capabilities(7) - overview of Linux capabilities



  • CAPABILITIES(7) 		  Linux Programmer's Manual		      CAPABILITIES(7)
    
    NAME
           capabilities - overview of Linux capabilities
    
    DESCRIPTION
           For the purpose of performing permission checks, traditional UNIX implementations dis‐
           tinguish 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 unprivi‐
           leged processes are subject to full permission checking based on the process's creden‐
           tials (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 abbrevia‐
    	      tion 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;
    	      * 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 capabil‐
    	      ity from the calling thread's bounding set to its inheritable set;  drop	capa‐
    	      bilities	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),  setre‐
    		suid(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), swapon(2), swapoff(2),  sethostname(2),  and  setdomain‐
    		name(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 capabil‐
    		ity);
    	      * 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;
    	      * use ptrace(2) PTRACE_SECCOMP_GET_FILTER to dump a tracees seccomp filters;
    	      * 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 ran‐
    		dom(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 sec‐
    		comp protections (i.e., the PTRACE_O_SUSPEND_SECCOMP flag).
    	      * perform administrative operations on many device drivers.
    
           CAP_SYS_BOOT
    	      Use reboot(2) and kexec_load(2).
    
           CAP_SYS_CHROOT
    	      Use chroot(2).
    
           CAP_SYS_MODULE
    	      * Load and unload kernel modules (see init_module(2) and delete_module(2));
    	      * in  kernels  before 2.6.25: drop capabilities from the system-wide capability
    		bounding set.
    
           CAP_SYS_NICE
    	      * Raise process nice value (nice(2), setpriority(2)) and change the nice	value
    		for arbitrary processes;
    	      * set  real-time	scheduling  policies  for calling process, and set scheduling
    		policies  and  priorities  for	arbitrary  processes  (sched_setscheduler(2),
    		sched_setparam(2), shed_setattr(2));
    	      * set CPU affinity for arbitrary processes (sched_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 (hard‐
    	      ware) clock.
    
           CAP_SYS_TTY_CONFIG
    	      Use vhangup(2); employ various privileged ioctl(2) operations on virtual termi‐
    	      nals.
    
           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 fea‐
    	  ture, 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 use along  with  the  new
    	  feature.   If  the  new feature is useless without these other features, you should
    	  use the same capability as the other features.
    
           *  Don't choose CAP_SYS_ADMIN if you can possibly avoid	it!   A  vast  proportion  of
    	  existing  capability	checks	are  associated with this capability (see the partial
    	  list above).	It can plausibly be called "the new root", since on the one hand,  it
    	  confers  a  wide range of powers, and on the other hand, its broad scope means that
    	  this is the capability that is required by many privileged  programs.   Don't  make
    	  the  problem	worse.	 The  only  new  features  that  should  be  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 exam‐
    	  ple, the addition of the highly specific  CAP_SYS_PACCT  was	probably  a  mistake.
    	  Instead,  try to identify and name your new capability as a broader silo into which
    	  other related future use cases might fit.
    
       Thread capability sets
           Each thread has three capability sets containing zero or more of the  above  capabili‐
           ties:
    
           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 capabil‐
    	      ity 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 capa‐
    	      bilities	remain	inheritable when executing any program, and inheritable capa‐
    	      bilities are added to the permitted set when executing a program that  has  the
    	      corresponding bits set in the file inheritable set.
    
    	      Because  inheritable  capabilities are not generally preserved across execve(2)
    	      when running as a non-root user, applications that wish to run helper  programs
    	      with   elevated	capabilities  should  consider	using  ambient	capabilities,
    	      described below.
    
           Effective:
    	      This is the set of capabilities used by the kernel to perform permission checks
    	      for the thread.
    
           Ambient (since Linux 4.3):
    	      This  is a set of capabilities that are preserved across an execve(2) of a pro‐
    	      gram 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 inheri‐
    	      table.
    
    	      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.
    
           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  exe‐
           cutable	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  inheri‐
    	      table  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 dur‐
    	      ing  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  per‐
    	      mitted  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.
    
       Transformation of capabilities during execve()
           During an execve(2), the kernel calculates the new capabilities of the  process	using
           the following algorithm:
    
    	   P'(ambient)	   = (file is privileged) ? 0 : P(ambient)
    
    	   P'(permitted)   = (P(inheritable) & F(inheritable)) |
    			     (F(permitted) & cap_bset) | P'(ambient)
    
    	   P'(effective)   = F(effective) ? P'(permitted) : P'(ambient)
    
    	   P'(inheritable) = P(inheritable)    [i.e., unchanged]
    
           where:
    
    	   P	     denotes the value of a thread capability set before the execve(2)
    
    	   P'	     denotes the value of a thread capability set after the execve(2)
    
    	   F	     denotes a file capability set
    
    	   cap_bset  is the value of the capability bounding set (described below).
    
           A  privileged file is one that has capabilities or has the set-user-ID or set-group-ID
           bit set.
    
           Note: the capability transitions described above may  not  be  performed  (i.e.,  file
           capabilities  may be ignored) for the same reasons that the set-user-ID and set-group-
           ID bits are ignored; see execve(2).
    
           Note: according to the rules above, if a process with nonzero  user  IDs  performs  an
           execve(2)  then	any capabilities that are present in its permitted and effective sets
           will be cleared.  For the treatment of 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 capabili‐
           ties,  but has not been converted to use the libcap(3) API to manipulate its capabili‐
           ties.  (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 capa‐
           bility 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 capa‐
           bilities 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  exe‐
           cuted  with  less  privilege that it needs.  Note that, by definition, the application
           could not itself recognize this problem, since it does not employ the libcap(3) API.
    
       Capabilities and execution of programs by root
           In order to provide an all-powerful root using capability sets, during an execve(2):
    
           1. If a set-user-ID-root program is being executed, or the real or effective  user  ID
    	  of the process is 0 (root) then the file inheritable and permitted sets are defined
    	  to be all ones (i.e., all capabilities enabled).
    
           2. If a set-user-ID-root program is being executed, or the effective user  ID  of  the
    	  process is 0 (root) then the file effective bit is defined to be one (enabled).
    
           The  upshot  of	the  above  rules,  combined  with  the  capabilities transformations
           described above, is as follows:
    
           *  When a process execve(2)s a set-user-ID-root program, or when  a  process  with  an
    	  effective UID of 0 execve(2)s a program, it gains all capabilities in its permitted
    	  and effective capability sets, except those masked out by the  capability  bounding
    	  set.
    
           *  When a process with a real UID of 0 execve(2)s a program, it gains all capabilities
    	  in its permitted capability set, except those masked out by the capability bounding
    	  set.
    
           The  above  steps  yield  semantics that are the same as those provided by traditional
           UNIX systems.
    
       Set-user-ID-root programs that have file capabilities
           Executing a program that is both set-user-ID root and has file capabilities will cause
           the  process to gain just the capabilities granted by the program (i.e., not all capa‐
           bilities, 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 permit‐
    	 ted capability set.  The capability bounding set thus places a limit on the  permit‐
    	 ted 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  inheri‐
           table capabilities.  If a thread maintains a capability in its inheritable set that is
           not in its bounding set, then it can still gain that capability in its  permitted  set
           by executing a file that has the capability in its inheritable set.
    
           Depending  on  the kernel version, the capability bounding set is either a system-wide
           attribute, or a per-process attribute.
    
           Capability bounding set prior to Linux 2.6.25
    
           In kernels before 2.6.25, the capability bounding set is a system-wide attribute  that
           affects	all  threads  on  the  system.	 The  bounding set is accessible via the file
           /proc/sys/kernel/cap-bound.  (Confusingly, this bit mask parameter is expressed	as  a
           signed decimal number in /proc/sys/kernel/cap-bound.)
    
           Only  the init process may set capabilities in the capability bounding set; other than
           that, the superuser (more precisely: a process with the CAP_SYS_MODULE capability) may
           only clear capabilities from this set.
    
           On  a  standard	system	the  capability bounding set always masks out the CAP_SETPCAP
           capability.  To	remove	this  restriction  (dangerous!),  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 ker‐
           nel version 2.2.11.
    
           Capability bounding set from Linux 2.6.25 onward
    
           From Linux 2.6.25, the capability bounding set is a per-thread attribute.   (There  is
           no longer a system-wide capability bounding set.)
    
           The  bounding  set  is inherited at fork(2) from the thread's parent, and is preserved
           across an execve(2).
    
           A thread may remove capabilities from its capability bounding set using	the  prctl(2)
           PR_CAPBSET_DROP	operation,  provided it has the CAP_SETPCAP capability.  Once a capa‐
           bility 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.
    
       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),  setre‐
           suid(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_OVER‐
    	  RIDE, 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 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 spe‐
           cial 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	capa‐
    	      bilities	in  its permitted and effective sets when it switches all of its UIDs
    	      to nonzero values.  If this flag is not set, then such a UID switch causes  the
    	      thread  to lose all capabilities in those sets.  This flag is always cleared on
    	      an execve(2).
    
    	      The   setting   of   the	 SECBIT_KEEP_CAPS   flag   is	 ignored    if	  the
    	      SECBIT_NO_SETUID_FIXUP  flag  is	set.  (The latter flag provides a superset of
    	      the effect of the former flag.)
    
    	      This flag provides the same functionality as the older prctl(2) PR_SET_KEEPCAPS
    	      operation.
    
           SECBIT_NO_SETUID_FIXUP
    	      Setting  this  flag  stops  the  kernel from adjusting the process's permitted,
    	      effective, and ambient capability sets when the thread's effective and filesys‐
    	      tem  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_AMBI‐
           ENT_RAISE_LOCKED.
    
           The  securebits	flags  can   be   modified   and   retrieved   using   the   prctl(2)
           PR_SET_SECUREBITS  and  PR_GET_SECUREBITS  operations.	The CAP_SETPCAP capability is
           required to modify the flags.
    
           The securebits flags are inherited by child processes.  During an  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  pro‐
           gram with associated file capabilities:
    
    	   prctl(PR_SET_SECUREBITS,
    		/* SECBIT_KEEP_CAPS off */
    		   SECBIT_KEEP_CAPS_LOCKED |
    		   SECBIT_NO_SETUID_FIXUP |
    		   SECBIT_NO_SETUID_FIXUP_LOCKED |
    		   SECBIT_NOROOT |
    		   SECBIT_NOROOT_LOCKED);
    		   /* Setting/locking SECBIT_NO_CAP_AMBIENT_RAISE
    		      is not required */
    
       Interaction with user namespaces
           For  a  discussion  of  the  interaction  of  capabilities  and	user  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	⟨http://wt.tuxomania.net/publications
           /posix.1e/⟩.
    
    NOTES
           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 configura‐
           tion 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)  pro‐
           grams.  It can be found at
           ⟨http://www.kernel.org/pub/linux/libs/security/linux-privs⟩.
    
           Before kernel 2.6.24, and from kernel 2.6.24 to kernel 2.6.32 if file capabilities are
           not enabled, a thread with the CAP_SETPCAP capability can manipulate the  capabilities
           of  threads other than itself.  However, this is only theoretically possible, since no
           thread ever has CAP_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 rebuilding.
    
           * If file capabilities are disabled in the current implementation,  then  init  starts
    	 out  with this capability removed from its per-process bounding set, and that bound‐
    	 ing 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 4.15 of the Linux man-pages project.  A description of
           the project, information about reporting bugs, and the latest version  of  this	page,
           can be found at https://www.kernel.org/doc/man-pages/.
    
    Linux					  2018-02-02			      CAPABILITIES(7)
    

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