capabilities(7)



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

NAME
       capabilities - overview of Linux capabilities

DESCRIPTION
       For  the  purpose of performing permission checks, traditional UNIX 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).

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

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

       CAP_DAC_READ_SEARCH
              * Bypass file read permission checks and directory read and exe-
                cute permission checks;
              * invoke open_by_handle_at(2);
              * use the linkat(2) AT_EMPTY_PATH flag to create  a  link  to  a
                file referred to by a file descriptor.

       CAP_FOWNER
              * Bypass  permission  checks on operations that normally require
                the filesystem UID of the process to match the UID of the file
                (e.g., chmod(2), utime(2)), excluding those operations covered
                by CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH;
              * set inode flags (see ioctl_iflags(2)) on arbitrary files;
              * set Access Control Lists (ACLs) on arbitrary files;
              * ignore directory sticky bit on file deletion;
              * specify O_NOATIME for arbitrary files in open(2) and fcntl(2).

       CAP_FSETID
              * Don't clear set-user-ID and set-group-ID mode bits when a file
                is modified;
              * set  the  set-group-ID bit for a file whose GID does not match
                the filesystem or any of the supplementary GIDs of the calling
                process.

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

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

       CAP_KILL
              Bypass  permission  checks  for  sending  signals (see kill(2)).
              This includes use of the ioctl(2) KDSIGACCEPT operation.

       CAP_LEASE (since Linux 2.4)
              Establish leases on arbitrary files (see fcntl(2)).

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

       CAP_MAC_ADMIN (since Linux 2.6.25)
              Allow  MAC  configuration or state changes.  Implemented for the
              Smack Linux Security Module (LSM).

       CAP_MAC_OVERRIDE (since Linux 2.6.25)
              Override Mandatory Access Control (MAC).   Implemented  for  the
              Smack LSM.

       CAP_MKNOD (since Linux 2.4)
              Create special files using mknod(2).

       CAP_NET_ADMIN
              Perform various network-related operations:
              * interface configuration;
              * administration of IP firewall, masquerading, and accounting;
              * modify routing tables;
              * bind to any address for transparent proxying;
              * set type-of-service (TOS)
              * clear driver statistics;
              * set promiscuous mode;
              * enabling multicasting;
              * use  setsockopt(2) to set the following socket options: SO_DE-
                BUG, 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_name-
                spaces(7)).

       CAP_SETFCAP (since Linux 2.6.24)
              Set arbitrary capabilities on a file.

       CAP_SETPCAP
              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.)

       CAP_SETUID
              * Make  arbitrary  manipulations of process UIDs (setuid(2), se-
                treuid(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_name-
                spaces(7)).

       CAP_SYS_ADMIN
              Note: this capability is overloaded; see Notes to kernel  devel-
              opers, below.

              * Perform a range of system administration operations including:
                quotactl(2),  mount(2),  umount(2),   swapon(2),   swapoff(2),
                sethostname(2), and setdomainname(2);
              * perform  privileged  syslog(2) operations (since Linux 2.6.37,
                CAP_SYSLOG should be used to permit such operations);
              * perform VM86_REQUEST_IRQ vm86(2) command;
              * perform IPC_SET and IPC_RMID operations on arbitrary System  V
                IPC objects;
              * override RLIMIT_NPROC resource limit;
              * perform operations on trusted and security Extended Attributes
                (see xattr(7));
              * use lookup_dcookie(2);
              * use ioprio_set(2) to assign IOPRIO_CLASS_RT and (before  Linux
                2.6.25) IOPRIO_CLASS_IDLE I/O scheduling classes;
              * forge  PID  when  passing  socket  credentials via UNIX domain
                sockets;
              * exceed /proc/sys/fs/file-max, the  system-wide  limit  on  the
                number  of  open files, in system calls that open files (e.g.,
                accept(2), execve(2), open(2), pipe(2));
              * employ CLONE_* flags that create new namespaces with  clone(2)
                and unshare(2) (but, since Linux 3.8, creating user namespaces
                does not require any capability);
              * call perf_event_open(2);
              * access privileged perf event information;
              * call setns(2) (requires  CAP_SYS_ADMIN  in  the  target  name-
                space);
              * call fanotify_init(2);
              * call bpf(2);
              * perform  privileged  KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2)
                operations;
              * 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
                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  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.

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

       CAP_SYS_CHROOT
              Use chroot(2).

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

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

       CAP_SYS_PACCT
              Use acct(2).

       CAP_SYS_PTRACE
              * Trace arbitrary processes using ptrace(2);
              * apply get_robust_list(2) to arbitrary processes;
              * transfer data to or from the memory of arbitrary processes us-
                ing 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  de-
                vices.

       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 de-
                scriptors to another process via a  UNIX  domain  socket  (see
                unix(7));
              * override the /proc/sys/fs/pipe-size-max limit when setting the
                capacity of a pipe using the F_SETPIPE_SZ fcntl(2) command.
              * use F_SETPIPE_SZ to increase the capacity of a pipe above  the
                limit specified by /proc/sys/fs/pipe-max-size;
              * override  /proc/sys/fs/mqueue/queues_max  limit  when creating
                POSIX message queues (see mq_overview(7));
              * employ the prctl(2) PR_SET_MM operation;
              * set /proc/[pid]/oom_score_adj to a value lower than the  value
                last set by a process with CAP_SYS_RESOURCE.

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

       CAP_SYS_TTY_CONFIG
              Use vhangup(2); employ various privileged ioctl(2) operations on
              virtual terminals.

       CAP_SYSLOG (since Linux 2.6.37)
              * Perform  privileged  syslog(2)  operations.  See syslog(2) for
                information on which operations require privilege.
              * View kernel addresses exposed via /proc and  other  interfaces
                when /proc/sys/kernel/kptr_restrict has the value 1.  (See the
                discussion of the kptr_restrict in proc(5).)

       CAP_WAKE_ALARM (since Linux 3.0)
              Trigger something that will wake up the system (set  CLOCK_REAL-
              TIME_ALARM and CLOCK_BOOTTIME_ALARM timers).

   Past and current implementation
       A full implementation of capabilities requires that:

       1. For  all  privileged  operations,  the kernel must check whether the
          thread has the required capability in its effective set.

       2. The kernel must provide system calls allowing a thread's  capability
          sets to be changed and retrieved.

       3. The  filesystem must support attaching capabilities to an executable
          file, so that a process gains those capabilities when  the  file  is
          executed.

       Before kernel 2.6.24, only the first two of these requirements are met;
       since kernel 2.6.24, all three requirements are met.

   Notes to kernel developers
       When adding a new kernel feature that should be governed by a  capabil-
       ity, consider the following points.

       *  The  goal  of  capabilities  is  divide  the power of superuser into
          pieces, such that if a program that has one or more capabilities  is
          compromised, its power to do damage to the system would be less than
          the same program running with root privilege.

       *  You have the choice of either creating a new capability for your new
          feature,  or  associating the feature with one of the existing capa-
          bilities.  In order to keep the set of capabilities to a  manageable
          size,  the  latter option is preferable, unless there are compelling
          reasons to take the former  option.   (There  is  also  a  technical
          limit: the size of capability sets is currently limited to 64 bits.)

       *  To determine which existing capability might best be associated with
          your new feature, review the list of capabilities above in order  to
          find  a  "silo" into which your new feature best fits.  One approach
          to take is to determine if there are other features requiring  capa-
          bilities that will always be use along with the new feature.  If the
          new feature is useless without these other features, you should  use
          the same capability as the other features.

       *  Don't  choose  CAP_SYS_ADMIN  if  you can possibly avoid it!  A vast
          proportion of existing capability checks are  associated  with  this
          capability (see the partial list above).  It can plausibly be called
          "the new root", since on the one hand, it confers a  wide  range  of
          powers,  and  on  the other hand, its broad scope means that this is
          the capability that is required by many privileged programs.   Don't
          make  the problem worse.  The only new features that should be asso-
          ciated with CAP_SYS_ADMIN are ones that closely match existing  uses
          in that silo.

       *  If  you  have determined that it really is necessary to create a new
          capability for your feature, don't make or name it as a "single-use"
          capability.   Thus, for example, the addition of the highly specific
          CAP_SYS_PACCT was probably a mistake.  Instead, try to identify  and
          name  your new capability as a broader silo into which other related
          future use cases might fit.

   Thread capability sets
       Each thread has three capability sets containing zero or  more  of  the
       above capabilities:

       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  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).

       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  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.

       A child created via fork(2) inherits copies of its parent's  capability
       sets.  See below for a discussion of the treatment of capabilities dur-
       ing execve(2).

       Using capset(2), a thread may manipulate its own capability  sets  (see
       below).

       Since Linux 3.2, the file /proc/sys/kernel/cap_last_cap exposes the 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).

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

              Enabling the file effective capability bit implies that any file
              permitted or inheritable capability that causes a thread to  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 mask versioning
       To allow extensibility, the kernel supports a scheme to encode  a  ver-
       sion  number  inside the security.capability extended attribute that is
       used to implement file capabilities.  These version numbers are  inter-
       nal  to  the implementation, and not directly visible to user-space ap-
       plications.  To date, the following versions are supported:

       VFS_CAP_REVISION_1
              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 capability mask that could  be  at-
       tached  to a file was a VFS_CAP_REVISION_2 mask.  Since Linux 4.14, the
       version of the capability mask that is attached to a  file  depends  on
       the  circumstances  in which the security.capability extended attribute
       was created.

       Starting with Linux 4.14, a security.capability extended  attribute  is
       automatically  created  as (or converted to) a version 3 (VFS_CAP_REVI-
       SION_3) attribute if both of the following are true:

       (1) The thread writing the attribute resides in a noninitial namespace.
           (More  precisely: the thread resides in a user namespace other than
           the one from which the underlying filesystem was mounted.)

       (2) The thread has the CAP_SETFCAP  capability  over  the  file  inode,
           meaning  that  (a) the thread has the CAP_SETFCAP capability in its
           own user namespace; and (b) the UID and GID of the file inode  have
           mappings in the writer's user namespace.

       When  a  VFS_CAP_REVISION_3  security.capability  extended attribute is
       created, the root user ID of the creating thread's  user  namespace  is
       saved in the extended attribute.

       By  contrast,  creating a security.capability extended attribute from a
       privileged (CAP_SETFCAP) thread that resides in the namespace where the
       underlying filesystem was mounted (this normally means the initial user
       namespace) automatically results in a  version  2  (VFS_CAP_REVISION_2)
       attribute.

       Note  that  a  file  can  have  either a version 2 or a version 3 secu-
       rity.capability extended attribute associated with it,  but  not  both:
       creation  or modification of the security.capability extended attribute
       will automatically modify the version according to the circumstances in
       which the extended attribute is created or modified.

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

           P'(ambient)     = (file is privileged) ? 0 : P(ambient)

           P'(permitted)   = (P(inheritable) & F(inheritable)) |
                             (F(permitted) & cap_bset) | P'(ambient)

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

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

       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 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 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  ef-
          fective user ID of the process is 0 (root) then the file inheritable
          and permitted sets are defined to be all ones (i.e.,  all  capabili-
          ties enabled).

       2. If  a  set-user-ID-root  program is being executed, or the effective
          user ID of the process is 0 (root) then the file  effective  bit  is
          defined to be one (enabled).

       The upshot of the above rules, combined with the capabilities transfor-
       mations described above, is as follows:

       *  When a process execve(2)s a  set-user-ID-root  program,  or  when  a
          process  with  an  effective UID of 0 execve(2)s a program, it gains
          all capabilities in its permitted and effective capability sets, ex-
          cept those masked out by the capability bounding set.

       *  When  a  process with a real UID of 0 execve(2)s a program, it gains
          all capabilities in  its  permitted  capability  set,  except  those
          masked out by the capability bounding set.

       The  above steps yield semantics that are the same as those provided by
       traditional UNIX systems.

   Set-user-ID-root programs that have file capabilities
       Executing a program that is both set-user-ID root and has file capabil-
       ities  will  cause the process to gain just the capabilities granted by
       the program (i.e., not all capabilities, as would occur when  executing
       a set-user-ID-root program that does not have any associated file capa-
       bilities).  Note that one can assign empty capability sets to a program
       file, and thus it is possible to create a set-user-ID-root program that
       changes the effective and saved set-user-ID of the  process  that  exe-
       cutes the program to 0, but confers no capabilities to that process.

   Capability bounding set
       The capability bounding set is a security mechanism that can be used to
       limit the capabilities that can be gained  during  an  execve(2).   The
       bounding set is used in the following ways:

       * During  an  execve(2),  the capability bounding set is ANDed with the
         file permitted capability set, and the result of  this  operation  is
         assigned  to  the  thread's permitted capability set.  The capability
         bounding set thus places a limit on the permitted  capabilities  that
         may be granted by an executable file.

       * (Since  Linux  2.6.25) The capability bounding set acts as a limiting
         superset for the capabilities that a thread can add to its  inherita-
         ble  set  using capset(2).  This means that if a capability is not in
         the bounding set, then a thread can't add this capability to its  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
         set.

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

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

       Capability bounding set prior to Linux 2.6.25

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

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

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

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

       Capability bounding set from Linux 2.6.25 onward

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

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

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

       Removing capabilities from the bounding set is supported only  if  file
       capabilities  are  compiled  into  the kernel.  In kernels before Linux
       2.6.33, file capabilities were an optional feature configurable via the
       CONFIG_SECURITY_FILE_CAPABILITIES option.  Since Linux 2.6.33, the con-
       figuration option has been removed and  file  capabilities  are  always
       part  of the kernel.  When file capabilities are compiled into the ker-
       nel, the init process (the ancestor of all  processes)  begins  with  a
       full bounding set.  If file capabilities are not compiled into the ker-
       nel, then init begins with a full bounding set minus  CAP_SETPCAP,  be-
       cause  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  inheritable set.  However it does prevent the capability from
       being added back into the thread's inheritable set in the future.

   Effect of user ID changes on capabilities
       To preserve the traditional semantics for  transitions  between  0  and
       nonzero  user IDs, the kernel makes the following changes to a thread's
       capability sets on changes to the thread's real, effective, saved  set,
       and filesystem user IDs (using setuid(2), setresuid(2), or similar):

       1. If one or more of the real, effective or saved set user IDs was pre-
          viously 0, and as a result of the UID changes all of these IDs  have
          a  nonzero value, then all capabilities are cleared from the permit-
          ted, effective, and ambient capability sets.

       2. If the effective user ID is changed from 0 to nonzero, then all  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  capability  sets  using  the
       capget(2)   and   capset(2)   system   calls.    However,  the  use  of
       cap_get_proc(3) and cap_set_proc(3), both provided in the libcap  pack-
       age, is preferred for this purpose.  The following rules govern changes
       to the thread capability sets:

       1. If the caller does not have the CAP_SETPCAP capability, the new  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:

       SECBIT_KEEP_CAPS
              Setting this flag allows a thread that has one or more 0 UIDs to
              retain capabilities in its permitted and effective sets when  it
              switches all of its UIDs to nonzero values.  If this flag is not
              set, then such a UID switch causes the thread to lose all  capa-
              bilities  in  those sets.  This flag is always cleared on an ex-
              ecve(2).

              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.

       SECBIT_NO_SETUID_FIXUP
              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.)

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

       SECBIT_NO_CAP_AMBIENT_RAISE
              Setting this flag disallows raising ambient capabilities via the
              prctl(2) PR_CAP_AMBIENT_RAISE operation.

       Each  of the above "base" flags has a companion "locked" flag.  Setting
       any of the "locked" flags is irreversible, and has the effect  of  pre-
       venting  further  changes to the corresponding "base" flag.  The locked
       flags  are:   SECBIT_KEEP_CAPS_LOCKED,   SECBIT_NO_SETUID_FIXUP_LOCKED,
       SECBIT_NOROOT_LOCKED, and SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED.

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

       The  securebits  flags are inherited by child processes.  During an 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:

           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 SECBIT_NO_CAP_AMBIENT_RAISE
                      is not required */

   Interaction with user namespaces
       For  a  discussion  of  the  interaction of capabilities and user name-
       spaces, see user_namespaces(7).

   Namespaced file capabilities
       Traditional (i.e., version 2) file capabilities associate only a set of
       capability  masks  with  a binary executable file.  When a process exe-
       cutes a binary with such capabilities, it gains the associated capabil-
       ities  (within  its user namespace) as per the rules described above in
       "Transformation of capabilities during execve()".

       Because version 2 file capabilities confer capabilities to the  execut-
       ing  process  regardless  of  which  user namespace it resides in, only
       privileged processes are permitted to  associate  capabilities  with  a
       file.   Here, "privileged" means a process that has the CAP_SETFCAP 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 when a process that  resides
       in  a noninitial user namespace associates (setxattr(2)) file capabili-
       ties with a file whose user ID matches the user ID of  the  creator  of
       the  namespace.  In this case, the kernel records not just the capabil-
       ity masks in the extended attribute, but also the namespace  root  user
       ID.  For further details, see File capability mask versioning, above.

       As  with  a binary that has VFS_CAP_REVISION_2 file capabilities, a 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  descendant  of
       such a namespace.

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

NOTES
       From kernel 2.5.27 to kernel 2.6.26, capabilities were an optional ker-
       nel component, and  could  be  enabled/disabled  via  the  CONFIG_SECU-
       RITY_CAPABILITIES kernel configuration option.

       The /proc/[pid]/task/TID/status file can be used to view the capability
       sets of a thread.  The /proc/[pid]/status  file  shows  the  capability
       sets  of  a process's main thread.  Before Linux 3.8, nonexistent capa-
       bilities were shown as being enabled (1) in these  sets.   Since  Linux
       3.8,  all  nonexistent  capabilities  (above CAP_LAST_CAP) are shown as
       disabled (0).

       The libcap package provides a suite of routines for setting and getting
       capabilities  that  is  more comfortable and less likely to change than
       the interface provided by capset(2) and capget(2).  This  package  also
       provides the setcap(8) and getcap(8) programs.  It can be found at
       <http://www.kernel.org/pub/linux/libs/security/linux-privs>.

       Before  kernel  2.6.24, and from kernel 2.6.24 to kernel 2.6.32 if file
       capabilities are not enabled, a thread with the CAP_SETPCAP  capability
       can manipulate the capabilities of threads other than itself.  However,
       this is only theoretically possible, since no thread ever has CAP_SETP-
       CAP in either of these cases:

       * In  the pre-2.6.25 implementation the system-wide capability bounding
         set, /proc/sys/kernel/cap-bound, always masks  out  this  capability,
         and  this  can not be changed without modifying the kernel source and
         rebuilding.

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

SEE ALSO
       capsh(1),     setpriv(1),    prctl(2),    setfsuid(2),    cap_clear(3),
       cap_copy_ext(3),  cap_from_text(3),  cap_get_file(3),  cap_get_proc(3),
       cap_init(3),   capgetp(3),   capsetp(3),  libcap(3),  proc(5),  creden-
       tials(7), pthreads(7), user_namespaces(7), captest(8), filecap(8), get-
       cap(8), netcap(8), pscap(8), setcap(8)

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

COLOPHON
       This  page  is  part of release 4.16 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2018-02-02                   CAPABILITIES(7)

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