path: root/man7/capabilities.7

.\" Copyright (c) 2002 by Michael Kerrisk <mtk.manpages@gmail.com>
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.\" 6 Aug 2002 - Initial Creation
.\" Modified 2003-05-23, Michael Kerrisk, <mtk.manpages@gmail.com>
.\" Modified 2004-05-27, Michael Kerrisk, <mtk.manpages@gmail.com>
.\" 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 <serue@us.bbm.com>
.\"     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 2015-05-07 "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_READ " (since Linux 3.16)"
.\" commit a29b694aa1739f9d76538e34ae25524f9c549d59
.\" commit 3a101b8de0d39403b2c7e5c23fd0b005668acf48
Allow reading the audit log via a multicast netlink socket.
.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
.PD 0
.RS
.IP * 2
Bypass file read permission checks and
directory read and execute permission checks;
.IP *
Invoke
.BR open_by_handle_at (2).
.RE
.PD

.TP
.B CAP_FOWNER
.PD 0
.RS
.IP * 2
Bypass permission checks on operations that normally
require the filesystem 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 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.
.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
inode 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;
write a group ID mapping in a user namespace (see
.BR user_namespaces (7)).
.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));
forge UID when passing socket credentials via UNIX domain sockets;
write a user ID mapping in a user namespace (see
.BR user_namespaces (7)).
.\" 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 *
override
.B RLIMIT_NPROC
resource limit;
.IP *
perform operations on
.I trusted
and
.I security
Extended Attributes (see
.BR xattr (7));
.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 PID when passing socket credentials via UNIX domain sockets;
.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)
(but, since Linux 3.8,
creating user namespaces does not require any capability);
.IP *
call
.BR perf_event_open (2);
.IP *
access privileged
.I perf
event information;
.IP *
call
.BR setns (2)
(requires
.B CAP_SYS_ADMIN
in the
.I target
namespace);
.IP *
call
.BR fanotify_init (2);
.IP *
call
.BR bpf (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 filesystem
.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),
.BR shed_setattr (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);
.\" Document this.
.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
.PD 0
.RS
.IP * 3
Trace arbitrary processes using
.BR ptrace (2);
.IP *
apply
.BR get_robust_list (2)
to arbitrary processes;
.IP *
transfer data to or from the memory of arbitrary processes using
.BR process_vm_readv (2)
and
.BR process_vm_writev (2).
.IP *
inspect processes using
.BR kcmp (2).
.RE
.PD
.TP
.B CAP_SYS_RAWIO
.PD 0
.RS
.IP * 2
Perform I/O port operations
.RB ( iopl (2)
and
.BR ioperm (2));
.IP *
access
.IR /proc/kcore ;
.IP *
employ the
.B FIBMAP
.BR ioctl (2)
operation;
.IP *
open devices for accessing x86 model-specific registers (MSRs, see
.BR msr (4))
.IP *
update
.IR /proc/sys/vm/mmap_min_addr ;
.IP *
create memory mappings at addresses below the value specified by
.IR /proc/sys/vm/mmap_min_addr ;
.IP *
map files in
.IR /proc/bus/pci ;
.IP *
open
.IR /dev/mem
and
.IR /dev/kmem ;
.IP *
perform various SCSI device commands;
.IP *
perform certain operations on
.BR hpsa (4)
and
.BR cciss (4)
devices;
.IP *
perform a range of device-specific operations on other devices.
.RE
.PD
.TP
.B CAP_SYS_RESOURCE
.PD 0
.RS
.IP * 2
Use reserved space on ext2 filesystems;
.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;
.IP *
set
.IR /proc/PID/oom_score_adj
to a value lower than the value last set by a process with
.BR CAP_SYS_RESOURCE .
.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)"
.RS
.PD 0
.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).)
.PD
.RE
.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 filesystem 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).
Inheritable capabilities remain inheritable when executing any program,
and inheritable capabilities are added to the permitted set when executing
a program that has the corresponding bits set in the file inheritable set.
.IP
Because inheritable capabilities are not generally preserved across
.BR 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.
.TP
.IR Effective :
This is the set of capabilities used by the kernel to
perform permission checks for the thread.
.TP
.IR Ambient " (since Linux 4.3):"
This is a set of capabilities that are preserved across an
.BR execve (2)
of a program that does not have file capabilities.  The ambient capability
set obeys the invariant that no capability can ever be ambient if it is
not both permitted and inheritable.  Ambient capabilities are
preserved in the permitted set and added to the effective
set when
.BR execve (2)
is called.  The ambient capability set is modified using
.BR prctl (2).
Executing a program that changes uid or gid due to the setuid or setgid
bits or executing a program that has any file capabilities set will clear
the ambient set.
.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).
.PP
Since Linux 3.2, the file
.I /proc/sys/kernel/cap_last_cap
.\" commit 73efc0394e148d0e15583e13712637831f926720
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.
.\"
.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'(ambient) = (file has capabilities or is setuid or setgid) ? 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]

.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 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
.B CONFIG_SECURITY_FILE_CAPABILITIES
option.
Since Linux 2.6.33,
.\" commit b3a222e52e4d4be77cc4520a57af1a4a0d8222d1
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 filesystem 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 filesystem 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 2.6.30),
.BR CAP_MAC_OVERRIDE ,
and
.B 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.
.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 or the
.B SECBIT_KEEP_CAPS
securebits flag described below.
.\"
.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 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.
.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 filesystem 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" .)
.TP
.B SECBIT_NO_CAP_AMBIENT_RAISE
Setting this flag disallows
.BR PR_CAP_AMBIENT_RAISE .
.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 ,
.BR SECBIT_NOROOT_LOCKED ,
and
.BR SECBIT_NO_CAP_AMBIENT_RAISE .
.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
.SS Interaction with user namespaces
For a discussion of the interaction of capabilities and user namespaces, see
.BR user_namespaces (7).
.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.tuxomania.net\:/publications\:/posix.1e/
.UE .
.SH NOTES
From kernel 2.5.27 to kernel 2.6.26,
.\" commit 5915eb53861c5776cfec33ca4fcc1fd20d66dd27 removed
.\" CONFIG_SECURITY_CAPABILITIES
capabilities were an optional kernel component,
and can be enabled/disabled via the
.B 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.
Before Linux 3.8, nonexistent capabilities were shown as being
enabled (1) in these sets.
Since Linux 3.8,
.\" 7b9a7ec565505699f503b4fcf61500dceb36e744
all nonexistent capabilities (above
.BR CAP_LAST_CAP )
are shown as disabled (0).

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 from kernel 2.6.24 to kernel 2.6.32 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 capsh (1),
.BR setpriv (1),
.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 user_namespaces (7),
.BR pthreads (7),
.BR getcap (8),
.BR setcap (8)
.PP
.I include/linux/capability.h
in the Linux kernel source tree