man7/capabilities.7

   1 .\" Copyright (c) 2002 by Michael Kerrisk <mtk.manpages@gmail.com>
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  12 .\" Since the Linux kernel and libraries are constantly changing, this
  13 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
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  22 .\"
  23 .\" 6 Aug 2002 - Initial Creation
  24 .\" Modified 2003-05-23, Michael Kerrisk, <mtk.manpages@gmail.com>
  25 .\" Modified 2004-05-27, Michael Kerrisk, <mtk.manpages@gmail.com>
  26 .\" 2004-12-08, mtk Added O_NOATIME for CAP_FOWNER
  27 .\" 2005-08-16, mtk, Added CAP_AUDIT_CONTROL and CAP_AUDIT_WRITE
  28 .\" 2008-07-15, Serge Hallyn <serue@us.bbm.com>
  29 .\"     Document file capabilities, per-process capability
  30 .\"     bounding set, changed semantics for CAP_SETPCAP,
  31 .\"     and other changes in 2.6.2[45].
  32 .\"     Add CAP_MAC_ADMIN, CAP_MAC_OVERRIDE, CAP_SETFCAP.
  33 .\" 2008-07-15, mtk
  34 .\"     Add text describing circumstances in which CAP_SETPCAP
  35 .\"     (theoretically) permits a thread to change the
  36 .\"     capability sets of another thread.
  37 .\"     Add section describing rules for programmatically
  38 .\"     adjusting thread capability sets.
  39 .\"     Describe rationale for capability bounding set.
  40 .\"     Document "securebits" flags.
  41 .\"     Add text noting that if we set the effective flag for one file
  42 .\"     capability, then we must also set the effective flag for all
  43 .\"     other capabilities where the permitted or inheritable bit is set.
  44 .\" 2011-09-07, mtk/Serge hallyn: Add CAP_SYSLOG
  45 .\"
  46 .TH CAPABILITIES 7 2012-12-22 "Linux" "Linux Programmer's Manual"
  47 .SH NAME
  48 capabilities \- overview of Linux capabilities
  49 .SH DESCRIPTION
  50 For the purpose of performing permission checks,
  51 traditional UNIX implementations distinguish two categories of processes:
  52 .I privileged
  53 processes (whose effective user ID is 0, referred to as superuser or root),
  54 and
  55 .I unprivileged
  56 processes (whose effective UID is nonzero).
  57 Privileged processes bypass all kernel permission checks,
  58 while unprivileged processes are subject to full permission
  59 checking based on the process's credentials
  60 (usually: effective UID, effective GID, and supplementary group list).
  61
  62 Starting with kernel 2.2, Linux divides the privileges traditionally
  63 associated with superuser into distinct units, known as
  64 .IR capabilities ,
  65 which can be independently enabled and disabled.
  66 Capabilities are a per-thread attribute.
  67 .\"
  68 .SS Capabilities list
  69 The following list shows the capabilities implemented on Linux,
  70 and the operations or behaviors that each capability permits:
  71 .TP
  72 .BR CAP_AUDIT_CONTROL " (since Linux 2.6.11)"
  73 Enable and disable kernel auditing; change auditing filter rules;
  74 retrieve auditing status and filtering rules.
  75 .TP
  76 .BR CAP_AUDIT_WRITE " (since Linux 2.6.11)"
  77 Write records to kernel auditing log.
  78 .TP
  79 .BR CAP_BLOCK_SUSPEND " (since Linux 3.5)"
  80 Employ features that can block system suspend
  81 .RB ( epoll (7)
  82 .BR EPOLLWAKEUP ,
  83 .IR /proc/sys/wake_lock ).
  84 .TP
  85 .B CAP_CHOWN
  86 Make arbitrary changes to file UIDs and GIDs (see
  87 .BR chown (2)).
  88 .TP
  89 .B CAP_DAC_OVERRIDE
  90 Bypass file read, write, and execute permission checks.
  91 (DAC is an abbreviation of "discretionary access control".)
  92 .TP
  93 .B CAP_DAC_READ_SEARCH
  94 Bypass file read permission checks and
  95 directory read and execute permission checks.
  96 .TP
  97 .B CAP_FOWNER
  98 .PD 0
  99 .RS
 100 .IP * 2
 101 Bypass permission checks on operations that normally
 102 require the file system UID of the process to match the UID of
 103 the file (e.g.,
 104 .BR chmod (2),
 105 .BR utime (2)),
 106 excluding those operations covered by
 107 .B CAP_DAC_OVERRIDE
 108 and
 109 .BR CAP_DAC_READ_SEARCH ;
 110 .IP *
 111 set extended file attributes (see
 112 .BR chattr (1))
 113 on arbitrary files;
 114 .IP *
 115 set Access Control Lists (ACLs) on arbitrary files;
 116 .IP *
 117 ignore directory sticky bit on file deletion;
 118 .IP *
 119 specify
 120 .B O_NOATIME
 121 for arbitrary files in
 122 .BR open (2)
 123 and
 124 .BR fcntl (2).
 125 .RE
 126 .PD
 127 .TP
 128 .B CAP_FSETID
 129 Don't clear set-user-ID and set-group-ID permission
 130 bits when a file is modified;
 131 set the set-group-ID bit for a file whose GID does not match
 132 the file system or any of the supplementary GIDs of the calling process.
 133 .TP
 134 .B CAP_IPC_LOCK
 135 .\" FIXME As at Linux 3.2, there are some strange uses of this capability
 136 .\" in other places; they probably should be replaced with something else.
 137 Lock memory
 138 .RB ( mlock (2),
 139 .BR mlockall (2),
 140 .BR mmap (2),
 141 .BR shmctl (2)).
 142 .TP
 143 .B CAP_IPC_OWNER
 144 Bypass permission checks for operations on System V IPC objects.
 145 .TP
 146 .B CAP_KILL
 147 Bypass permission checks for sending signals (see
 148 .BR kill (2)).
 149 This includes use of the
 150 .BR ioctl (2)
 151 .B KDSIGACCEPT
 152 operation.
 153 .\" FIXME CAP_KILL also has an effect for threads + setting child
 154 .\"       termination signal to other than SIGCHLD: without this
 155 .\"       capability, the termination signal reverts to SIGCHLD
 156 .\"       if the child does an exec().  What is the rationale
 157 .\"       for this?
 158 .TP
 159 .BR CAP_LEASE " (since Linux 2.4)"
 160 Establish leases on arbitrary files (see
 161 .BR fcntl (2)).
 162 .TP
 163 .B CAP_LINUX_IMMUTABLE
 164 Set the
 165 .B FS_APPEND_FL
 166 and
 167 .B FS_IMMUTABLE_FL
 168 .\" These attributes are now available on ext2, ext3, Reiserfs, XFS, JFS
 169 i-node flags (see
 170 .BR chattr (1)).
 171 .TP
 172 .BR CAP_MAC_ADMIN " (since Linux 2.6.25)"
 173 Override Mandatory Access Control (MAC).
 174 Implemented for the Smack Linux Security Module (LSM).
 175 .TP
 176 .BR CAP_MAC_OVERRIDE " (since Linux 2.6.25)"
 177 Allow MAC configuration or state changes.
 178 Implemented for the Smack LSM.
 179 .TP
 180 .BR CAP_MKNOD " (since Linux 2.4)"
 181 Create special files using
 182 .BR mknod (2).
 183 .TP
 184 .B CAP_NET_ADMIN
 185 Perform various network-related operations:
 186 .PD 0
 187 .RS
 188 .IP * 2
 189 interface configuration;
 190 .IP *
 191 administration of IP firewall, masquerading, and accounting
 192 .IP *
 193 modify routing tables;
 194 .IP *
 195 bind to any address for transparent proxying;
 196 .IP *
 197 set type-of-service (TOS)
 198 .IP *
 199 clear driver statistics;
 200 .IP *
 201 set promiscuous mode;
 202 .IP *
 203 enabling multicasting;
 204 .IP *
 205 use
 206 .BR setsockopt (2)
 207 to set the following socket options:
 208 .BR SO_DEBUG ,
 209 .BR SO_MARK ,
 210 .BR SO_PRIORITY
 211 (for a priority outside the range 0 to 6),
 212 .BR SO_RCVBUFFORCE ,
 213 and
 214 .BR SO_SNDBUFFORCE .
 215 .RE
 216 .PD
 217 .TP
 218 .B CAP_NET_BIND_SERVICE
 219 Bind a socket to Internet domain privileged ports
 220 (port numbers less than 1024).
 221 .TP
 222 .B CAP_NET_BROADCAST
 223 (Unused)  Make socket broadcasts, and listen to multicasts.
 224 .TP
 225 .B CAP_NET_RAW
 226 .PD 0
 227 .RS
 228 .IP * 2
 229 use RAW and PACKET sockets;
 230 .IP *
 231 bind to any address for transparent proxying.
 232 .RE
 233 .PD
 234 .\" Also various IP options and setsockopt(SO_BINDTODEVICE)
 235 .TP
 236 .B CAP_SETGID
 237 Make arbitrary manipulations of process GIDs and supplementary GID list;
 238 forge GID when passing socket credentials via UNIX domain sockets.
 239 .TP
 240 .BR CAP_SETFCAP " (since Linux 2.6.24)"
 241 Set file capabilities.
 242 .TP
 243 .B CAP_SETPCAP
 244 If file capabilities are not supported:
 245 grant or remove any capability in the
 246 caller's permitted capability set to or from any other process.
 247 (This property of
 248 .B CAP_SETPCAP
 249 is not available when the kernel is configured to support
 250 file capabilities, since
 251 .B CAP_SETPCAP
 252 has entirely different semantics for such kernels.)
 253
 254 If file capabilities are supported:
 255 add any capability from the calling thread's bounding set
 256 to its inheritable set;
 257 drop capabilities from the bounding set (via
 258 .BR prctl (2)
 259 .BR PR_CAPBSET_DROP );
 260 make changes to the
 261 .I securebits
 262 flags.
 263 .TP
 264 .B CAP_SETUID
 265 Make arbitrary manipulations of process UIDs
 266 .RB ( setuid (2),
 267 .BR setreuid (2),
 268 .BR setresuid (2),
 269 .BR setfsuid (2));
 270 make forged UID when passing socket credentials via UNIX domain sockets.
 271 .\" FIXME CAP_SETUID also an effect in exec(); document this.
 272 .TP
 273 .B CAP_SYS_ADMIN
 274 .PD 0
 275 .RS
 276 .IP * 2
 277 Perform a range of system administration operations including:
 278 .BR quotactl (2),
 279 .BR mount (2),
 280 .BR umount (2),
 281 .BR swapon (2),
 282 .BR swapoff (2),
 283 .BR sethostname (2),
 284 and
 285 .BR setdomainname (2);
 286 .IP *
 287 perform privileged
 288 .BR syslog (2)
 289 operations (since Linux 2.6.37,
 290 .BR CAP_SYSLOG
 291 should be used to permit such operations);
 292 .IP *
 293 perform
 294 .B VM86_REQUEST_IRQ
 295 .BR vm86 (2)
 296 command;
 297 .IP *
 298 perform
 299 .B IPC_SET
 300 and
 301 .B IPC_RMID
 302 operations on arbitrary System V IPC objects;
 303 .IP *
 304 perform operations on
 305 .I trusted
 306 and
 307 .I security
 308 Extended Attributes (see
 309 .BR attr (5));
 310 .IP *
 311 use
 312 .BR lookup_dcookie (2);
 313 .IP *
 314 use
 315 .BR ioprio_set (2)
 316 to assign
 317 .B IOPRIO_CLASS_RT
 318 and (before Linux 2.6.25)
 319 .B IOPRIO_CLASS_IDLE
 320 I/O scheduling classes;
 321 .IP *
 322 forge UID when passing socket credentials;
 323 .IP *
 324 exceed
 325 .IR /proc/sys/fs/file-max ,
 326 the system-wide limit on the number of open files,
 327 in system calls that open files (e.g.,
 328 .BR accept (2),
 329 .BR execve (2),
 330 .BR open (2),
 331 .BR pipe (2));
 332 .IP *
 333 employ
 334 .B CLONE_*
 335 flags that create new namespaces with
 336 .BR clone (2)
 337 and
 338 .BR unshare (2);
 339 .IP *
 340 call
 341 .BR perf_event_open (2);
 342 .IP *
 343 access privileged
 344 .I perf
 345 event information;
 346 .IP *
 347 call
 348 .BR setns (2);
 349 .IP *
 350 call
 351 .BR fanotify_init (2);
 352 .IP *
 353 perform
 354 .B KEYCTL_CHOWN
 355 and
 356 .B KEYCTL_SETPERM
 357 .BR keyctl (2)
 358 operations;
 359 .IP *
 360 perform
 361 .BR madvise (2)
 362 .B MADV_HWPOISON
 363 operation;
 364 .IP *
 365 employ the
 366 .B TIOCSTI
 367 .BR ioctl (2)
 368 to insert characters into the input queue of a terminal other than
 369 the caller's controlling terminal.
 370 .IP *
 371 employ the obsolete
 372 .BR nfsservctl (2)
 373 system call;
 374 .IP *
 375 employ the obsolete
 376 .BR bdflush (2)
 377 system call;
 378 .IP *
 379 perform various privileged block-device
 380 .BR ioctl (2)
 381 operations;
 382 .IP *
 383 perform various privileged file-system
 384 .BR ioctl (2)
 385 operations;
 386 .IP *
 387 perform administrative operations on many device drivers.
 388 .RE
 389 .PD
 390 .TP
 391 .B CAP_SYS_BOOT
 392 Use
 393 .BR reboot (2)
 394 and
 395 .BR kexec_load (2).
 396 .TP
 397 .B CAP_SYS_CHROOT
 398 Use
 399 .BR chroot (2).
 400 .TP
 401 .B CAP_SYS_MODULE
 402 Load and unload kernel modules
 403 (see
 404 .BR init_module (2)
 405 and
 406 .BR delete_module (2));
 407 in kernels before 2.6.25:
 408 drop capabilities from the system-wide capability bounding set.
 409 .TP
 410 .B CAP_SYS_NICE
 411 .PD 0
 412 .RS
 413 .IP * 2
 414 Raise process nice value
 415 .RB ( nice (2),
 416 .BR setpriority (2))
 417 and change the nice value for arbitrary processes;
 418 .IP *
 419 set real-time scheduling policies for calling process,
 420 and set scheduling policies and priorities for arbitrary processes
 421 .RB ( sched_setscheduler (2),
 422 .BR sched_setparam (2));
 423 .IP *
 424 set CPU affinity for arbitrary processes
 425 .RB ( sched_setaffinity (2));
 426 .IP *
 427 set I/O scheduling class and priority for arbitrary processes
 428 .RB ( ioprio_set (2));
 429 .IP *
 430 apply
 431 .BR migrate_pages (2)
 432 to arbitrary processes and allow processes
 433 to be migrated to arbitrary nodes;
 434 .\" FIXME CAP_SYS_NICE also has the following effect for
 435 .\" migrate_pages(2):
 436 .\"     do_migrate_pages(mm, &old, &new,
 437 .\"         capable(CAP_SYS_NICE) ? MPOL_MF_MOVE_ALL : MPOL_MF_MOVE);
 438 .IP *
 439 apply
 440 .BR move_pages (2)
 441 to arbitrary processes;
 442 .IP *
 443 use the
 444 .B MPOL_MF_MOVE_ALL
 445 flag with
 446 .BR mbind (2)
 447 and
 448 .BR move_pages (2).
 449 .RE
 450 .PD
 451 .TP
 452 .B CAP_SYS_PACCT
 453 Use
 454 .BR acct (2).
 455 .TP
 456 .B CAP_SYS_PTRACE
 457 Trace arbitrary processes using
 458 .BR ptrace (2);
 459 apply
 460 .BR get_robust_list (2)
 461 to arbitrary processes;
 462 inspect processes using
 463 .BR kcmp (2).
 464 .TP
 465 .B CAP_SYS_RAWIO
 466 Perform I/O port operations
 467 .RB ( iopl (2)
 468 and
 469 .BR ioperm (2));
 470 access
 471 .IR /proc/kcore ;
 472 employ the
 473 .B FIBMAP
 474 .BR ioctl (2)
 475 operation.
 476 .TP
 477 .B CAP_SYS_RESOURCE
 478 .PD 0
 479 .RS
 480 .IP * 2
 481 Use reserved space on ext2 file systems;
 482 .IP *
 483 make
 484 .BR ioctl (2)
 485 calls controlling ext3 journaling;
 486 .IP *
 487 override disk quota limits;
 488 .IP *
 489 increase resource limits (see
 490 .BR setrlimit (2));
 491 .IP *
 492 override
 493 .B RLIMIT_NPROC
 494 resource limit;
 495 .IP *
 496 override maximum number of consoles on console allocation;
 497 .IP *
 498 override maximum number of keymaps;
 499 .IP *
 500 allow more than 64hz interrupts from the real-time clock;
 501 .IP *
 502 raise
 503 .I msg_qbytes
 504 limit for a System V message queue above the limit in
 505 .I /proc/sys/kernel/msgmnb
 506 (see
 507 .BR msgop (2)
 508 and
 509 .BR msgctl (2));
 510 .IP *
 511 override the
 512 .I /proc/sys/fs/pipe-size-max
 513 limit when setting the capacity of a pipe using the
 514 .B F_SETPIPE_SZ
 515 .BR fcntl (2)
 516 command.
 517 .IP *
 518 use
 519 .BR F_SETPIPE_SZ
 520 to increase the capacity of a pipe above the limit specified by
 521 .IR /proc/sys/fs/pipe-max-size ;
 522 .IP *
 523 override
 524 .I /proc/sys/fs/mqueue/queues_max
 525 limit when creating POSIX message queues (see
 526 .BR mq_overview (7));
 527 .IP *
 528 employ
 529 .BR prctl (2)
 530 .B PR_SET_MM
 531 operation.
 532 .RE
 533 .PD
 534 .TP
 535 .B CAP_SYS_TIME
 536 Set system clock
 537 .RB ( settimeofday (2),
 538 .BR stime (2),
 539 .BR adjtimex (2));
 540 set real-time (hardware) clock.
 541 .TP
 542 .B CAP_SYS_TTY_CONFIG
 543 Use
 544 .BR vhangup (2);
 545 employ various privileged
 546 .BR ioctl (2)
 547 operations on virtual terminals.
 548 .TP
 549 .BR CAP_SYSLOG " (since Linux 2.6.37)"
 550 .IP * 3
 551 Perform privileged
 552 .BR syslog (2)
 553 operations.
 554 See
 555 .BR syslog (2)
 556 for information on which operations require privilege.
 557 .IP *
 558 View kernel addresses exposed via
 559 .I /proc
 560 and other interfaces when
 561 .IR /proc/sys/kernel/kptr_restrict
 562 has the value 1.
 563 (See the discussion of the
 564 .I kptr_restrict
 565 in
 566 .BR proc (5).)
 567 .TP
 568 .BR CAP_WAKE_ALARM " (since Linux 3.0)"
 569 Trigger something that will wake up the system (set
 570 .B CLOCK_REALTIME_ALARM
 571 and
 572 .B CLOCK_BOOTTIME_ALARM
 573 timers).
 574 .\"
 575 .SS Past and current implementation
 576 A full implementation of capabilities requires that:
 577 .IP 1. 3
 578 For all privileged operations,
 579 the kernel must check whether the thread has the required
 580 capability in its effective set.
 581 .IP 2.
 582 The kernel must provide system calls allowing a thread's capability sets to
 583 be changed and retrieved.
 584 .IP 3.
 585 The file system must support attaching capabilities to an executable file,
 586 so that a process gains those capabilities when the file is executed.
 587 .PP
 588 Before kernel 2.6.24, only the first two of these requirements are met;
 589 since kernel 2.6.24, all three requirements are met.
 590 .\"
 591 .SS Thread capability sets
 592 Each thread has three capability sets containing zero or more
 593 of the above capabilities:
 594 .TP
 595 .IR Permitted :
 596 This is a limiting superset for the effective
 597 capabilities that the thread may assume.
 598 It is also a limiting superset for the capabilities that
 599 may be added to the inheritable set by a thread that does not have the
 600 .B CAP_SETPCAP
 601 capability in its effective set.
 602
 603 If a thread drops a capability from its permitted set,
 604 it can never reacquire that capability (unless it
 605 .BR execve (2)s
 606 either a set-user-ID-root program, or
 607 a program whose associated file capabilities grant that capability).
 608 .TP
 609 .IR Inheritable :
 610 This is a set of capabilities preserved across an
 611 .BR execve (2).
 612 It provides a mechanism for a process to assign capabilities
 613 to the permitted set of the new program during an
 614 .BR execve (2).
 615 .TP
 616 .IR Effective :
 617 This is the set of capabilities used by the kernel to
 618 perform permission checks for the thread.
 619 .PP
 620 A child created via
 621 .BR fork (2)
 622 inherits copies of its parent's capability sets.
 623 See below for a discussion of the treatment of capabilities during
 624 .BR execve (2).
 625 .PP
 626 Using
 627 .BR capset (2),
 628 a thread may manipulate its own capability sets (see below).
 629 .\"
 630 .SS File capabilities
 631 Since kernel 2.6.24, the kernel supports
 632 associating capability sets with an executable file using
 633 .BR setcap (8).
 634 The file capability sets are stored in an extended attribute (see
 635 .BR setxattr (2))
 636 named
 637 .IR "security.capability" .
 638 Writing to this extended attribute requires the
 639 .BR CAP_SETFCAP
 640 capability.
 641 The file capability sets,
 642 in conjunction with the capability sets of the thread,
 643 determine the capabilities of a thread after an
 644 .BR execve (2).
 645
 646 The three file capability sets are:
 647 .TP
 648 .IR Permitted " (formerly known as " forced ):
 649 These capabilities are automatically permitted to the thread,
 650 regardless of the thread's inheritable capabilities.
 651 .TP
 652 .IR Inheritable " (formerly known as " allowed ):
 653 This set is ANDed with the thread's inheritable set to determine which
 654 inheritable capabilities are enabled in the permitted set of
 655 the thread after the
 656 .BR execve (2).
 657 .TP
 658 .IR Effective :
 659 This is not a set, but rather just a single bit.
 660 If this bit is set, then during an
 661 .BR execve (2)
 662 all of the new permitted capabilities for the thread are
 663 also raised in the effective set.
 664 If this bit is not set, then after an
 665 .BR execve (2),
 666 none of the new permitted capabilities is in the new effective set.
 667
 668 Enabling the file effective capability bit implies
 669 that any file permitted or inheritable capability that causes a
 670 thread to acquire the corresponding permitted capability during an
 671 .BR execve (2)
 672 (see the transformation rules described below) will also acquire that
 673 capability in its effective set.
 674 Therefore, when assigning capabilities to a file
 675 .RB ( setcap (8),
 676 .BR cap_set_file (3),
 677 .BR cap_set_fd (3)),
 678 if we specify the effective flag as being enabled for any capability,
 679 then the effective flag must also be specified as enabled
 680 for all other capabilities for which the corresponding permitted or
 681 inheritable flags is enabled.
 682 .\"
 683 .SS Transformation of capabilities during execve()
 684 .PP
 685 During an
 686 .BR execve (2),
 687 the kernel calculates the new capabilities of
 688 the process using the following algorithm:
 689 .in +4n
 690 .nf
 691
 692 P'(permitted) = (P(inheritable) & F(inheritable)) |
 693                 (F(permitted) & cap_bset)
 694
 695 P'(effective) = F(effective) ? P'(permitted) : 0
 696
 697 P'(inheritable) = P(inheritable)    [i.e., unchanged]
 698
 699 .fi
 700 .in
 701 where:
 702 .RS 4
 703 .IP P 10
 704 denotes the value of a thread capability set before the
 705 .BR execve (2)
 706 .IP P'
 707 denotes the value of a capability set after the
 708 .BR execve (2)
 709 .IP F
 710 denotes a file capability set
 711 .IP cap_bset
 712 is the value of the capability bounding set (described below).
 713 .RE
 714 .\"
 715 .SS Capabilities and execution of programs by root
 716 In order to provide an all-powerful
 717 .I root
 718 using capability sets, during an
 719 .BR execve (2):
 720 .IP 1. 3
 721 If a set-user-ID-root program is being executed,
 722 or the real user ID of the process is 0 (root)
 723 then the file inheritable and permitted sets are defined to be all ones
 724 (i.e., all capabilities enabled).
 725 .IP 2.
 726 If a set-user-ID-root program is being executed,
 727 then the file effective bit is defined to be one (enabled).
 728 .PP
 729 The upshot of the above rules,
 730 combined with the capabilities transformations described above,
 731 is that when a process
 732 .BR execve (2)s
 733 a set-user-ID-root program, or when a process with an effective UID of 0
 734 .BR execve (2)s
 735 a program,
 736 it gains all capabilities in its permitted and effective capability sets,
 737 except those masked out by the capability bounding set.
 738 .\" If a process with real UID 0, and nonzero effective UID does an
 739 .\" exec(), then it gets all capabilities in its
 740 .\" permitted set, and no effective capabilities
 741 This provides semantics that are the same as those provided by
 742 traditional UNIX systems.
 743 .SS Capability bounding set
 744 The capability bounding set is a security mechanism that can be used
 745 to limit the capabilities that can be gained during an
 746 .BR execve (2).
 747 The bounding set is used in the following ways:
 748 .IP * 2
 749 During an
 750 .BR execve (2),
 751 the capability bounding set is ANDed with the file permitted
 752 capability set, and the result of this operation is assigned to the
 753 thread's permitted capability set.
 754 The capability bounding set thus places a limit on the permitted
 755 capabilities that may be granted by an executable file.
 756 .IP *
 757 (Since Linux 2.6.25)
 758 The capability bounding set acts as a limiting superset for
 759 the capabilities that a thread can add to its inheritable set using
 760 .BR capset (2).
 761 This means that if a capability is not in the bounding set,
 762 then a thread can't add this capability to its
 763 inheritable set, even if it was in its permitted capabilities,
 764 and thereby cannot have this capability preserved in its
 765 permitted set when it
 766 .BR execve (2)s
 767 a file that has the capability in its inheritable set.
 768 .PP
 769 Note that the bounding set masks the file permitted capabilities,
 770 but not the inherited capabilities.
 771 If a thread maintains a capability in its inherited set
 772 that is not in its bounding set,
 773 then it can still gain that capability in its permitted set
 774 by executing a file that has the capability in its inherited set.
 775 .PP
 776 Depending on the kernel version, the capability bounding set is either
 777 a system-wide attribute, or a per-process attribute.
 778 .PP
 779 .B "Capability bounding set prior to Linux 2.6.25"
 780 .PP
 781 In kernels before 2.6.25, the capability bounding set is a system-wide
 782 attribute that affects all threads on the system.
 783 The bounding set is accessible via the file
 784 .IR /proc/sys/kernel/cap-bound .
 785 (Confusingly, this bit mask parameter is expressed as a
 786 signed decimal number in
 787 .IR /proc/sys/kernel/cap-bound .)
 788
 789 Only the
 790 .B init
 791 process may set capabilities in the capability bounding set;
 792 other than that, the superuser (more precisely: programs with the
 793 .B CAP_SYS_MODULE
 794 capability) may only clear capabilities from this set.
 795
 796 On a standard system the capability bounding set always masks out the
 797 .B CAP_SETPCAP
 798 capability.
 799 To remove this restriction (dangerous!), modify the definition of
 800 .B CAP_INIT_EFF_SET
 801 in
 802 .I include/linux/capability.h
 803 and rebuild the kernel.
 804
 805 The system-wide capability bounding set feature was added
 806 to Linux starting with kernel version 2.2.11.
 807 .\"
 808 .PP
 809 .B "Capability bounding set from Linux 2.6.25 onward"
 810 .PP
 811 From Linux 2.6.25, the
 812 .I "capability bounding set"
 813 is a per-thread attribute.
 814 (There is no longer a system-wide capability bounding set.)
 815
 816 The bounding set is inherited at
 817 .BR fork (2)
 818 from the thread's parent, and is preserved across an
 819 .BR execve (2).
 820
 821 A thread may remove capabilities from its capability bounding set using the
 822 .BR prctl (2)
 823 .B PR_CAPBSET_DROP
 824 operation, provided it has the
 825 .B CAP_SETPCAP
 826 capability.
 827 Once a capability has been dropped from the bounding set,
 828 it cannot be restored to that set.
 829 A thread can determine if a capability is in its bounding set using the
 830 .BR prctl (2)
 831 .B PR_CAPBSET_READ
 832 operation.
 833
 834 Removing capabilities from the bounding set is only supported if file
 835 capabilities are compiled into the kernel.
 836 In kernels before Linux 2.6.33,
 837 file capabilities were an optional feature configurable via the
 838 CONFIG_SECURITY_FILE_CAPABILITIES
 839 option.
 840 Since Linux 2.6.33, the configuration option has been removed
 841 and file capabilities are always part of the kernel.
 842 When file capabilities are compiled into the kernel, the
 843 .B init
 844 process (the ancestor of all processes) begins with a full bounding set.
 845 If file capabilities are not compiled into the kernel, then
 846 .B init
 847 begins with a full bounding set minus
 848 .BR CAP_SETPCAP ,
 849 because this capability has a different meaning when there are
 850 no file capabilities.
 851
 852 Removing a capability from the bounding set does not remove it
 853 from the thread's inherited set.
 854 However it does prevent the capability from being added
 855 back into the thread's inherited set in the future.
 856 .\"
 857 .\"
 858 .SS Effect of user ID changes on capabilities
 859 To preserve the traditional semantics for transitions between
 860 0 and nonzero user IDs,
 861 the kernel makes the following changes to a thread's capability
 862 sets on changes to the thread's real, effective, saved set,
 863 and file system user IDs (using
 864 .BR setuid (2),
 865 .BR setresuid (2),
 866 or similar):
 867 .IP 1. 3
 868 If one or more of the real, effective or saved set user IDs
 869 was previously 0, and as a result of the UID changes all of these IDs
 870 have a nonzero value,
 871 then all capabilities are cleared from the permitted and effective
 872 capability sets.
 873 .IP 2.
 874 If the effective user ID is changed from 0 to nonzero,
 875 then all capabilities are cleared from the effective set.
 876 .IP 3.
 877 If the effective user ID is changed from nonzero to 0,
 878 then the permitted set is copied to the effective set.
 879 .IP 4.
 880 If the file system user ID is changed from 0 to nonzero (see
 881 .BR setfsuid (2))
 882 then the following capabilities are cleared from the effective set:
 883 .BR CAP_CHOWN ,
 884 .BR CAP_DAC_OVERRIDE ,
 885 .BR CAP_DAC_READ_SEARCH ,
 886 .BR CAP_FOWNER ,
 887 .BR CAP_FSETID ,
 888 .B CAP_LINUX_IMMUTABLE
 889 (since Linux 2.2.30),
 890 .BR CAP_MAC_OVERRIDE ,
 891 and
 892 .B CAP_MKNOD
 893 (since Linux 2.2.30).
 894 If the file system UID is changed from nonzero to 0,
 895 then any of these capabilities that are enabled in the permitted set
 896 are enabled in the effective set.
 897 .PP
 898 If a thread that has a 0 value for one or more of its user IDs wants
 899 to prevent its permitted capability set being cleared when it resets
 900 all of its user IDs to nonzero values, it can do so using the
 901 .BR prctl (2)
 902 .B PR_SET_KEEPCAPS
 903 operation.
 904 .\"
 905 .SS Programmatically adjusting capability sets
 906 A thread can retrieve and change its capability sets using the
 907 .BR capget (2)
 908 and
 909 .BR capset (2)
 910 system calls.
 911 However, the use of
 912 .BR cap_get_proc (3)
 913 and
 914 .BR cap_set_proc (3),
 915 both provided in the
 916 .I libcap
 917 package,
 918 is preferred for this purpose.
 919 The following rules govern changes to the thread capability sets:
 920 .IP 1. 3
 921 If the caller does not have the
 922 .B CAP_SETPCAP
 923 capability,
 924 the new inheritable set must be a subset of the combination
 925 of the existing inheritable and permitted sets.
 926 .IP 2.
 927 (Since kernel 2.6.25)
 928 The new inheritable set must be a subset of the combination of the
 929 existing inheritable set and the capability bounding set.
 930 .IP 3.
 931 The new permitted set must be a subset of the existing permitted set
 932 (i.e., it is not possible to acquire permitted capabilities
 933 that the thread does not currently have).
 934 .IP 4.
 935 The new effective set must be a subset of the new permitted set.
 936 .SS The """securebits"" flags: establishing a capabilities-only environment
 937 .\" For some background:
 938 .\"       see http://lwn.net/Articles/280279/ and
 939 .\"       http://article.gmane.org/gmane.linux.kernel.lsm/5476/
 940 Starting with kernel 2.6.26,
 941 and with a kernel in which file capabilities are enabled,
 942 Linux implements a set of per-thread
 943 .I securebits
 944 flags that can be used to disable special handling of capabilities for UID 0
 945 .RI ( root ).
 946 These flags are as follows:
 947 .TP
 948 .B SECBIT_KEEP_CAPS
 949 Setting this flag allows a thread that has one or more 0 UIDs to retain
 950 its capabilities when it switches all of its UIDs to a nonzero value.
 951 If this flag is not set,
 952 then such a UID switch causes the thread to lose all capabilities.
 953 This flag is always cleared on an
 954 .BR execve (2).
 955 (This flag provides the same functionality as the older
 956 .BR prctl (2)
 957 .B PR_SET_KEEPCAPS
 958 operation.)
 959 .TP
 960 .B SECBIT_NO_SETUID_FIXUP
 961 Setting this flag stops the kernel from adjusting capability sets when
 962 the threads's effective and file system UIDs are switched between
 963 zero and nonzero values.
 964 (See the subsection
 965 .IR "Effect of User ID Changes on Capabilities" .)
 966 .TP
 967 .B SECBIT_NOROOT
 968 If this bit is set, then the kernel does not grant capabilities
 969 when a set-user-ID-root program is executed, or when a process with
 970 an effective or real UID of 0 calls
 971 .BR execve (2).
 972 (See the subsection
 973 .IR "Capabilities and execution of programs by root" .)
 974 .PP
 975 Each of the above "base" flags has a companion "locked" flag.
 976 Setting any of the "locked" flags is irreversible,
 977 and has the effect of preventing further changes to the
 978 corresponding "base" flag.
 979 The locked flags are:
 980 .BR SECBIT_KEEP_CAPS_LOCKED ,
 981 .BR SECBIT_NO_SETUID_FIXUP_LOCKED ,
 982 and
 983 .BR SECBIT_NOROOT_LOCKED .
 984 .PP
 985 The
 986 .I securebits
 987 flags can be modified and retrieved using the
 988 .BR prctl (2)
 989 .B PR_SET_SECUREBITS
 990 and
 991 .B PR_GET_SECUREBITS
 992 operations.
 993 The
 994 .B CAP_SETPCAP
 995 capability is required to modify the flags.
 996
 997 The
 998 .I securebits
 999 flags are inherited by child processes.
1000 During an
1001 .BR execve (2),
1002 all of the flags are preserved, except
1003 .B SECBIT_KEEP_CAPS
1004 which is always cleared.
1005
1006 An application can use the following call to lock itself,
1007 and all of its descendants,
1008 into an environment where the only way of gaining capabilities
1009 is by executing a program with associated file capabilities:
1010 .in +4n
1011 .nf
1012
1013 prctl(PR_SET_SECUREBITS,
1014         SECBIT_KEEP_CAPS_LOCKED |
1015         SECBIT_NO_SETUID_FIXUP |
1016         SECBIT_NO_SETUID_FIXUP_LOCKED |
1017         SECBIT_NOROOT |
1018         SECBIT_NOROOT_LOCKED);
1019 .fi
1020 .in
1021 .SH CONFORMING TO
1022 .PP
1023 No standards govern capabilities, but the Linux capability implementation
1024 is based on the withdrawn POSIX.1e draft standard; see
1025 .UR http://wt.tuxomania.net\:/publications\:/posix.1e/
1026 .UE .
1027 .SH NOTES
1028 Since kernel 2.5.27, capabilities are an optional kernel component,
1029 and can be enabled/disabled via the CONFIG_SECURITY_CAPABILITIES
1030 kernel configuration option.
1031
1032 The
1033 .I /proc/PID/task/TID/status
1034 file can be used to view the capability sets of a thread.
1035 The
1036 .I /proc/PID/status
1037 file shows the capability sets of a process's main thread.
1038
1039 The
1040 .I libcap
1041 package provides a suite of routines for setting and
1042 getting capabilities that is more comfortable and less likely
1043 to change than the interface provided by
1044 .BR capset (2)
1045 and
1046 .BR capget (2).
1047 This package also provides the
1048 .BR setcap (8)
1049 and
1050 .BR getcap (8)
1051 programs.
1052 It can be found at
1053 .br
1054 .UR http://www.kernel.org\:/pub\:/linux\:/libs\:/security\:/linux-privs
1055 .UE .
1056
1057 Before kernel 2.6.24, and since kernel 2.6.24 if
1058 file capabilities are not enabled, a thread with the
1059 .B CAP_SETPCAP
1060 capability can manipulate the capabilities of threads other than itself.
1061 However, this is only theoretically possible,
1062 since no thread ever has
1063 .BR CAP_SETPCAP
1064 in either of these cases:
1065 .IP * 2
1066 In the pre-2.6.25 implementation the system-wide capability bounding set,
1067 .IR /proc/sys/kernel/cap-bound ,
1068 always masks out this capability, and this can not be changed
1069 without modifying the kernel source and rebuilding.
1070 .IP *
1071 If file capabilities are disabled in the current implementation, then
1072 .B init
1073 starts out with this capability removed from its per-process bounding
1074 set, and that bounding set is inherited by all other processes
1075 created on the system.
1076 .SH SEE ALSO
1077 .BR capget (2),
1078 .BR prctl (2),
1079 .BR setfsuid (2),
1080 .BR cap_clear (3),
1081 .BR cap_copy_ext (3),
1082 .BR cap_from_text (3),
1083 .BR cap_get_file (3),
1084 .BR cap_get_proc (3),
1085 .BR cap_init (3),
1086 .BR capgetp (3),
1087 .BR capsetp (3),
1088 .BR libcap (3),
1089 .BR credentials (7),
1090 .BR pthreads (7),
1091 .BR getcap (8),
1092 .BR setcap (8)
1093 .PP
1094 .I include/linux/capability.h
1095 in the Linux kernel source tree