.\" Copyright (c) 2002 by Michael Kerrisk .\" .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" .\" 6 Aug 2002 - Initial Creation .\" Modified 2003-05-23, Michael Kerrisk, .\" Modified 2004-05-27, Michael Kerrisk, .\" 2004-12-08, mtk Added O_NOATIME for CAP_FOWNER .\" 2005-08-16, mtk, Added CAP_AUDIT_CONTROL and CAP_AUDIT_WRITE .\" 2008-07-15, Serge Hallyn .\" Document file capabilities, per-process capability .\" bounding set, changed semantics for CAP_SETPCAP, .\" and other changes in 2.6.2[45]. .\" Add CAP_MAC_ADMIN, CAP_MAC_OVERRIDE, CAP_SETFCAP. .\" 2008-07-15, mtk .\" Add text describing circumstances in which CAP_SETPCAP .\" (theoretically) permits a thread to change the .\" capability sets of another thread. .\" Add section describing rules for programmatically .\" adjusting thread capability sets. .\" Describe rationale for capability bounding set. .\" Document "securebits" flags. .\" Add text noting that if we set the effective flag for one file .\" capability, then we must also set the effective flag for all .\" other capabilities where the permitted or inheritable bit is set. .\" 2011-09-07, mtk/Serge hallyn: Add CAP_SYSLOG .\" .TH CAPABILITIES 7 2012-10-15 "Linux" "Linux Programmer's Manual" .SH NAME capabilities \- overview of Linux capabilities .SH DESCRIPTION For the purpose of performing permission checks, traditional UNIX implementations distinguish two categories of processes: .I privileged processes (whose effective user ID is 0, referred to as superuser or root), and .I unprivileged processes (whose effective UID is nonzero). Privileged processes bypass all kernel permission checks, while unprivileged processes are subject to full permission checking based on the process's credentials (usually: effective UID, effective GID, and supplementary group list). Starting with kernel 2.2, Linux divides the privileges traditionally associated with superuser into distinct units, known as .IR capabilities , which can be independently enabled and disabled. Capabilities are a per-thread attribute. .\" .SS Capabilities List The following list shows the capabilities implemented on Linux, and the operations or behaviors that each capability permits: .TP .BR CAP_AUDIT_CONTROL " (since Linux 2.6.11)" Enable and disable kernel auditing; change auditing filter rules; retrieve auditing status and filtering rules. .TP .BR CAP_AUDIT_WRITE " (since Linux 2.6.11)" Write records to kernel auditing log. .TP .BR CAP_BLOCK_SUSPEND " (since Linux 3.5)" Employ features that can block system suspend .RB ( epoll (7) .BR EPOLLWAKEUP , .IR /proc/sys/wake_lock ). .TP .B CAP_CHOWN Make arbitrary changes to file UIDs and GIDs (see .BR chown (2)). .TP .B CAP_DAC_OVERRIDE Bypass file read, write, and execute permission checks. (DAC is an abbreviation of "discretionary access control".) .TP .B CAP_DAC_READ_SEARCH Bypass file read permission checks and directory read and execute permission checks. .TP .B CAP_FOWNER .PD 0 .RS .IP * 2 Bypass permission checks on operations that normally require the file system UID of the process to match the UID of the file (e.g., .BR chmod (2), .BR utime (2)), excluding those operations covered by .B CAP_DAC_OVERRIDE and .BR CAP_DAC_READ_SEARCH ; .IP * set extended file attributes (see .BR chattr (1)) on arbitrary files; .IP * set Access Control Lists (ACLs) on arbitrary files; .IP * ignore directory sticky bit on file deletion; .IP * specify .B O_NOATIME for arbitrary files in .BR open (2) and .BR fcntl (2). .RE .PD .TP .B CAP_FSETID Don't clear set-user-ID and set-group-ID permission bits when a file is modified; set the set-group-ID bit for a file whose GID does not match the file system or any of the supplementary GIDs of the calling process. .TP .B CAP_IPC_LOCK .\" FIXME As at Linux 3.2, there are some strange uses of this capability .\" in other places; they probably should be replaced with something else. Lock memory .RB ( mlock (2), .BR mlockall (2), .BR mmap (2), .BR shmctl (2)). .TP .B CAP_IPC_OWNER Bypass permission checks for operations on System V IPC objects. .TP .B CAP_KILL Bypass permission checks for sending signals (see .BR kill (2)). This includes use of the .BR ioctl (2) .B KDSIGACCEPT operation. .\" FIXME CAP_KILL also has an effect for threads + setting child .\" termination signal to other than SIGCHLD: without this .\" capability, the termination signal reverts to SIGCHLD .\" if the child does an exec(). What is the rationale .\" for this? .TP .BR CAP_LEASE " (since Linux 2.4)" Establish leases on arbitrary files (see .BR fcntl (2)). .TP .B CAP_LINUX_IMMUTABLE Set the .B FS_APPEND_FL and .B FS_IMMUTABLE_FL .\" These attributes are now available on ext2, ext3, Reiserfs, XFS, JFS i-node flags (see .BR chattr (1)). .TP .BR CAP_MAC_ADMIN " (since Linux 2.6.25)" Override Mandatory Access Control (MAC). Implemented for the Smack Linux Security Module (LSM). .TP .BR CAP_MAC_OVERRIDE " (since Linux 2.6.25)" Allow MAC configuration or state changes. Implemented for the Smack LSM. .TP .BR CAP_MKNOD " (since Linux 2.4)" Create special files using .BR mknod (2). .TP .B CAP_NET_ADMIN Perform various network-related operations: .PD 0 .RS .IP * 2 interface configuration; .IP * administration of IP firewall, masquerading, and accounting .IP * modify routing tables; .IP * bind to any address for transparent proxying; .IP * set type-of-service (TOS) .IP * clear driver statistics; .IP * set promiscuous mode; .IP * enabling multicasting; .IP * use .BR setsockopt (2) to set the following socket options: .BR SO_DEBUG , .BR SO_MARK , .BR SO_PRIORITY (for a priority outside the range 0 to 6), .BR SO_RCVBUFFORCE , and .BR SO_SNDBUFFORCE . .RE .PD .TP .B CAP_NET_BIND_SERVICE Bind a socket to Internet domain privileged ports (port numbers less than 1024). .TP .B CAP_NET_BROADCAST (Unused) Make socket broadcasts, and listen to multicasts. .TP .B CAP_NET_RAW .PD 0 .RS .IP * 2 use RAW and PACKET sockets; .IP * bind to any address for transparent proxying. .RE .PD .\" Also various IP options and setsockopt(SO_BINDTODEVICE) .TP .B CAP_SETGID Make arbitrary manipulations of process GIDs and supplementary GID list; forge GID when passing socket credentials via UNIX domain sockets. .TP .BR CAP_SETFCAP " (since Linux 2.6.24)" Set file capabilities. .TP .B CAP_SETPCAP If file capabilities are not supported: grant or remove any capability in the caller's permitted capability set to or from any other process. (This property of .B CAP_SETPCAP is not available when the kernel is configured to support file capabilities, since .B CAP_SETPCAP has entirely different semantics for such kernels.) If file capabilities are supported: add any capability from the calling thread's bounding set to its inheritable set; drop capabilities from the bounding set (via .BR prctl (2) .BR PR_CAPBSET_DROP ); make changes to the .I securebits flags. .TP .B CAP_SETUID Make arbitrary manipulations of process UIDs .RB ( setuid (2), .BR setreuid (2), .BR setresuid (2), .BR setfsuid (2)); make forged UID when passing socket credentials via UNIX domain sockets. .\" FIXME CAP_SETUID also an effect in exec(); document this. .TP .B CAP_SYS_ADMIN .PD 0 .RS .IP * 2 Perform a range of system administration operations including: .BR quotactl (2), .BR mount (2), .BR umount (2), .BR swapon (2), .BR swapoff (2), .BR sethostname (2), and .BR setdomainname (2); .IP * perform privileged .BR syslog (2) operations (since Linux 2.6.37, .BR CAP_SYSLOG should be used to permit such operations); .IP * perform .B VM86_REQUEST_IRQ .BR vm86 (2) command; .IP * perform .B IPC_SET and .B IPC_RMID operations on arbitrary System V IPC objects; .IP * perform operations on .I trusted and .I security Extended Attributes (see .BR attr (5)); .IP * use .BR lookup_dcookie (2); .IP * use .BR ioprio_set (2) to assign .B IOPRIO_CLASS_RT and (before Linux 2.6.25) .B IOPRIO_CLASS_IDLE I/O scheduling classes; .IP * forge UID when passing socket credentials; .IP * exceed .IR /proc/sys/fs/file-max , the system-wide limit on the number of open files, in system calls that open files (e.g., .BR accept (2), .BR execve (2), .BR open (2), .BR pipe (2)); .IP * employ .B CLONE_* flags that create new namespaces with .BR clone (2) and .BR unshare (2); .IP * call .BR perf_event_open (2); .IP * access privileged .I perf event information; .IP * call .BR setns (2); .IP * call .BR fanotify_init (2); .IP * perform .B KEYCTL_CHOWN and .B KEYCTL_SETPERM .BR keyctl (2) operations; .IP * perform .BR madvise (2) .B MADV_HWPOISON operation; .IP * employ the .B TIOCSTI .BR ioctl (2) to insert characters into the input queue of a terminal other than the caller's controlling terminal. .IP * employ the obsolete .BR nfsservctl (2) system call; .IP * employ the obsolete .BR bdflush (2) system call; .IP * perform various privileged block-device .BR ioctl (2) operations; .IP * perform various privileged file-system .BR ioctl (2) operations; .IP * perform administrative operations on many device drivers. .RE .PD .TP .B CAP_SYS_BOOT Use .BR reboot (2) and .BR kexec_load (2). .TP .B CAP_SYS_CHROOT Use .BR chroot (2). .TP .B CAP_SYS_MODULE Load and unload kernel modules (see .BR init_module (2) and .BR delete_module (2)); in kernels before 2.6.25: drop capabilities from the system-wide capability bounding set. .TP .B CAP_SYS_NICE .PD 0 .RS .IP * 2 Raise process nice value .RB ( nice (2), .BR setpriority (2)) and change the nice value for arbitrary processes; .IP * set real-time scheduling policies for calling process, and set scheduling policies and priorities for arbitrary processes .RB ( sched_setscheduler (2), .BR sched_setparam (2)); .IP * set CPU affinity for arbitrary processes .RB ( sched_setaffinity (2)); .IP * set I/O scheduling class and priority for arbitrary processes .RB ( ioprio_set (2)); .IP * apply .BR migrate_pages (2) to arbitrary processes and allow processes to be migrated to arbitrary nodes; .\" FIXME CAP_SYS_NICE also has the following effect for .\" migrate_pages(2): .\" do_migrate_pages(mm, &old, &new, .\" capable(CAP_SYS_NICE) ? MPOL_MF_MOVE_ALL : MPOL_MF_MOVE); .IP * apply .BR move_pages (2) to arbitrary processes; .IP * use the .B MPOL_MF_MOVE_ALL flag with .BR mbind (2) and .BR move_pages (2). .RE .PD .TP .B CAP_SYS_PACCT Use .BR acct (2). .TP .B CAP_SYS_PTRACE Trace arbitrary processes using .BR ptrace (2); apply .BR get_robust_list (2) to arbitrary processes. .TP .B CAP_SYS_RAWIO Perform I/O port operations .RB ( iopl (2) and .BR ioperm (2)); access .IR /proc/kcore ; employ the .B FIBMAP .BR ioctl (2) operation. .TP .B CAP_SYS_RESOURCE .PD 0 .RS .IP * 2 Use reserved space on ext2 file systems; .IP * make .BR ioctl (2) calls controlling ext3 journaling; .IP * override disk quota limits; .IP * increase resource limits (see .BR setrlimit (2)); .IP * override .B RLIMIT_NPROC resource limit; .IP * override maximum number of consoles on console allocation; .IP * override maximum number of keymaps; .IP * allow more than 64hz interrupts from the real-time clock; .IP * raise .I msg_qbytes limit for a System V message queue above the limit in .I /proc/sys/kernel/msgmnb (see .BR msgop (2) and .BR msgctl (2)); .IP * override the .I /proc/sys/fs/pipe-size-max limit when setting the capacity of a pipe using the .B F_SETPIPE_SZ .BR fcntl (2) command. .IP * use .BR F_SETPIPE_SZ to increase the capacity of a pipe above the limit specified by .IR /proc/sys/fs/pipe-max-size ; .IP * override .I /proc/sys/fs/mqueue/queues_max limit when creating POSIX message queues (see .BR mq_overview (7)); .IP * employ .BR prctl (2) .B PR_SET_MM operation. .RE .PD .TP .B CAP_SYS_TIME Set system clock .RB ( settimeofday (2), .BR stime (2), .BR adjtimex (2)); set real-time (hardware) clock. .TP .B CAP_SYS_TTY_CONFIG Use .BR vhangup (2); employ various privileged .BR ioctl (2) operations on virtual terminals. .TP .BR CAP_SYSLOG " (since Linux 2.6.37)" .IP * 3 Perform privileged .BR syslog (2) operations. See .BR syslog (2) for information on which operations require privilege. .IP * View kernel addresses exposed via .I /proc and other interfaces when .IR /proc/sys/kernel/kptr_restrict has the value 1. (See the discussion of the .I kptr_restrict in .BR proc (5).) .TP .BR CAP_WAKE_ALARM " (since Linux 3.0)" Trigger something that will wake up the system (set .B CLOCK_REALTIME_ALARM and .B CLOCK_BOOTTIME_ALARM timers). .\" .SS Past and Current Implementation A full implementation of capabilities requires that: .IP 1. 3 For all privileged operations, the kernel must check whether the thread has the required capability in its effective set. .IP 2. The kernel must provide system calls allowing a thread's capability sets to be changed and retrieved. .IP 3. The file system must support attaching capabilities to an executable file, so that a process gains those capabilities when the file is executed. .PP Before kernel 2.6.24, only the first two of these requirements are met; since kernel 2.6.24, all three requirements are met. .\" .SS Thread Capability Sets Each thread has three capability sets containing zero or more of the above capabilities: .TP .IR 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 .B 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 .BR execve (2)s either a set-user-ID-root program, or a program whose associated file capabilities grant that capability). .TP .IR Inheritable : This is a set of capabilities preserved across an .BR execve (2). It provides a mechanism for a process to assign capabilities to the permitted set of the new program during an .BR execve (2). .TP .IR Effective : This is the set of capabilities used by the kernel to perform permission checks for the thread. .PP A child created via .BR fork (2) inherits copies of its parent's capability sets. See below for a discussion of the treatment of capabilities during .BR execve (2). .PP Using .BR capset (2), a thread may manipulate its own capability sets (see below). .\" .SS File Capabilities Since kernel 2.6.24, the kernel supports associating capability sets with an executable file using .BR setcap (8). The file capability sets are stored in an extended attribute (see .BR setxattr (2)) named .IR "security.capability" . Writing to this extended attribute requires the .BR CAP_SETFCAP capability. The file capability sets, in conjunction with the capability sets of the thread, determine the capabilities of a thread after an .BR execve (2). The three file capability sets are: .TP .IR Permitted " (formerly known as " forced ): These capabilities are automatically permitted to the thread, regardless of the thread's inheritable capabilities. .TP .IR 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 .BR execve (2). .TP .IR Effective : This is not a set, but rather just a single bit. If this bit is set, then during an .BR 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 .BR 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 .BR execve (2) (see the transformation rules described below) will also acquire that capability in its effective set. Therefore, when assigning capabilities to a file .RB ( setcap (8), .BR cap_set_file (3), .BR 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. .\" .SS Transformation of Capabilities During execve() .PP During an .BR execve (2), the kernel calculates the new capabilities of the process using the following algorithm: .in +4n .nf P'(permitted) = (P(inheritable) & F(inheritable)) | (F(permitted) & cap_bset) P'(effective) = F(effective) ? P'(permitted) : 0 P'(inheritable) = P(inheritable) [i.e., unchanged] .fi .in where: .RS 4 .IP P 10 denotes the value of a thread capability set before the .BR execve (2) .IP P' denotes the value of a capability set after the .BR execve (2) .IP F denotes a file capability set .IP cap_bset is the value of the capability bounding set (described below). .RE .\" .SS Capabilities and execution of programs by root In order to provide an all-powerful .I root using capability sets, during an .BR execve (2): .IP 1. 3 If a set-user-ID-root program is being executed, or the real 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). .IP 2. If a set-user-ID-root program is being executed, then the file effective bit is defined to be one (enabled). .PP The upshot of the above rules, combined with the capabilities transformations described above, is that when a process .BR execve (2)s a set-user-ID-root program, or when a process with an effective UID of 0 .BR 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. .\" If a process with real UID 0, and nonzero effective UID does an .\" exec(), then it gets all capabilities in its .\" permitted set, and no effective capabilities This provides semantics that are the same as those provided by traditional UNIX systems. .SS 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 .BR execve (2). The bounding set is used in the following ways: .IP * 2 During an .BR 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. .IP * (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 .BR 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 .BR execve (2)s a file that has the capability in its inheritable set. .PP 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. .PP Depending on the kernel version, the capability bounding set is either a system-wide attribute, or a per-process attribute. .PP .B "Capability bounding set prior to Linux 2.6.25" .PP 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 .IR /proc/sys/kernel/cap-bound . (Confusingly, this bit mask parameter is expressed as a signed decimal number in .IR /proc/sys/kernel/cap-bound .) Only the .B init process may set capabilities in the capability bounding set; other than that, the superuser (more precisely: programs with the .B CAP_SYS_MODULE capability) may only clear capabilities from this set. On a standard system the capability bounding set always masks out the .B CAP_SETPCAP capability. To remove this restriction (dangerous!), modify the definition of .B CAP_INIT_EFF_SET in .I 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. .\" .PP .B "Capability bounding set from Linux 2.6.25 onward" .PP From Linux 2.6.25, the .I "capability bounding set" is a per-thread attribute. (There is no longer a system-wide capability bounding set.) The bounding set is inherited at .BR fork (2) from the thread's parent, and is preserved across an .BR execve (2). A thread may remove capabilities from its capability bounding set using the .BR prctl (2) .B PR_CAPBSET_DROP operation, provided it has the .B CAP_SETPCAP capability. Once a capability has been dropped from the bounding set, it cannot be restored to that set. A thread can determine if a capability is in its bounding set using the .BR prctl (2) .B PR_CAPBSET_READ operation. Removing capabilities from the bounding set is only supported 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 .B init process (the ancestor of all processes) begins with a full bounding set. If file capabilities are not compiled into the kernel, then .B init begins with a full bounding set minus .BR 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 inherited set. However it does prevent the capability from being added back into the thread's inherited set in the future. .\" .\" .SS 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 file system user IDs (using .BR setuid (2), .BR setresuid (2), or similar): .IP 1. 3 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 and effective capability sets. .IP 2. If the effective user ID is changed from 0 to nonzero, then all capabilities are cleared from the effective set. .IP 3. If the effective user ID is changed from nonzero to 0, then the permitted set is copied to the effective set. .IP 4. If the file system user ID is changed from 0 to nonzero (see .BR setfsuid (2)) then the following capabilities are cleared from the effective set: .BR CAP_CHOWN , .BR CAP_DAC_OVERRIDE , .BR CAP_DAC_READ_SEARCH , .BR CAP_FOWNER , .BR CAP_FSETID , .B CAP_LINUX_IMMUTABLE (since Linux 2.2.30), .BR CAP_MAC_OVERRIDE , and .B CAP_MKNOD (since Linux 2.2.30). If the file system 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. .PP 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 .BR prctl (2) .B PR_SET_KEEPCAPS operation. .\" .SS Programmatically adjusting capability sets A thread can retrieve and change its capability sets using the .BR capget (2) and .BR capset (2) system calls. However, the use of .BR cap_get_proc (3) and .BR cap_set_proc (3), both provided in the .I libcap package, is preferred for this purpose. The following rules govern changes to the thread capability sets: .IP 1. 3 If the caller does not have the .B CAP_SETPCAP capability, the new inheritable set must be a subset of the combination of the existing inheritable and permitted sets. .IP 2. (Since kernel 2.6.25) The new inheritable set must be a subset of the combination of the existing inheritable set and the capability bounding set. .IP 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). .IP 4. The new effective set must be a subset of the new permitted set. .SS The """securebits"" flags: establishing a capabilities-only environment .\" For some background: .\" see http://lwn.net/Articles/280279/ and .\" http://article.gmane.org/gmane.linux.kernel.lsm/5476/ Starting with kernel 2.6.26, and with a kernel in which file capabilities are enabled, Linux implements a set of per-thread .I securebits flags that can be used to disable special handling of capabilities for UID 0 .RI ( root ). These flags are as follows: .TP .B SECBIT_KEEP_CAPS 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 .BR execve (2). (This flag provides the same functionality as the older .BR prctl (2) .B PR_SET_KEEPCAPS operation.) .TP .B SECBIT_NO_SETUID_FIXUP Setting this flag stops the kernel from adjusting capability sets when the threads's effective and file system UIDs are switched between zero and nonzero values. (See the subsection .IR "Effect of User ID Changes on Capabilities" .) .TP .B 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 .BR execve (2). (See the subsection .IR "Capabilities and execution of programs by root" .) .PP 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: .BR SECBIT_KEEP_CAPS_LOCKED , .BR SECBIT_NO_SETUID_FIXUP_LOCKED , and .BR SECBIT_NOROOT_LOCKED . .PP The .I securebits flags can be modified and retrieved using the .BR prctl (2) .B PR_SET_SECUREBITS and .B PR_GET_SECUREBITS operations. The .B CAP_SETPCAP capability is required to modify the flags. The .I securebits flags are inherited by child processes. During an .BR execve (2), all of the flags are preserved, except .B SECBIT_KEEP_CAPS which is always cleared. An application can use the following call to lock itself, and all of its descendants, into an environment where the only way of gaining capabilities is by executing a program with associated file capabilities: .in +4n .nf prctl(PR_SET_SECUREBITS, SECBIT_KEEP_CAPS_LOCKED | SECBIT_NO_SETUID_FIXUP | SECBIT_NO_SETUID_FIXUP_LOCKED | SECBIT_NOROOT | SECBIT_NOROOT_LOCKED); .fi .in .SH "CONFORMING TO" .PP No standards govern capabilities, but the Linux capability implementation is based on the withdrawn POSIX.1e draft standard; see .UR http://wt.xpilot.org\:/publications\:/posix.1e/ .UE . .SH NOTES Since kernel 2.5.27, capabilities are an optional kernel component, and can be enabled/disabled via the CONFIG_SECURITY_CAPABILITIES kernel configuration option. The .I /proc/PID/task/TID/status file can be used to view the capability sets of a thread. The .I /proc/PID/status file shows the capability sets of a process's main thread. The .I 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 .BR capset (2) and .BR capget (2). This package also provides the .BR setcap (8) and .BR getcap (8) programs. It can be found at .br .UR http://www.kernel.org\:/pub\:/linux\:/libs\:/security\:/linux-privs .UE . Before kernel 2.6.24, and since kernel 2.6.24 if file capabilities are not enabled, a thread with the .B CAP_SETPCAP capability can manipulate the capabilities of threads other than itself. However, this is only theoretically possible, since no thread ever has .BR CAP_SETPCAP in either of these cases: .IP * 2 In the pre-2.6.25 implementation the system-wide capability bounding set, .IR /proc/sys/kernel/cap-bound , always masks out this capability, and this can not be changed without modifying the kernel source and rebuilding. .IP * If file capabilities are disabled in the current implementation, then .B init starts out with this capability removed from its per-process bounding set, and that bounding set is inherited by all other processes created on the system. .SH "SEE ALSO" .BR capget (2), .BR prctl (2), .BR setfsuid (2), .BR cap_clear (3), .BR cap_copy_ext (3), .BR cap_from_text (3), .BR cap_get_file (3), .BR cap_get_proc (3), .BR cap_init (3), .BR capgetp (3), .BR capsetp (3), .BR libcap (3), .BR credentials (7), .BR pthreads (7), .BR getcap (8), .BR setcap (8) .PP .I include/linux/capability.h in the Linux kernel source tree .SH COLOPHON This page is part of release 3.44 of the Linux .I man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.