capabilities

CAPABILITIES(7)            Linux Programmer's Manual           CAPABILITIES(7)



NAME
       capabilities - overview of Linux capabilities

DESCRIPTION
       For the purpose of performing permission checks, traditional UNIX
       implementations 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_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_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;
              * 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);
              * call perf_event_open(2);
              * access privileged perf event information;
              * call setns(2) (requires CAP_SYS_ADMIN in the target
                namespace);
              * call fanotify_init(2);
              * call bpf(2);
              * perform privileged KEYCTL_CHOWN and KEYCTL_SETPERM keyctl(2)
                operations;
              * 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
              * Raise process nice value (nice(2), setpriority(2)) and change
                the nice value for arbitrary processes;
              * set real-time scheduling policies for calling process, and set
                scheduling policies and priorities for arbitrary processes
                (sched_setscheduler(2), sched_setparam(2), 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 limit when creating
                POSIX message queues (see mq_overview(7));
              * employ the prctl(2) PR_SET_MM operation;
              * set /proc/[pid]/oom_score_adj to a value lower than the value
                last set by a process with CAP_SYS_RESOURCE.

       CAP_SYS_TIME
              Set system clock (settimeofday(2), stime(2), adjtimex(2)); set
              real-time (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
       ⟨https://archive.org/details/posix_1003.1e-990310⟩.

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
       ⟨https://git.kernel.org/pub/scm/libs/libcap/libcap.git/refs/⟩.

       Before kernel 2.6.24, and from kernel 2.6.24 to kernel 2.6.32 if file
       capabilities are not enabled, a thread with the CAP_SETPCAP capability
       can manipulate the capabilities of threads other than itself.  However,
       this is only theoretically possible, since no thread ever has
       CAP_SETPCAP in either of these cases:

       * In the pre-2.6.25 implementation the system-wide capability bounding
         set, /proc/sys/kernel/cap-bound, always masks out this capability,
         and this can not be changed without modifying the kernel source and
         rebuilding.

       * If file capabilities are disabled in the current implementation, then
         init starts out with this capability removed from its per-process
         bounding set, and that 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), netcap(8), pscap(8), setcap(8)

       include/linux/capability.h in the Linux kernel source tree

COLOPHON
       This page is part of release 5.03 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                             2019-08-02                   CAPABILITIES(7)