CAPABILITIES
Section: Linux Programmer's Manual (7)
Updated: 2021-03-22
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NAME
capabilities - overview of Linux capabilities
DESCRIPTION
For the purpose of performing permission checks,
traditional UNIX implementations distinguish two categories of processes:
privileged
processes (whose effective user ID is 0, referred to as superuser or root),
and
unprivileged
processes (whose effective UID is nonzero).
Privileged processes bypass all kernel permission checks,
while 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
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_BPF (since Linux 5.8)
-
Employ privileged BPF operations; see
bpf(2)
and
bpf-helpers(7).
-
This capability was added in Linux 5.8 to separate out
BPF functionality from the overloaded
CAP_SYS_ADMIN
capability.
- CAP_CHECKPOINT_RESTORE (since Linux 5.9)
-
-
- *
-
Update
/proc/sys/kernel/ns_last_pid
(see
pid_namespaces(7));
- *
-
employ the
set_tid
feature of
clone3(2);
- *
-
read the contents of the symbolic links in
/proc/[pid]/map_files
for other processes.
-
This capability was added in Linux 5.9 to separate out
checkpoint/restore functionality from the overloaded
CAP_SYS_ADMIN
capability.
- 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 abbreviation 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;
- *
-
modify
user
extended attributes on sticky directory owned by any user;
- *
-
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_PERFMON (since Linux 5.8)
-
Employ various performance-monitoring mechanisms, including:
-
- *
-
call
perf_event_open(2);
- *
-
employ various BPF operations that have performance implications.
-
This capability was added in Linux 5.8 to separate out
performance monitoring functionality from the overloaded
CAP_SYS_ADMIN
capability.
See also the kernel source file
Documentation/admin-guide/perf-security.rst.
- 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 capability from the calling thread's bounding set
to its inheritable set;
drop capabilities 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),
setresuid(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),
pivot_root(2),
swapon(2),
swapoff(2),
sethostname(2),
and
setdomainname(2);
- *
-
perform privileged
syslog(2)
operations (since Linux 2.6.37,
CAP_SYSLOG
should be used to permit such operations);
- *
-
perform
VM86_REQUEST_IRQ
vm86(2)
command;
- *
-
access the same checkpoint/restore functionality that is governed by
CAP_CHECKPOINT_RESTORE
(but the latter, weaker capability is preferred for accessing
that functionality).
- *
-
perform the same BPF operations as are governed by
CAP_BPF
(but the latter, weaker capability is preferred for accessing
that functionality).
- *
-
employ the same performance monitoring mechanisms as are governed by
CAP_PERFMON
(but the latter, weaker capability is preferred for accessing
that functionality).
- *
-
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 capability);
- *
-
access privileged
perf
event information;
- *
-
call
setns(2)
(requires
CAP_SYS_ADMIN
in the
target
namespace);
- *
-
call
fanotify_init(2);
- *
-
perform privileged
KEYCTL_CHOWN
and
KEYCTL_SETPERM
keyctl(2)
operations;
- *
-
perform
madvise(2)
MADV_HWPOISON
operation;
- *
-
employ the
TIOCSTI
ioctl(2)
to insert characters into the input queue of a terminal other than
the caller's controlling 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
random(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 seccomp protections (i.e., the
PTRACE_O_SUSPEND_SECCOMP
flag);
- *
-
perform administrative operations on many device drivers;
- *
-
modify autogroup nice values by writing to
/proc/[pid]/autogroup
(see
sched(7)).
- CAP_SYS_BOOT
-
Use
reboot(2)
and
kexec_load(2).
- CAP_SYS_CHROOT
-
-
- *
-
Use
chroot(2);
- *
-
change mount namespaces using
setns(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
-
-
- *
-
Lower the 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),
sched_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,
/proc/sys/fs/mqueue/msg_max,
and
/proc/sys/fs/mqueue/msgsize_max
limits 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 (hardware) clock.
- CAP_SYS_TTY_CONFIG
-
Use
vhangup(2);
employ various privileged
ioctl(2)
operations on virtual terminals.
- 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 feature, 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 used 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 example, 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 the following capability sets containing zero or more
of the above capabilities:
- 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
capability in its effective set.
-
If a thread drops a capability from its permitted set,
it can never reacquire that capability (unless it
execve(2)s
either a set-user-ID-root program, or
a program whose associated file capabilities grant that capability).
- Inheritable
-
This is a set of capabilities preserved across an
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.
-
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.
- Bounding (per-thread since Linux 2.6.25)
-
The capability bounding set is a mechanism that can be used
to limit the capabilities that are gained during
execve(2).
-
Since Linux 2.6.25, this is a per-thread capability set.
In older kernels, the capability bounding set was a system wide attribute
shared by all threads on the system.
-
For more details on the capability bounding set, see below.
- Ambient (since Linux 4.3)
-
This is a set of capabilities that are preserved across an
execve(2)
of a program 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 inheritable.
-
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.
If ambient capabilities cause a process's permitted and effective
capabilities to increase during an
execve(2),
this does not trigger the secure-execution mode described in
ld.so(8).
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 executable 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
inheritable 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 during 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 permitted capability during an
execve(2)
(see the transformation rules described below) will also acquire that
capability in its effective set.
Therefore, when assigning capabilities to a file
(setcap(8),
cap_set_file(3),
cap_set_fd(3)),
if we specify the effective flag as being enabled for any capability,
then the effective flag must also be specified as enabled
for all other capabilities for which the corresponding permitted or
inheritable flags is enabled.
File capability extended attribute versioning
To allow extensibility,
the kernel supports a scheme to encode a version number inside the
security.capability
extended attribute that is used to implement file capabilities.
These version numbers are internal to the implementation,
and not directly visible to user-space applications.
To date, the following versions are supported:
- VFS_CAP_REVISION_1
-
This was the original file capability implementation,
which supported 32-bit masks for file capabilities.
- VFS_CAP_REVISION_2 (since Linux 2.6.25)
-
This version allows for file capability masks that are 64 bits in size,
and was necessary as the number of supported capabilities grew beyond 32.
The kernel transparently continues to support the execution of files
that have 32-bit version 1 capability masks,
but when adding capabilities to files that did not previously
have capabilities, or modifying the capabilities of existing files,
it automatically uses the version 2 scheme
(or possibly the version 3 scheme, as described below).
- VFS_CAP_REVISION_3 (since Linux 4.14)
-
Version 3 file capabilities are provided
to support namespaced file capabilities (described below).
-
As with version 2 file capabilities,
version 3 capability masks are 64 bits in size.
But in addition, the root user ID of namespace is encoded in the
security.capability
extended attribute.
(A namespace's root user ID is the value that user ID 0
inside that namespace maps to in the initial user namespace.)
-
Version 3 file capabilities are designed to coexist
with version 2 capabilities;
that is, on a modern Linux system,
there may be some files with version 2 capabilities
while others have version 3 capabilities.
Before Linux 4.14,
the only kind of file capability extended attribute
that could be attached to a file was a
VFS_CAP_REVISION_2
attribute.
Since Linux 4.14,
the version of the
security.capability
extended attribute that is attached to a file
depends on the circumstances in which the attribute was created.
Starting with Linux 4.14, a
security.capability
extended attribute is automatically created as (or converted to)
a version 3
(VFS_CAP_REVISION_3)
attribute if both of the following are true:
- (1)
-
The thread writing the attribute resides in a noninitial user namespace.
(More precisely: the thread resides in a user namespace other
than the one from which the underlying filesystem was mounted.)
- (2)
-
The thread has the
CAP_SETFCAP
capability over the file inode,
meaning that (a) the thread has the
CAP_SETFCAP
capability in its own user namespace;
and (b) the UID and GID of the file inode have mappings in
the writer's user namespace.
When a
VFS_CAP_REVISION_3
security.capability
extended attribute is created, the root user ID of the creating thread's
user namespace is saved in the extended attribute.
By contrast, creating or modifying a
security.capability
extended attribute from a privileged
(CAP_SETFCAP)
thread that resides in the
namespace where the underlying filesystem was mounted
(this normally means the initial user namespace)
automatically results in the creation of a version 2
(VFS_CAP_REVISION_2)
attribute.
Note that the creation of a version 3
security.capability
extended attribute is automatic.
That is to say, when a user-space application writes
(setxattr(2))
a
security.capability
attribute in the version 2 format,
the kernel will automatically create a version 3 attribute
if the attribute is created in the circumstances described above.
Correspondingly, when a version 3
security.capability
attribute is retrieved
(getxattr(2))
by a process that resides inside a user namespace that was created by the
root user ID (or a descendant of that user namespace),
the returned attribute is (automatically)
simplified to appear as a version 2 attribute
(i.e., the returned value is the size of a version 2 attribute and does
not include the root user ID).
These automatic translations mean that no changes are required to
user-space tools (e.g.,
setcap(1)
and
getcap(1))
in order for those tools to be used to create and retrieve version 3
security.capability
attributes.
Note that a file can have either a version 2 or a version 3
security.capability
extended attribute associated with it, but not both:
creation or modification of the
security.capability
extended attribute will automatically modify the version
according to the circumstances in which the extended attribute is
created or modified.
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) & P(bounding)) | P'(ambient)
P'(effective) = F(effective) ? P'(permitted) : P'(ambient)
P'(inheritable) = P(inheritable) [i.e., unchanged]
P'(bounding) = P(bounding) [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
Note the following details relating to the above capability
transformation rules:
- *
-
The ambient capability set is present only since Linux 4.3.
When determining the transformation of the ambient set during
execve(2),
a privileged file is one that has capabilities or
has the set-user-ID or set-group-ID bit set.
- *
-
Prior to Linux 2.6.25,
the bounding set was a system-wide attribute shared by all threads.
That system-wide value was employed to calculate the new permitted set during
execve(2)
in the same manner as shown above for
P(bounding).
Note:
during the capability transitions described above,
file capabilities may be ignored (treated as empty) for the same reasons
that the set-user-ID and set-group-ID bits are ignored; see
execve(2).
File capabilities are similarly ignored if the kernel was booted with the
no_file_caps
option.
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 capabilities, but has not been converted to use the
libcap(3)
API to manipulate its capabilities.
(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 capability 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 capabilities 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 executed 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 mirror traditional UNIX semantics,
the kernel performs special treatment of file capabilities when
a process with UID 0 (root) executes a program and
when a set-user-ID-root program is executed.
After having performed any changes to the process effective ID that
were triggered by the set-user-ID mode bit of the binary---e.g.,
switching the effective user ID to 0 (root) because
a set-user-ID-root program was executed---the
kernel calculates the file capability sets as follows:
- 1.
-
If the real or effective user ID of the process is 0 (root),
then the file inheritable and permitted sets are ignored;
instead they are notionally considered to be all ones
(i.e., all capabilities enabled).
(There is one exception to this behavior, described below in
Set-user-ID-root programs that have file capabilities.)
- 2.
-
If the effective user ID of the process is 0 (root) or
the file effective bit is in fact enabled,
then the file effective bit is notionally defined to be one (enabled).
These notional values for the file's capability sets are then used
as described above to calculate the transformation of the process's
capabilities during
execve(2).
Thus, when a process with nonzero UIDs
execve(2)s
a set-user-ID-root program that does not have capabilities attached,
or when a process whose real and effective UIDs are zero
execve(2)s
a program, the calculation of the process's new
permitted capabilities simplifies to:
P'(permitted) = P(inheritable) | P(bounding)
P'(effective) = P'(permitted)
Consequently, the process gains all capabilities in its permitted and
effective capability sets,
except those masked out by the capability bounding set.
(In the calculation of P'(permitted),
the P'(ambient) term can be simplified away because it is by
definition a proper subset of P(inheritable).)
The special treatments of user ID 0 (root) described in this subsection
can be disabled using the securebits mechanism described below.
Set-user-ID-root programs that have file capabilities
There is one exception to the behavior described under
Capabilities and execution of programs by root.
If (a) the binary that is being executed has capabilities attached and
(b) the real user ID of the process is
not
0 (root) and
(c) the effective user ID of the process
is
0 (root), then the file capability bits are honored
(i.e., they are not notionally considered to be all ones).
The usual way in which this situation can arise is when executing
a set-UID-root program that also has file capabilities.
When such a program is executed,
the process gains just the capabilities granted by the program
(i.e., not all capabilities,
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 permitted capability set.
The capability bounding set thus places a limit on the permitted
capabilities that may be granted by an executable file.
- *
-
(Since Linux 2.6.25)
The capability bounding set acts as a limiting superset for
the capabilities that a thread can add to its 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 inheritable 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 from Linux 2.6.25 onward
From Linux 2.6.25, the
capability bounding set
is a per-thread attribute.
(The system-wide capability bounding set described below no longer exists.)
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 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
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.
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 kernel version 2.2.11.
Effect of user ID changes on capabilities
To preserve the traditional semantics for transitions between
0 and nonzero user IDs,
the kernel makes the following changes to a thread's capability
sets on changes to the thread's real, effective, saved set,
and filesystem user IDs (using
setuid(2),
setresuid(2),
or similar):
- 1.
-
If one or more of the real, effective, or saved set user IDs
was 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_OVERRIDE,
CAP_DAC_READ_SEARCH,
CAP_FOWNER,
CAP_FSETID,
CAP_LINUX_IMMUTABLE
(since Linux 2.6.30),
CAP_MAC_OVERRIDE,
and
CAP_MKNOD
(since Linux 2.6.30).
If the filesystem UID is changed from nonzero to 0,
then any of these 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 permitted, effective, and inheritable
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 special 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
capabilities in its permitted set
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 permitted capabilities.
This flag is always cleared on an
execve(2).
-
Note that even with the
SECBIT_KEEP_CAPS
flag set, the effective capabilities of a thread are cleared when it
switches its effective UID to a nonzero value.
However,
if the thread has set this flag and its effective UID is already nonzero,
and the thread subsequently switches all other UIDs to nonzero values,
then the effective capabilities will not be cleared.
-
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 filesystem 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_AMBIENT_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.
Note that the
SECBIT_*
constants are available only after including the
<linux/securebits.h>
header file.
The
securebits
flags are inherited by child processes.
During an
execve(2),
all of the flags are preserved, except
SECBIT_KEEP_CAPS
which is always cleared.
An application can use the following call to lock itself,
and all of its descendants,
into an environment where the only way of gaining capabilities
is by executing a program with associated file 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 */
Per-user-namespace set-user-ID-root programs
A set-user-ID program whose UID matches the UID that
created a user namespace will confer capabilities
in the process's permitted and effective sets
when executed by any process inside that namespace
or any descendant user namespace.
The rules about the transformation of the process's capabilities during the
execve(2)
are exactly as described in the subsections
Transformation of capabilities during execve()
and
Capabilities and execution of programs by root,
with the difference that, in the latter subsection, "root"
is the UID of the creator of the user namespace.
Namespaced file capabilities
Traditional (i.e., version 2) file capabilities associate
only a set of capability masks with a binary executable file.
When a process executes a binary with such capabilities,
it gains the associated capabilities (within its user namespace)
as per the rules described above in
"Transformation of capabilities during execve()".
Because version 2 file capabilities confer capabilities to
the executing process regardless of which user namespace it resides in,
only privileged processes are permitted to associate capabilities with a file.
Here, "privileged" means a process that has the
CAP_SETFCAP
capability in the user namespace where the filesystem was mounted
(normally the initial user namespace).
This limitation renders file capabilities useless for certain use cases.
For example, in user-namespaced containers,
it can be desirable to be able to create a binary that
confers capabilities only to processes executed inside that container,
but not to processes that are executed outside the container.
Linux 4.14 added so-called namespaced file capabilities
to support such use cases.
Namespaced file capabilities are recorded as version 3 (i.e.,
VFS_CAP_REVISION_3)
security.capability
extended attributes.
Such an attribute is automatically created in the circumstances described
above under "File capability extended attribute versioning".
When a version 3
security.capability
extended attribute is created,
the kernel records not just the capability masks in the extended attribute,
but also the namespace root user ID.
As with a binary that has
VFS_CAP_REVISION_2
file capabilities, a binary with
VFS_CAP_REVISION_3
file capabilities confers capabilities to a process during
execve().
However, capabilities are conferred only if the binary is executed by
a process that resides in a user namespace whose
UID 0 maps to the root user ID that is saved in the extended attribute,
or when executed by a process that resides in a descendant of such a namespace.
Interaction with user namespaces
For further information on 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
NOTES
When attempting to
strace(1)
binaries that have capabilities (or set-user-ID-root binaries),
you may find the
-u <username>
option useful.
Something like:
$ sudo strace -o trace.log -u ceci ./myprivprog
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 configuration 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)
programs.
It can be found at
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 the
CAP_SETPCAP
capability, and this can not be changed
without modifying the kernel source and rebuilding the kernel.
- *
-
If file capabilities are disabled (i.e., the kernel
CONFIG_SECURITY_FILE_CAPABILITIES
option is disabled), then
init
starts out with the
CAP_SETPCAP
capability removed from its per-process bounding
set, and that bounding 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),
getpcaps(8),
netcap(8),
pscap(8),
setcap(8)
include/linux/capability.h
in the Linux kernel source tree
COLOPHON
This page is part of release 5.11 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/.
Index
- NAME
-
- DESCRIPTION
-
- Capabilities list
-
- Past and current implementation
-
- Notes to kernel developers
-
- Thread capability sets
-
- File capabilities
-
- File capability extended attribute versioning
-
- Transformation of capabilities during execve()
-
- Safety checking for capability-dumb binaries
-
- Capabilities and execution of programs by root
-
- Set-user-ID-root programs that have file capabilities
-
- Capability bounding set
-
- Effect of user ID changes on capabilities
-
- Programmatically adjusting capability sets
-
- The securebits flags: establishing a capabilities-only environment
-
- Per-user-namespace set-user-ID-root programs
-
- Namespaced file capabilities
-
- Interaction with user namespaces
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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Time: 06:22:49 GMT, May 09, 2021