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));
* Allocate memory using huge pages (memfd_create(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.
Since Linux 5.12, this capability is also needed to map
user ID 0 in a new user namespace; see user_namespaces(7)
for details.
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).
