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           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 unpriv-
           ileged processes are subject to full permission checking based  on  the
           process's  credentials (usually: effective UID, effective GID, and sup-
           plementary group list).
           Starting with kernel 2.2, Linux divides  the  privileges  traditionally
           associated  with  superuser into distinct units, known as capabilities,
           which can be independently enabled and disabled.   Capabilities  are  a
           per-thread attribute.
       Capabilities list
           The following list shows the capabilities implemented on Linux, and the
           operations or behaviors that each capability permits:
           CAP_AUDIT_CONTROL (since Linux 2.6.11)
                  Enable and  disable  kernel  auditing;  change  auditing  filter
                  rules; retrieve auditing status and filtering rules.
           CAP_AUDIT_WRITE (since Linux 2.6.11)
                  Write records to kernel auditing log.
           CAP_BLOCK_SUSPEND (since Linux 3.5)
                  Employ  features  that can block system suspend (epoll(7) EPOLL-
                  WAKEUP, /proc/sys/wake_lock).
                  Make arbitrary changes to file UIDs and GIDs (see chown(2)).
                  Bypass file read, write, and execute permission checks.  (DAC is
                  an abbreviation of "discretionary access control".)
                  * Bypass file read permission checks and directory read and exe-
                    cute permission checks;
                  * Invoke open_by_handle_at(2).
                  * 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  extended  file  attributes  (see  chattr(1)) on arbitrary
                  * set Access Control Lists (ACLs) on arbitrary files;
                  * ignore directory sticky bit on file deletion;
                  Bypass  permission  checks  for  sending  signals (see kill(2)).
                  This includes use of the ioctl(2) KDSIGACCEPT operation.
           CAP_LEASE (since Linux 2.4)
                  Establish leases on arbitrary files (see fcntl(2)).
                  Set the  FS_APPEND_FL  and  FS_IMMUTABLE_FL  i-node  flags  (see
           CAP_MAC_ADMIN (since Linux 2.6.25)
                  Override  Mandatory  Access  Control (MAC).  Implemented for the
                  Smack Linux Security Module (LSM).
           CAP_MAC_OVERRIDE (since Linux 2.6.25)
                  Allow MAC configuration or state changes.  Implemented  for  the
                  Smack LSM.
           CAP_MKNOD (since Linux 2.4)
                  Create special files using mknod(2).
                  Perform various network-related operations:
                  * interface configuration;
                  * administration of IP firewall, masquerading, and accounting;
                  * modify routing tables;
                  * bind to any address for transparent proxying;
                  * set type-of-service (TOS)
                  * clear driver statistics;
                  * set promiscuous mode;
                  * enabling multicasting;
                  * use   setsockopt(2)  to  set  the  following  socket  options:
                    SO_DEBUG, SO_MARK, SO_PRIORITY (for  a  priority  outside  the
                    range 0 to 6), SO_RCVBUFFORCE, and SO_SNDBUFFORCE.
                  Bind  a socket to Internet domain privileged ports (port numbers
                  less than 1024).
                  (Unused)  Make socket broadcasts, and listen to multicasts.
                  * use RAW and PACKET sockets;
                  * bind to any address for transparent proxying.
                  Make arbitrary manipulations of process GIDs  and  supplementary
                  GID  list;  forge  GID  when passing socket credentials via UNIX
                  domain sockets.
                  make changes to the securebits flags.
                  Make  arbitrary  manipulations  of  process   UIDs   (setuid(2),
                  setreuid(2),  setresuid(2),  setfsuid(2));  make forged UID when
                  passing socket credentials via UNIX domain sockets.
                  * Perform a range of system administration operations including:
                    quotactl(2),   mount(2),   umount(2),  swapon(2),  swapoff(2),
                    sethostname(2), and setdomainname(2);
                  * perform privileged syslog(2) operations (since  Linux  2.6.37,
                    CAP_SYSLOG should be used to permit such operations);
                  * perform VM86_REQUEST_IRQ vm86(2) command;
                  * perform  IPC_SET and IPC_RMID operations on arbitrary System V
                    IPC objects;
                  * perform operations on trusted and security Extended Attributes
                    (see attr(5));
                  * 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 UID when passing socket credentials;
                  * 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);
                  * call perf_event_open(2);
                  * access privileged perf event information;
                  * call setns(2);
                  * call fanotify_init(2);
                  * perform 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
                  * 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 administrative operations on many device drivers.
                  Use reboot(2) and kexec_load(2).
                  Use chroot(2).
                  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.
                  * use the MPOL_MF_MOVE_ALL flag with mbind(2) and move_pages(2).
                  Use acct(2).
                  Trace    arbitrary    processes    using    ptrace(2);     apply
                  get_robust_list(2)  to  arbitrary  processes;  inspect processes
                  using kcmp(2).
                  * Perform I/O port operations (iopl(2) and ioperm(2));
                  * access /proc/kcore;
                  * employ the FIBMAP ioctl(2) operation;
                  * open devices for accessing x86 model-specific registers (MSRs,
                    see msr(4))
                  * update /proc/sys/vm/mmap_min_addr;
                  * create  memory mappings at addresses below the value specified
                    by /proc/sys/vm/mmap_min_addr;
                  * map files in /proc/bus/pci;
                  * open /dev/mem and /dev/kmem;
                  * perform various SCSI device commands;
                  * perform certain operations on hpsa(4) and cciss(4) devices;
                  * perform  a  range  of  device-specific  operations  on   other
                  * 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));
                  * 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 prctl(2) PR_SET_MM operation;
                  * set /proc/PID/oom_score_adj to a value lower  than  the  value
                    last set by a process with CAP_SYS_RESOURCE.
                  Set  system  clock (settimeofday(2), stime(2), adjtimex(2)); set
                  real-time (hardware) clock.
              TIME_ALARM and CLOCK_BOOTTIME_ALARM timers).
       Past and current implementation
           A full implementation of capabilities requires that:
           1. For  all  privileged  operations,  the kernel must check whether the
              thread has the required capability in its effective set.
           2. The kernel must provide system calls allowing a thread's  capability
              sets to be changed and retrieved.
           3. The  filesystem must support attaching capabilities to an executable
              file, so that a process gains those capabilities when  the  file  is
           Before kernel 2.6.24, only the first two of these requirements are met;
           since kernel 2.6.24, all three requirements are met.
       Thread capability sets
           Each thread has three capability sets containing zero or  more  of  the
           above capabilities:
                  This  is a limiting superset for the effective capabilities that
                  the thread may assume.  It is also a limiting superset  for  the
                  capabilities  that  may  be  added  to  the inheritable set by a
                  thread that does not have  the  CAP_SETPCAP  capability  in  its
                  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).
                  This is a set of capabilities preserved across an execve(2).  It
                  provides a mechanism for a process to assign capabilities to the
                  permitted set of the new program during an execve(2).
                  This is the set of capabilities used by the  kernel  to  perform
                  permission checks for the thread.
           A  child created via fork(2) inherits copies of its parent's capability
           sets.  See below for a discussion of the treatment of capabilities dur-
           ing execve(2).
           Using  capset(2),  a thread may manipulate its own capability sets (see
           Since Linux 3.2, the  file  /proc/sys/kernel/cap_last_cap  exposes  the
           numerical value of the highest capability supported by the running ker-
           Permitted (formerly known as forced):
                  These  capabilities  are  automatically permitted to the thread,
                  regardless of the thread's inheritable capabilities.
           Inheritable (formerly known as allowed):
                  This set is ANDed with the thread's inheritable set to determine
                  which  inheritable capabilities are enabled in the permitted set
                  of the thread after the execve(2).
                  This is not a set, but rather just a single bit.  If this bit is
                  set, then during an execve(2) all of the new permitted capabili-
                  ties for the thread are also raised in the  effective  set.   If
                  this  bit  is  not set, then after an execve(2), none of the new
                  permitted capabilities is in the new effective set.
                  Enabling the file effective capability bit implies that any file
                  permitted  or  inheritable  capability  that  causes a thread to
                  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  capabili-
                  ties  for which the corresponding permitted or inheritable flags
                  is enabled.
       Transformation of capabilities during execve()
           During an execve(2), the kernel calculates the new capabilities of  the
           process using the following algorithm:
               P'(permitted) = (P(inheritable) & F(inheritable)) |
                               (F(permitted) & cap_bset)
               P'(effective) = F(effective) ? P'(permitted) : 0
               P'(inheritable) = P(inheritable)    [i.e., unchanged]
               P         denotes  the  value of a thread capability set before the
               P'        denotes the value of a capability set after the execve(2)
               F         denotes a file capability set
               cap_bset  is  the  value  of the capability bounding set (described
       Capabilities and execution of programs by root
           effective capability sets, except those masked out  by  the  capability
           bounding  set.  This provides semantics that are the same as those pro-
           vided by traditional UNIX systems.
       Capability bounding set
           The capability bounding set is a security mechanism that can be used to
           limit  the  capabilities  that  can be gained during an execve(2).  The
           bounding set is used in the following ways:
           * During an execve(2), the capability bounding set is  ANDed  with  the
             file  permitted  capability  set, and the result of this operation is
             assigned to the thread's permitted capability  set.   The  capability
             bounding  set  thus places a limit on the permitted capabilities that
             may be granted by an executable file.
           * (Since Linux 2.6.25) The capability bounding set acts as  a  limiting
             superset  for the capabilities that a thread can add to its inherita-
             ble set using capset(2).  This means that if a capability is  not  in
             the  bounding  set,  then  a  thread can't add this capability to its
             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
           Note  that  the bounding set masks the file permitted capabilities, but
           not the inherited capabilities.  If a thread maintains a capability  in
           its  inherited  set  that is not in its bounding set, then it can still
           gain that capability in its permitted set by executing a file that  has
           the capability in its inherited set.
           Depending  on the kernel version, the capability bounding set is either
           a system-wide attribute, or a per-process attribute.
           Capability bounding set prior to Linux 2.6.25
           In kernels before 2.6.25, the capability bounding set is a  system-wide
           attribute  that affects all threads on the system.  The bounding set is
           accessible via the file /proc/sys/kernel/cap-bound.  (Confusingly, this
           bit  mask  parameter  is  expressed  as  a  signed  decimal  number  in
           Only the init process may set capabilities in the  capability  bounding
           set;  other than that, the superuser (more precisely: programs with the
           CAP_SYS_MODULE capability) may only clear capabilities from this set.
           On a standard system the capability bounding set always masks  out  the
           CAP_SETPCAP  capability.  To remove this restriction (dangerous!), mod-
           ify the definition of  CAP_INIT_EFF_SET  in  include/linux/capability.h
           and rebuild the kernel.
           The  system-wide  capability  bounding  set  feature was added to Linux
           starting with kernel version 2.2.11.
           Removing capabilities from the bounding set is supported only  if  file
           capabilities  are  compiled  into  the kernel.  In kernels before Linux
           2.6.33, file capabilities were an optional feature configurable via the
           CONFIG_SECURITY_FILE_CAPABILITIES option.  Since Linux 2.6.33, the con-
           figuration option has been removed and  file  capabilities  are  always
           part  of the kernel.  When file capabilities are compiled into the ker-
           nel, the init process (the ancestor of all  processes)  begins  with  a
           full bounding set.  If file capabilities are not compiled into the ker-
           nel, then init begins with  a  full  bounding  set  minus  CAP_SETPCAP,
           because  this capability has a different meaning when there are no file
           Removing a capability from the bounding set does not remove it from the
           thread's  inherited  set.   However it does prevent the capability from
           being added back into the thread's inherited set in the future.
       Effect of user ID changes on capabilities
           To preserve the traditional semantics for  transitions  between  0  and
           nonzero  user IDs, the kernel makes the following changes to a thread's
           capability sets on changes to the thread's real, effective, saved  set,
           and filesystem user IDs (using setuid(2), setresuid(2), or similar):
           1. If one or more of the real, effective or saved set user IDs was pre-
              viously 0, and as a result of the UID changes all of these IDs  have
              a  nonzero value, then all capabilities are cleared from the permit-
              ted and effective capability sets.
           2. If the effective user ID is changed from  0  to  nonzero,  then  all
              capabilities are cleared from the effective set.
           3. If the effective user ID is changed from nonzero to 0, then the per-
              mitted set is copied to the effective set.
           4. If the filesystem user ID is changed from 0 to  nonzero  (see  setf-
              suid(2)) then the following capabilities are cleared from the effec-
              tive   set:   CAP_CHOWN,   CAP_DAC_OVERRIDE,    CAP_DAC_READ_SEARCH,
              CAP_FOWNER,  CAP_FSETID,  CAP_LINUX_IMMUTABLE  (since Linux 2.6.30),
              CAP_MAC_OVERRIDE,  and  CAP_MKNOD  (since  Linux  2.6.30).   If  the
              filesystem UID is changed from nonzero to 0, then any of these capa-
              bilities that are enabled in the permitted set are  enabled  in  the
              effective set.
           If a thread that has a 0 value for one or more of its user IDs wants to
           prevent its permitted capability set being cleared when it  resets  all
           of  its  user  IDs  to  nonzero values, it can do so using the prctl(2)
           PR_SET_KEEPCAPS operation.
       Programmatically adjusting capability sets
           A thread  can  retrieve  and  change  its  capability  sets  using  the
           capget(2)   and   capset(2)   system   calls.    However,  the  use  of
              thread does not currently have).
           4. The new effective set must be a subset of the new permitted set.
       The securebits flags: establishing a capabilities-only environment
           Starting with kernel 2.6.26, and with a kernel in which file  capabili-
           ties are enabled, Linux implements a set of per-thread securebits flags
           that can be used to disable special handling of capabilities for UID  0
           (root).  These flags are as follows:
                  Setting this flag allows a thread that has one or more 0 UIDs to
                  retain its capabilities when it switches all of its  UIDs  to  a
                  nonzero  value.  If this flag is not set, then such a UID switch
                  causes the thread to lose all capabilities.  This flag is always
                  cleared on an execve(2).  (This flag provides the same function-
                  ality as the older prctl(2) PR_SET_KEEPCAPS operation.)
                  Setting this flag stops the  kernel  from  adjusting  capability
                  sets  when  the  threads's  effective  and  filesystem  UIDs are
                  switched between zero and nonzero values.  (See  the  subsection
                  Effect of User ID Changes on Capabilities.)
                  If  this bit is set, then the kernel does not grant capabilities
                  when a set-user-ID-root program is executed, or when  a  process
                  with  an  effective  or real UID of 0 calls execve(2).  (See the
                  subsection Capabilities and execution of programs by root.)
           Each of the above "base" flags has a companion "locked" flag.   Setting
           any  of  the "locked" flags is irreversible, and has the effect of pre-
           venting further changes to the corresponding "base" flag.   The  locked
           The securebits flags can be modified and retrieved using  the  prctl(2)
           PR_SET_SECUREBITS  and  PR_GET_SECUREBITS  operations.  The CAP_SETPCAP
           capability is required to modify the flags.
           The securebits flags are  inherited  by  child  processes.   During  an
           execve(2),  all  of  the  flags  are preserved, except SECBIT_KEEP_CAPS
           which is always cleared.
           An application can use the following call to lock itself,  and  all  of
           its  descendants,  into  an  environment  where the only way of gaining
           capabilities is by executing a program with associated  file  capabili-
                       SECBIT_KEEP_CAPS_LOCKED |
                       SECBIT_NO_SETUID_FIXUP |
           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
           The libcap package provides a suite of routines for setting and getting
           capabilities that is more comfortable and less likely  to  change  than
           the  interface  provided by capset(2) and capget(2).  This package also
           provides the setcap(8) and getcap(8) programs.  It can be found at
           Before kernel 2.6.24, and since kernel 2.6.24 if file capabilities  are
           not  enabled,  a  thread with the CAP_SETPCAP capability can manipulate
           the capabilities of threads other than itself.  However, this  is  only
           theoretically  possible, since no thread ever has CAP_SETPCAP in either
           of these cases:
           * In the pre-2.6.25 implementation the system-wide capability  bounding
             set,  /proc/sys/kernel/cap-bound,  always  masks out this capability,
             and this can not be changed without modifying the kernel  source  and
           * If file capabilities are disabled in the current implementation, then
             init starts out with this capability  removed  from  its  per-process
             bounding  set,  and  that bounding set is inherited by all other pro-
             cesses created on the system.


           capget(2),  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),  credentials(7),  pthreads(7),  get-
           cap(8), setcap(8)
           include/linux/capability.h in the Linux kernel source tree

    Linux 2013-09-27 CAPABILITIES(7)


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