       gram  was  executed--the  kernel calculates the file capability sets as

       1. If the real or effective user ID of the process is  0  (root),  then
          the  file  inheritable  and permitted sets are ignored; instead they
          are notionally considered to be all ones (i.e., all capabilities en-
          abled).   (There  is one exception to this behavior, described below
          in Set-user-ID-root programs that have file capabilities.)

       2. If the effective user ID of the process is 0 (root) or the file  ef-
          fective  bit  is in fact enabled, then the file effective bit is no-
          tionally defined to be one (enabled).

       These notional values for the file's capability sets are then  used  as
       described  above to calculate the transformation of the process's capa-
       bilities during execve(2).

       Thus, when a process with nonzero UIDs  execve(2)s  a  set-user-ID-root
       program  that  does  not  have capabilities attached, or when a process
       whose real and effective UIDs are zero execve(2)s a program, the calcu-
       lation of the process's new permitted capabilities simplifies to:

           P'(permitted)   = P(inheritable) | P(bounding)

           P'(effective)   = P'(permitted)

       Consequently,  the  process gains all capabilities in its permitted and
       effective capability sets, except those masked out  by  the  capability
       bounding  set.   (In  the calculation of P'(permitted), the P'(ambient)
       term can be simplified away because it is by definition a proper subset
       of P(inheritable).)

       The special treatments of user ID 0 (root) described in this subsection
       can be disabled using the securebits mechanism described below.

   Set-user-ID-root programs that have file capabilities
       There is one exception to the behavior described under Capabilities and
       execution  of  programs  by root.  If (a) the binary that is being exe-
       cuted has capabilities attached and (b) the real user ID of the process
       is  not  0  (root)  and  (c)  the effective user ID of the process is 0
       (root), then the file capability bits are honored (i.e., they  are  not
       notionally  considered  to  be  all ones).  The usual way in which this
       situation can arise is when executing a set-UID-root program that  also
       has  file  capabilities.   When such a program is executed, the process
       gains just the capabilities granted by the program (i.e., not all capa-
       bilities, 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  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  in-
         heritable  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 from Linux 2.6.25 onward

       From  Linux  2.6.25, the capability bounding set is a per-thread attri-
       bute.  (The system-wide capability  bounding  set  described  below  no
       longer exists.)

       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, be-
       cause 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.

       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.

   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 ca-
          pabilities 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  ef-
          fective   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 permitted, effective, and inheri-
       table 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 in-
          heritable 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 set 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 permitted capabili-
              ties.  This flag is always cleared on an execve(2).

              Note that even with the SECBIT_KEEP_CAPS flag set, the effective
              capabilities of a thread are cleared when it switches its effec-
              tive UID to a nonzero value.  However, if  the  thread  has  set
              this  flag  and  its  effective  UID is already nonzero, and the
              thread subsequently switches all other UIDs to  nonzero  values,
              then the effective capabilities will not be cleared.

              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.   Note  that  the  SECBIT_*
       constants  are  available only after including the <linux/securebits.h>
       header file.

       The securebits flags are inherited by child processes.  During  an  ex-
       ecve(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 ca-
       pabilities is by executing a program with associated file capabilities:

                   /* SECBIT_KEEP_CAPS off */
                   SECBIT_KEEP_CAPS_LOCKED |
                   SECBIT_NO_SETUID_FIXUP |
                   SECBIT_NO_SETUID_FIXUP_LOCKED |
                   SECBIT_NOROOT |
                   /* Setting/locking SECBIT_NO_CAP_AMBIENT_RAISE
                      is not required */

   Per-user-namespace "set-user-ID-root" programs
       A set-user-ID program whose UID matches the UID  that  created  a  user
       namespace  will  confer capabilities in the process's permitted and ef-
       fective sets when executed by any process inside that namespace or  any
       descendant user namespace.

       The rules about the transformation of the process's capabilities during
       the execve(2) are exactly as described in the  subsections  Transforma-
       tion  of capabilities during execve() and Capabilities and execution of
       programs by root, with the difference that, in the  latter  subsection,
       "root" is the UID of the creator of the user namespace.

   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  ca-
       pability  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 in the circumstances de-
       scribed above under "File capability  extended  attribute  versioning".
       When a version 3 security.capability extended attribute is created, the
       kernel records not just the capability masks in the extended attribute,
       but also the namespace root user ID.

       As  with  a binary that has VFS_CAP_REVISION_2 file capabilities, a bi-
       nary 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 at-
       tribute, or when executed by a process that resides in a descendant  of
       such a namespace.

   Interaction with user namespaces
       For  further  information  on  the interaction of capabilities and user
       namespaces, see user_namespaces(7).

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

       When attempting to strace(1) binaries that have capabilities  (or  set-
       user-ID-root  binaries),  you may find the -u <username> option useful.
       Something like:

           $ sudo strace -o trace.log -u ceci ./myprivprog

       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 the CAP_SETPCAP ca-
         pability, and this can not be changed without  modifying  the  kernel
         source and rebuilding the kernel.

       * If  file  capabilities  are  disabled  (i.e., the kernel CONFIG_SECU-
         RITY_FILE_CAPABILITIES option is disabled), then init starts out with
         the CAP_SETPCAP capability removed from its per-process bounding set,
         and that bounding set is inherited by all other processes 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 5.07 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                             2019-08-02                   CAPABILITIES(7)

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