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NAME

       capabilities - overview of Linux capabilities

DESCRIPTION

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

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

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

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

       CAP_AUDIT_READ (since Linux 3.16)
              Allow reading the audit log via a multicast netlink socket.

       CAP_AUDIT_WRITE (since Linux 2.6.11)
              Write records to kernel auditing log.

       CAP_BLOCK_SUSPEND (since Linux 3.5)
              Employ  features   that   can   block   system   suspend   (epoll(7)   EPOLLWAKEUP,
              /proc/sys/wake_lock).

       CAP_BPF (since Linux 5.8)
              Employ privileged BPF operations; see bpf(2) and bpf-helpers(7).

              This  capability  was added in Linux 5.8 to separate out BPF functionality from the
              overloaded CAP_SYS_ADMIN capability.

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

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

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

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

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

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

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

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

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

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

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

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

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

       CAP_NET_ADMIN
              Perform various network-related operations:
              * interface configuration;
              * administration of IP firewall, masquerading, and accounting;
              * modify routing tables;
              * bind to any address for transparent proxying;
              * set type-of-service (TOS);
              * clear driver statistics;
              * set promiscuous mode;
              * enabling multicasting;
              * use setsockopt(2)  to  set  the  following  socket  options:  SO_DEBUG,  SO_MARK,
                SO_PRIORITY  (for  a  priority  outside  the  range  0 to 6), SO_RCVBUFFORCE, and
                SO_SNDBUFFORCE.

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

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

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

       CAP_PERFMON (since Linux 5.8)
              Employ various performance-monitoring mechanisms, including:
              *2     call perf_event_open(2);
              *      employ various BPF operations that have performance implications.

              This capability was added in Linux  5.8  to  separate  out  performance  monitoring
              functionality from the overloaded CAP_SYS_ADMIN capability.

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

       CAP_SETFCAP (since Linux 2.6.24)
              Set 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
              configured  to  support file capabilities, since CAP_SETPCAP has entirely different
              semantics for such kernels.)

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

       CAP_SYS_ADMIN
              Note: this capability is overloaded; see Notes to kernel developers, below.

              * Perform  a  range  of  system  administration  operations including: quotactl(2),
                mount(2), umount(2), 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 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);
              * employ various performance monitoring mechanisms (as for CAP_PERFMON);
              * access privileged perf event information;
              * call setns(2) (requires CAP_SYS_ADMIN in the target namespace);
              * call fanotify_init(2);
              * perform various BPF operations; see CAP_BPF;
              * 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 input queue of a
                terminal other than the caller's controlling terminal;
              * employ the obsolete nfsservctl(2) system call;
              * employ the obsolete bdflush(2) system call;
              * perform various privileged block-device ioctl(2) operations;
              * perform various privileged filesystem ioctl(2) operations;
              * perform privileged ioctl(2) operations on the /dev/random device (see random(4));
              * install a seccomp(2) filter without first having to set the  no_new_privs  thread
                attribute;
              * modify allow/deny rules for device control groups;
              * employ the ptrace(2) PTRACE_SECCOMP_GET_FILTER operation to dump tracee's seccomp
                filters;
              * employ the ptrace(2) PTRACE_SETOPTIONS operation to suspend the tracee's  seccomp
                protections (i.e., the PTRACE_O_SUSPEND_SECCOMP flag);
              * perform administrative operations on many device drivers.
              * Modify autogroup nice values by writing to /proc/[pid]/autogroup (see sched(7)).

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

       CAP_SYS_CHROOT
              * Use chroot(2);
              * change mount namespaces using setns(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
              * Lower the 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_setaffinity(2));
              * set I/O scheduling class and priority for arbitrary processes (ioprio_set(2));
              * apply  migrate_pages(2) to arbitrary processes and allow processes to be migrated
                to arbitrary nodes;
              * apply move_pages(2) to arbitrary processes;
              * use the MPOL_MF_MOVE_ALL flag with mbind(2) and move_pages(2).

       CAP_SYS_PACCT
              Use acct(2).

       CAP_SYS_PTRACE
              * Trace arbitrary processes using ptrace(2);
              * apply get_robust_list(2) to arbitrary processes;
              * transfer  data  to  or   from   the   memory   of   arbitrary   processes   using
                process_vm_readv(2) and process_vm_writev(2);
              * inspect processes using kcmp(2).

       CAP_SYS_RAWIO
              * Perform I/O port operations (iopl(2) and ioperm(2));
              * access /proc/kcore;
              * employ the FIBMAP ioctl(2) operation;
              * open devices for accessing x86 model-specific registers (MSRs, see msr(4));
              * update /proc/sys/vm/mmap_min_addr;
              * create   memory   mappings   at   addresses   below   the   value   specified  by
                /proc/sys/vm/mmap_min_addr;
              * map files in /proc/bus/pci;
              * open /dev/mem and /dev/kmem;
              * perform various SCSI device commands;
              * perform certain operations on hpsa(4) and cciss(4) devices;
              * perform a range of device-specific operations on other devices.

       CAP_SYS_RESOURCE
              * Use reserved space on ext2 filesystems;
              * make ioctl(2) calls controlling ext3 journaling;
              * override disk quota limits;
              * increase resource limits (see setrlimit(2));
              * override RLIMIT_NPROC resource limit;
              * override maximum number of consoles on console allocation;
              * override maximum number of keymaps;
              * allow more than 64hz interrupts from the real-time clock;
              * raise msg_qbytes  limit  for  a  System  V  message  queue  above  the  limit  in
                /proc/sys/kernel/msgmnb (see msgop(2) and msgctl(2));
              * allow  the  RLIMIT_NOFILE  resource  limit  on  the  number  of  "in-flight" file
                descriptors to be bypassed when passing file descriptors to another process via a
                UNIX domain socket (see unix(7));
              * override the /proc/sys/fs/pipe-size-max limit when setting the capacity of a pipe
                using the F_SETPIPE_SZ fcntl(2) command;
              * use F_SETPIPE_SZ to increase the capacity of a pipe above the limit specified  by
                /proc/sys/fs/pipe-max-size;
              * override    /proc/sys/fs/mqueue/queues_max,    /proc/sys/fs/mqueue/msg_max,   and
                /proc/sys/fs/mqueue/msgsize_max limits when creating POSIX  message  queues  (see
                mq_overview(7));
              * employ the prctl(2) PR_SET_MM operation;
              * set  /proc/[pid]/oom_score_adj  to  a  value  lower  than the value last set by a
                process with CAP_SYS_RESOURCE.

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

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

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

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

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

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

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

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

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

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

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

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

       *  To determine which existing capability might best be associated with your new  feature,
          review  the  list  of  capabilities above in order to find a "silo" into which your new
          feature best fits.  One approach to take is to determine if there  are  other  features
          requiring capabilities that will always be 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 associated with CAP_SYS_ADMIN are ones that  closely  match
          existing uses in that silo.

       *  If  you have determined that it really is necessary to create a new capability for your
          feature, don't make or name it as a "single-use" capability.  Thus,  for  example,  the
          addition  of the highly specific CAP_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:

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

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

       Inheritable
              This  is  a  set  of  capabilities  preserved  across  an  execve(2).   Inheritable
              capabilities  remain  inheritable  when  executing  any  program,  and  inheritable
              capabilities  are  added to the permitted set when executing a program that has the
              corresponding bits set in the file inheritable set.

              Because inheritable capabilities are not generally preserved across execve(2)  when
              running  as  a  non-root  user,  applications that wish to run helper programs with
              elevated capabilities should consider using ambient capabilities, described below.

       Effective
              This is the set of capabilities used by the kernel to perform permission checks for
              the thread.

       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 attribute 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 execve(2) of  a  program
              that  is  not  privileged.   The ambient capability set obeys the invariant that no
              capability can ever be ambient if it is not both permitted and inheritable.

              The ambient capability set  can  be  directly  modified  using  prctl(2).   Ambient
              capabilities  are automatically lowered if either of the corresponding permitted or
              inheritable capabilities is lowered.

              Executing a program that changes UID or GID due to the set-user-ID or  set-group-ID
              bits  or  executing  a  program  that  has any file capabilities set will clear the
              ambient set.  Ambient capabilities are added to the permitted set and  assigned  to
              the  effective  set  when  execve(2)  is  called.   If ambient capabilities cause a
              process's permitted and effective capabilities to  increase  during  an  execve(2),
              this does not trigger the secure-execution mode described in ld.so(8).

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

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

       Since Linux 3.2, the file /proc/sys/kernel/cap_last_cap exposes the numerical value of the
       highest  capability  supported  by  the  running kernel; this can be used to determine the
       highest bit that may be set in a capability set.

   File capabilities
       Since kernel 2.6.24, the kernel supports associating capability sets  with  an  executable
       file  using  setcap(8).  The file capability sets are stored in an extended attribute (see
       setxattr(2) 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 capabilities for the thread are also raised
              in the effective set.  If this bit is not set, then after an execve(2), none of the
              new permitted capabilities is in the new effective set.

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

   File capability extended attribute versioning
       To allow extensibility, the kernel supports a scheme to encode a version number inside the
       security.capability extended attribute that is used to implement file capabilities.  These
       version numbers are internal to the implementation, and not directly visible to user-space
       applications.  To date, the following versions are supported:

       VFS_CAP_REVISION_1
              This  was the original file capability implementation, which supported 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  capabilities grew beyond 32.  The kernel
              transparently continues to support the execution of files that have 32-bit  version
              1  capability  masks, but when adding capabilities to files that did not previously
              have  capabilities,  or  modifying  the  capabilities   of   existing   files,   it
              automatically  uses  the  version  2  scheme  (or possibly the version 3 scheme, as
              described below).

       VFS_CAP_REVISION_3 (since Linux 4.14)
              Version 3 file capabilities are provided to support  namespaced  file  capabilities
              (described below).

              As  with  version  2  file  capabilities, version 3 capability masks are 64 bits in
              size.  But  in  addition,  the  root  user  ID  of  namespace  is  encoded  in  the
              security.capability  extended  attribute.  (A namespace's root user ID is the value
              that user ID 0 inside that namespace maps to in the initial user 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 attribute 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_REVISION_3) attribute  if  both  of  the
       following are true:

       (1) The  thread  writing  the  attribute  resides  in  a noninitial user 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  or  modifying  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 the creation of a version 2 (VFS_CAP_REVISION_2) attribute.

       Note that the creation of a version 3 security.capability extended attribute 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.
       Correspondingly, 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 namespace), 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 automatic 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 security.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]

       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

       Note the following details relating to the above capability transformation 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 calculate 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 capabilities 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 capabilities 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 capabilities when a process  with  a  user  ID  of  zero
       performs an execve(2), see below under Capabilities and execution of programs by root.

   Safety checking for capability-dumb binaries
       A capability-dumb binary is an application that has been marked to have file capabilities,
       but has not been converted to use the libcap(3) API to manipulate its  capabilities.   (In
       other  words, this is a traditional set-user-ID-root program that has been switched to use
       file capabilities, but whose code has not been modified to understand capabilities.)   For
       such  applications,  the  effective  capability  bit  is set on the file, so that the file
       permitted capabilities are  automatically  enabled  in  the  process  effective  set  when
       executing  the  file.  The kernel recognizes a file which has the effective capability bit
       set as capability-dumb for the purpose of the check described here.

       When executing a capability-dumb binary, the kernel checks if  the  process  obtained  all
       permitted capabilities that were specified in the file permitted set, after the capability
       transformations described above have been performed.  (The typical reason why  this  might
       not  occur  is that the capability bounding set masked out some of the capabilities in the
       file permitted set.)  If the process did  not  obtain  the  full  set  of  file  permitted
       capabilities,  then execve(2) fails with the error EPERM.  This prevents possible security
       risks that could arise when a capability-dumb application is executed with less  privilege
       that  it needs.  Note that, by definition, the application could not itself recognize this
       problem, since it does not employ the libcap(3) API.

   Capabilities and execution of programs by root
       In order to mirror traditional UNIX semantics, the kernel performs  special  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.,  switching the effective user ID to 0 (root)
       because a set-user-ID-root program was executed—the kernel calculates the file  capability
       sets as follows:

       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 enabled).  (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  effective  bit  is  in
          fact enabled, then the file effective bit is notionally 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 capabilities 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  calculation  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 executed 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 capabilities, as would occur when executing a set-user-ID-root program  that  does
       not have any associated file capabilities).

       Note  that one can assign empty capability sets to a program file, and thus it is possible
       to create a set-user-ID-root program that changes the effective and saved  set-user-ID  of
       the process that executes the program to 0, but confers no capabilities to that process.

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

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

       * (Since Linux 2.6.25) The capability bounding set acts as a  limiting  superset  for  the
         capabilities  that  a thread can add to its inheritable set using capset(2).  This means
         that if a capability is not in the bounding set, then a thread can't add this capability
         to its inheritable set, even if it was in its permitted capabilities, and thereby cannot
         have this capability preserved in its permitted set when it execve(2)s a file  that  has
         the capability in its inheritable set.

       Note  that the bounding set masks the file permitted capabilities, but not the 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 attribute.  (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 capability is in  its  bounding  set  using  the  prctl(2)  PR_CAPBSET_READ
       operation.

       Removing  capabilities  from  the  bounding set is supported only if file capabilities are
       compiled into the kernel.  In kernels before  Linux  2.6.33,  file  capabilities  were  an
       optional  feature  configurable  via  the CONFIG_SECURITY_FILE_CAPABILITIES option.  Since
       Linux 2.6.33, the configuration option has been removed and file capabilities  are  always
       part of the kernel.  When file capabilities are compiled into the kernel, the init process
       (the ancestor of all processes) begins with a full bounding set.  If file capabilities are
       not compiled into the kernel, then init begins with a full bounding set minus CAP_SETPCAP,
       because this capability has a different meaning when there are no file capabilities.

       Removing a capability from  the  bounding  set  does  not  remove  it  from  the  thread's
       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 /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!),  modify   the   definition   of
       CAP_INIT_EFF_SET in include/linux/capability.h and rebuild the kernel.

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

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

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

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

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

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

       If a thread that has a 0 value for one or more of  its  user  IDs  wants  to  prevent  its
       permitted  capability  set  being  cleared  when  it resets all of its user IDs to nonzero
       values, it can do so using the SECBIT_KEEP_CAPS securebits flag described below.

   Programmatically adjusting capability sets
       A thread can retrieve and change its permitted, effective, and inheritable capability sets
       using  the  capget(2) and capset(2) system calls.  However, the use of cap_get_proc(3) and
       cap_set_proc(3), both provided in the libcap package, is preferred for this purpose.   The
       following rules govern changes to the thread capability sets:

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

       2. (Since Linux 2.6.25) The new inheritable set must be a subset of the combination of the
          existing inheritable set and the capability bounding set.

       3. The  new  permitted set must be a subset of the existing permitted set (i.e., it is not
          possible to acquire permitted capabilities that the thread does not currently have).

       4. The new effective set must be a subset of the new permitted set.

   The securebits flags: establishing a capabilities-only environment
       Starting with kernel 2.6.26, and with a kernel in which  file  capabilities  are  enabled,
       Linux  implements a set of per-thread securebits flags that can be used to disable special
       handling of capabilities for UID 0 (root).  These flags are as follows:

       SECBIT_KEEP_CAPS
              Setting this flag  allows  a  thread  that  has  one  or  more  0  UIDs  to  retain
              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 capabilities.  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 effective 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.

       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  preventing  further  changes  to  the
       corresponding    "base"    flag.    The   locked   flags   are:   SECBIT_KEEP_CAPS_LOCKED,
       SECBIT_NO_SETUID_FIXUP_LOCKED,                  SECBIT_NOROOT_LOCKED,                  and
       SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED.

       The  securebits  flags  can be modified and retrieved using the prctl(2) PR_SET_SECUREBITS
       and PR_GET_SECUREBITS operations.  The CAP_SETPCAP capability is required  to  modify  the
       flags.   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 execve(2),  all  of  the
       flags are preserved, except SECBIT_KEEP_CAPS which is always cleared.

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

           prctl(PR_SET_SECUREBITS,
                   /* SECBIT_KEEP_CAPS off */
                   SECBIT_KEEP_CAPS_LOCKED |
                   SECBIT_NO_SETUID_FIXUP |
                   SECBIT_NO_SETUID_FIXUP_LOCKED |
                   SECBIT_NOROOT |
                   SECBIT_NOROOT_LOCKED);
                   /* Setting/locking 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 effective 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 Transformation 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 executes a binary with such capabilities,
       it gains the associated  capabilities  (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  executing  process
       regardless of which user namespace it resides in, only privileged processes are  permitted
       to  associate  capabilities  with a file.  Here, "privileged" means a process that has the
       CAP_SETFCAP capability in the user namespace where the filesystem  was  mounted  (normally
       the  initial  user  namespace).   This  limitation  renders  file capabilities useless for
       certain use cases.  For example, in user-namespaced containers, it can be desirable to  be
       able  to  create a binary that confers capabilities only to processes executed inside that
       container, 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  described 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  binary  with
       VFS_CAP_REVISION_3 file capabilities confers capabilities to a  process  during  execve().
       However,  capabilities  are  conferred  only  if  the binary is executed by a process that
       resides in a user namespace whose UID 0 maps to the root user ID  that  is  saved  in  the
       extended  attribute,  or when executed by a process that resides in 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).

CONFORMING TO

       No  standards govern capabilities, but the Linux capability implementation is based on the
       withdrawn POSIX.1e draft standard; see ⟨https://archive.org/details/posix_1003.1e-990310⟩.

NOTES

       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 kernel component, and
       could  be  enabled/disabled  via  the  CONFIG_SECURITY_CAPABILITIES  kernel  configuration
       option.

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

       The libcap package provides a suite of routines for setting and getting capabilities  that
       is more comfortable and less likely to change than the interface provided by capset(2) and
       capget(2).  This package also provides the setcap(8) and getcap(8) programs.   It  can  be
       found at
       ⟨https://git.kernel.org/pub/scm/libs/libcap/libcap.git/refs/⟩.

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

       * In  the   pre-2.6.25   implementation   the   system-wide   capability   bounding   set,
         /proc/sys/kernel/cap-bound,  always  masks  out the CAP_SETPCAP capability, 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_SECURITY_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.

SEE ALSO

       capsh(1),    setpriv(1),    prctl(2),    setfsuid(2),    cap_clear(3),    cap_copy_ext(3),
       cap_from_text(3), cap_get_file(3), cap_get_proc(3), cap_init(3),  capgetp(3),  capsetp(3),
       libcap(3),   proc(5),   credentials(7),   pthreads(7),   user_namespaces(7),   captest(8),
       filecap(8), getcap(8), getpcaps(8), netcap(8), pscap(8), setcap(8)

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

COLOPHON

       This page is part of release 5.08 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/.