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NAME

       capabilities - overview of Linux capabilities

DESCRIPTION

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

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

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

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

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

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

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

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

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

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

       CAP_FOWNER
              * Bypass  permission  checks on operations that normally require the filesystem UID of the process
                to match the UID of the file (e.g., chmod(2), utime(2)), excluding those operations  covered  by
                CAP_DAC_OVERRIDE and CAP_DAC_READ_SEARCH;
              * set inode flags (see ioctl_iflags(2)) on arbitrary files;
              * set Access Control Lists (ACLs) on arbitrary files;
              * ignore directory sticky bit on file deletion;
              * 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_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);
              * call perf_event_open(2);
              * access privileged perf event information;
              * call setns(2) (requires CAP_SYS_ADMIN in the target namespace);
              * call fanotify_init(2);
              * call bpf(2);
              * perform privileged KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2) operations;
              * 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
              * Raise  process  nice  value  (nice(2),  setpriority(2))  and change the nice value for arbitrary
                processes;
              * set real-time  scheduling  policies  for  calling  process,  and  set  scheduling  policies  and
                priorities for arbitrary processes (sched_setscheduler(2), sched_setparam(2), sched_setattr(2));
              * set CPU affinity for arbitrary processes (sched_setaffinity(2));
              * set I/O scheduling class and priority for arbitrary processes (ioprio_set(2));
              * apply  migrate_pages(2)  to  arbitrary processes and allow processes to be migrated to arbitrary
                nodes;
              * apply move_pages(2) to arbitrary processes;
              * use the MPOL_MF_MOVE_ALL flag with mbind(2) and move_pages(2).

       CAP_SYS_PACCT
              Use acct(2).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       *  To determine which existing capability might best be associated with your new feature, review the list
          of  capabilities  above in order to find a "silo" into which your new feature best fits.  One approach
          to take is to determine if there are other features requiring capabilities that will  always  be  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), netcap(8), pscap(8),
       setcap(8)

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

COLOPHON

       This page is part of release 5.05 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/.