user_namespaces(7) — Linux manual page


user_namespaces(7)  Miscellaneous Information Manual  user_namespaces(7)

NAME         top

       user_namespaces - overview of Linux user namespaces

DESCRIPTION         top

       For an overview of namespaces, see namespaces(7).

       User namespaces isolate security-related identifiers and
       attributes, in particular, user IDs and group IDs (see
       credentials(7)), the root directory, keys (see keyrings(7)), and
       capabilities (see capabilities(7)).  A process's user and group
       IDs can be different inside and outside a user namespace.  In
       particular, a process can have a normal unprivileged user ID
       outside a user namespace while at the same time having a user ID
       of 0 inside the namespace; in other words, the process has full
       privileges for operations inside the user namespace, but is
       unprivileged for operations outside the namespace.

   Nested namespaces, namespace membership
       User namespaces can be nested; that is, each user namespace—
       except the initial ("root") namespace—has a parent user
       namespace, and can have zero or more child user namespaces.  The
       parent user namespace is the user namespace of the process that
       creates the user namespace via a call to unshare(2) or clone(2)
       with the CLONE_NEWUSER flag.

       The kernel imposes (since Linux 3.11) a limit of 32 nested levels
       of user namespaces.  Calls to unshare(2) or clone(2) that would
       cause this limit to be exceeded fail with the error EUSERS.

       Each process is a member of exactly one user namespace.  A
       process created via fork(2) or clone(2) without the CLONE_NEWUSER
       flag is a member of the same user namespace as its parent.  A
       single-threaded process can join another user namespace with
       setns(2) if it has the CAP_SYS_ADMIN in that namespace; upon
       doing so, it gains a full set of capabilities in that namespace.

       A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag
       makes the new child process (for clone(2)) or the caller (for
       unshare(2)) a member of the new user namespace created by the

       The NS_GET_PARENT ioctl(2) operation can be used to discover the
       parental relationship between user namespaces; see ioctl_ns(2).

       A task that changes one of its effective IDs will have its
       dumpability reset to the value in /proc/sys/fs/suid_dumpable.
       This may affect the ownership of proc files of child processes
       and may thus cause the parent to lack the permissions to write to
       mapping files of child processes running in a new user namespace.
       In such cases making the parent process dumpable, using
       PR_SET_DUMPABLE in a call to prctl(2), before creating a child
       process in a new user namespace may rectify this problem.  See
       prctl(2) and proc(5) for details on how ownership is affected.

       The child process created by clone(2) with the CLONE_NEWUSER flag
       starts out with a complete set of capabilities in the new user
       namespace.  Likewise, a process that creates a new user namespace
       using unshare(2) or joins an existing user namespace using
       setns(2) gains a full set of capabilities in that namespace.  On
       the other hand, that process has no capabilities in the parent
       (in the case of clone(2)) or previous (in the case of unshare(2)
       and setns(2)) user namespace, even if the new namespace is
       created or joined by the root user (i.e., a process with user ID
       0 in the root namespace).

       Note that a call to execve(2) will cause a process's capabilities
       to be recalculated in the usual way (see capabilities(7)).
       Consequently, unless the process has a user ID of 0 within the
       namespace, or the executable file has a nonempty inheritable
       capabilities mask, the process will lose all capabilities.  See
       the discussion of user and group ID mappings, below.

       A call to clone(2) or unshare(2) using the CLONE_NEWUSER flag or
       a call to setns(2) that moves the caller into another user
       namespace sets the "securebits" flags (see capabilities(7)) to
       their default values (all flags disabled) in the child (for
       clone(2)) or caller (for unshare(2) or setns(2)).  Note that
       because the caller no longer has capabilities in its original
       user namespace after a call to setns(2), it is not possible for a
       process to reset its "securebits" flags while retaining its user
       namespace membership by using a pair of setns(2) calls to move to
       another user namespace and then return to its original user

       The rules for determining whether or not a process has a
       capability in a particular user namespace are as follows:

       •  A process has a capability inside a user namespace if it is a
          member of that namespace and it has the capability in its
          effective capability set.  A process can gain capabilities in
          its effective capability set in various ways.  For example, it
          may execute a set-user-ID program or an executable with
          associated file capabilities.  In addition, a process may gain
          capabilities via the effect of clone(2), unshare(2), or
          setns(2), as already described.

       •  If a process has a capability in a user namespace, then it has
          that capability in all child (and further removed descendant)
          namespaces as well.

       •  When a user namespace is created, the kernel records the
          effective user ID of the creating process as being the "owner"
          of the namespace.  A process that resides in the parent of the
          user namespace and whose effective user ID matches the owner
          of the namespace has all capabilities in the namespace.  By
          virtue of the previous rule, this means that the process has
          all capabilities in all further removed descendant user
          namespaces as well.  The NS_GET_OWNER_UID ioctl(2) operation
          can be used to discover the user ID of the owner of the
          namespace; see ioctl_ns(2).

   Effect of capabilities within a user namespace
       Having a capability inside a user namespace permits a process to
       perform operations (that require privilege) only on resources
       governed by that namespace.  In other words, having a capability
       in a user namespace permits a process to perform privileged
       operations on resources that are governed by (nonuser) namespaces
       owned by (associated with) the user namespace (see the next

       On the other hand, there are many privileged operations that
       affect resources that are not associated with any namespace type,
       for example, changing the system (i.e., calendar) time (governed
       by CAP_SYS_TIME), loading a kernel module (governed by
       CAP_SYS_MODULE), and creating a device (governed by CAP_MKNOD).
       Only a process with privileges in the initial user namespace can
       perform such operations.

       Holding CAP_SYS_ADMIN within the user namespace that owns a
       process's mount namespace allows that process to create bind
       mounts and mount the following types of filesystems:

           •  /proc (since Linux 3.8)
           •  /sys (since Linux 3.8)
           •  devpts (since Linux 3.9)
           •  tmpfs(5) (since Linux 3.9)
           •  ramfs (since Linux 3.9)
           •  mqueue (since Linux 3.9)
           •  bpf (since Linux 4.4)
           •  overlayfs (since Linux 5.11)

       Holding CAP_SYS_ADMIN within the user namespace that owns a
       process's cgroup namespace allows (since Linux 4.6) that process
       to the mount the cgroup version 2 filesystem and cgroup version 1
       named hierarchies (i.e., cgroup filesystems mounted with the
       "none,name=" option).

       Holding CAP_SYS_ADMIN within the user namespace that owns a
       process's PID namespace allows (since Linux 3.8) that process to
       mount /proc filesystems.

       Note, however, that mounting block-based filesystems can be done
       only by a process that holds CAP_SYS_ADMIN in the initial user

   Interaction of user namespaces and other types of namespaces
       Starting in Linux 3.8, unprivileged processes can create user
       namespaces, and the other types of namespaces can be created with
       just the CAP_SYS_ADMIN capability in the caller's user namespace.

       When a nonuser namespace is created, it is owned by the user
       namespace in which the creating process was a member at the time
       of the creation of the namespace.  Privileged operations on
       resources governed by the nonuser namespace require that the
       process has the necessary capabilities in the user namespace that
       owns the nonuser namespace.

       If CLONE_NEWUSER is specified along with other CLONE_NEW* flags
       in a single clone(2) or unshare(2) call, the user namespace is
       guaranteed to be created first, giving the child (clone(2)) or
       caller (unshare(2)) privileges over the remaining namespaces
       created by the call.  Thus, it is possible for an unprivileged
       caller to specify this combination of flags.

       When a new namespace (other than a user namespace) is created via
       clone(2) or unshare(2), the kernel records the user namespace of
       the creating process as the owner of the new namespace.  (This
       association can't be changed.)  When a process in the new
       namespace subsequently performs privileged operations that
       operate on global resources isolated by the namespace, the
       permission checks are performed according to the process's
       capabilities in the user namespace that the kernel associated
       with the new namespace.  For example, suppose that a process
       attempts to change the hostname (sethostname(2)), a resource
       governed by the UTS namespace.  In this case, the kernel will
       determine which user namespace owns the process's UTS namespace,
       and check whether the process has the required capability
       (CAP_SYS_ADMIN) in that user namespace.

       The NS_GET_USERNS ioctl(2) operation can be used to discover the
       user namespace that owns a nonuser namespace; see ioctl_ns(2).

   User and group ID mappings: uid_map and gid_map
       When a user namespace is created, it starts out without a mapping
       of user IDs (group IDs) to the parent user namespace.  The
       /proc/pid/uid_map and /proc/pid/gid_map files (available since
       Linux 3.5) expose the mappings for user and group IDs inside the
       user namespace for the process pid.  These files can be read to
       view the mappings in a user namespace and written to (once) to
       define the mappings.

       The description in the following paragraphs explains the details
       for uid_map; gid_map is exactly the same, but each instance of
       "user ID" is replaced by "group ID".

       The uid_map file exposes the mapping of user IDs from the user
       namespace of the process pid to the user namespace of the process
       that opened uid_map (but see a qualification to this point
       below).  In other words, processes that are in different user
       namespaces will potentially see different values when reading
       from a particular uid_map file, depending on the user ID mappings
       for the user namespaces of the reading processes.

       Each line in the uid_map file specifies a 1-to-1 mapping of a
       range of contiguous user IDs between two user namespaces.  (When
       a user namespace is first created, this file is empty.)  The
       specification in each line takes the form of three numbers
       delimited by white space.  The first two numbers specify the
       starting user ID in each of the two user namespaces.  The third
       number specifies the length of the mapped range.  In detail, the
       fields are interpreted as follows:

       (1)  The start of the range of user IDs in the user namespace of
            the process pid.

       (2)  The start of the range of user IDs to which the user IDs
            specified by field one map.  How field two is interpreted
            depends on whether the process that opened uid_map and the
            process pid are in the same user namespace, as follows:

            (a)  If the two processes are in different user namespaces:
                 field two is the start of a range of user IDs in the
                 user namespace of the process that opened uid_map.

            (b)  If the two processes are in the same user namespace:
                 field two is the start of the range of user IDs in the
                 parent user namespace of the process pid.  This case
                 enables the opener of uid_map (the common case here is
                 opening /proc/self/uid_map) to see the mapping of user
                 IDs into the user namespace of the process that created
                 this user namespace.

       (3)  The length of the range of user IDs that is mapped between
            the two user namespaces.

       System calls that return user IDs (group IDs)—for example,
       getuid(2), getgid(2), and the credential fields in the structure
       returned by stat(2)—return the user ID (group ID) mapped into the
       caller's user namespace.

       When a process accesses a file, its user and group IDs are mapped
       into the initial user namespace for the purpose of permission
       checking and assigning IDs when creating a file.  When a process
       retrieves file user and group IDs via stat(2), the IDs are mapped
       in the opposite direction, to produce values relative to the
       process user and group ID mappings.

       The initial user namespace has no parent namespace, but, for
       consistency, the kernel provides dummy user and group ID mapping
       files for this namespace.  Looking at the uid_map file (gid_map
       is the same) from a shell in the initial namespace shows:

           $ cat /proc/$$/uid_map
                    0          0 4294967295

       This mapping tells us that the range starting at user ID 0 in
       this namespace maps to a range starting at 0 in the (nonexistent)
       parent namespace, and the length of the range is the largest
       32-bit unsigned integer.  This leaves 4294967295 (the 32-bit
       signed -1 value) unmapped.  This is deliberate: (uid_t) -1 is
       used in several interfaces (e.g., setreuid(2)) as a way to
       specify "no user ID".  Leaving (uid_t) -1 unmapped and unusable
       guarantees that there will be no confusion when using these

   Defining user and group ID mappings: writing to uid_map and gid_map
       After the creation of a new user namespace, the uid_map file of
       one of the processes in the namespace may be written to once to
       define the mapping of user IDs in the new user namespace.  An
       attempt to write more than once to a uid_map file in a user
       namespace fails with the error EPERM.  Similar rules apply for
       gid_map files.

       The lines written to uid_map (gid_map) must conform to the
       following validity rules:

       •  The three fields must be valid numbers, and the last field
          must be greater than 0.

       •  Lines are terminated by newline characters.

       •  There is a limit on the number of lines in the file.  In Linux
          4.14 and earlier, this limit was (arbitrarily) set at 5 lines.
          Since Linux 4.15, the limit is 340 lines.  In addition, the
          number of bytes written to the file must be less than the
          system page size, and the write must be performed at the start
          of the file (i.e., lseek(2) and pwrite(2) can't be used to
          write to nonzero offsets in the file).

       •  The range of user IDs (group IDs) specified in each line
          cannot overlap with the ranges in any other lines.  In the
          initial implementation (Linux 3.8), this requirement was
          satisfied by a simplistic implementation that imposed the
          further requirement that the values in both field 1 and field
          2 of successive lines must be in ascending numerical order,
          which prevented some otherwise valid maps from being created.
          Linux 3.9 and later fix this limitation, allowing any valid
          set of nonoverlapping maps.

       •  At least one line must be written to the file.

       Writes that violate the above rules fail with the error EINVAL.

       In order for a process to write to the /proc/pid/uid_map
       (/proc/pid/gid_map) file, all of the following permission
       requirements must be met:

       •  The writing process must have the CAP_SETUID (CAP_SETGID)
          capability in the user namespace of the process pid.

       •  The writing process must either be in the user namespace of
          the process pid or be in the parent user namespace of the
          process pid.

       •  The mapped user IDs (group IDs) must in turn have a mapping in
          the parent user namespace.

       •  If updating /proc/pid/uid_map to create a mapping that maps
          UID 0 in the parent namespace, then one of the following must
          be true:

          (a)  if writing process is in the parent user namespace, then
               it must have the CAP_SETFCAP capability in that user
               namespace; or

          (b)  if the writing process is in the child user namespace,
               then the process that created the user namespace must
               have had the CAP_SETFCAP capability when the namespace
               was created.

          This rule has been in place since Linux 5.12.  It eliminates
          an earlier security bug whereby a UID 0 process that lacks the
          CAP_SETFCAP capability, which is needed to create a binary
          with namespaced file capabilities (as described in
          capabilities(7)), could nevertheless create such a binary, by
          the following steps:

          (1)  Create a new user namespace with the identity mapping
               (i.e., UID 0 in the new user namespace maps to UID 0 in
               the parent namespace), so that UID 0 in both namespaces
               is equivalent to the same root user ID.

          (2)  Since the child process has the CAP_SETFCAP capability,
               it could create a binary with namespaced file
               capabilities that would then be effective in the parent
               user namespace (because the root user IDs are the same in
               the two namespaces).

       •  One of the following two cases applies:

          (a)  Either the writing process has the CAP_SETUID
               (CAP_SETGID) capability in the parent user namespace.

               •  No further restrictions apply: the process can make
                  mappings to arbitrary user IDs (group IDs) in the
                  parent user namespace.

          (b)  Or otherwise all of the following restrictions apply:

               •  The data written to uid_map (gid_map) must consist of
                  a single line that maps the writing process's
                  effective user ID (group ID) in the parent user
                  namespace to a user ID (group ID) in the user

               •  The writing process must have the same effective user
                  ID as the process that created the user namespace.

               •  In the case of gid_map, use of the setgroups(2) system
                  call must first be denied by writing "deny" to the
                  /proc/pid/setgroups file (see below) before writing to

       Writes that violate the above rules fail with the error EPERM.

   Project ID mappings: projid_map
       Similarly to user and group ID mappings, it is possible to create
       project ID mappings for a user namespace.  (Project IDs are used
       for disk quotas; see setquota(8) and quotactl(2).)

       Project ID mappings are defined by writing to the
       /proc/pid/projid_map file (present since Linux 3.7).

       The validity rules for writing to the /proc/pid/projid_map file
       are as for writing to the uid_map file; violation of these rules
       causes write(2) to fail with the error EINVAL.

       The permission rules for writing to the /proc/pid/projid_map file
       are as follows:

       •  The writing process must either be in the user namespace of
          the process pid or be in the parent user namespace of the
          process pid.

       •  The mapped project IDs must in turn have a mapping in the
          parent user namespace.

       Violation of these rules causes write(2) to fail with the error

   Interaction with system calls that change process UIDs or GIDs
       In a user namespace where the uid_map file has not been written,
       the system calls that change user IDs will fail.  Similarly, if
       the gid_map file has not been written, the system calls that
       change group IDs will fail.  After the uid_map and gid_map files
       have been written, only the mapped values may be used in system
       calls that change user and group IDs.

       For user IDs, the relevant system calls include setuid(2),
       setfsuid(2), setreuid(2), and setresuid(2).  For group IDs, the
       relevant system calls include setgid(2), setfsgid(2),
       setregid(2), setresgid(2), and setgroups(2).

       Writing "deny" to the /proc/pid/setgroups file before writing to
       /proc/pid/gid_map will permanently disable setgroups(2) in a user
       namespace and allow writing to /proc/pid/gid_map without having
       the CAP_SETGID capability in the parent user namespace.

   The /proc/pid/setgroups file
       The /proc/pid/setgroups file displays the string "allow" if
       processes in the user namespace that contains the process pid are
       permitted to employ the setgroups(2) system call; it displays
       "deny" if setgroups(2) is not permitted in that user namespace.
       Note that regardless of the value in the /proc/pid/setgroups file
       (and regardless of the process's capabilities), calls to
       setgroups(2) are also not permitted if /proc/pid/gid_map has not
       yet been set.

       A privileged process (one with the CAP_SYS_ADMIN capability in
       the namespace) may write either of the strings "allow" or "deny"
       to this file before writing a group ID mapping for this user
       namespace to the file /proc/pid/gid_map.  Writing the string
       "deny" prevents any process in the user namespace from employing

       The essence of the restrictions described in the preceding
       paragraph is that it is permitted to write to /proc/pid/setgroups
       only so long as calling setgroups(2) is disallowed because
       /proc/pid/gid_map has not been set.  This ensures that a process
       cannot transition from a state where setgroups(2) is allowed to a
       state where setgroups(2) is denied; a process can transition only
       from setgroups(2) being disallowed to setgroups(2) being allowed.

       The default value of this file in the initial user namespace is

       Once /proc/pid/gid_map has been written to (which has the effect
       of enabling setgroups(2) in the user namespace), it is no longer
       possible to disallow setgroups(2) by writing "deny" to
       /proc/pid/setgroups (the write fails with the error EPERM).

       A child user namespace inherits the /proc/pid/setgroups setting
       from its parent.

       If the setgroups file has the value "deny", then the setgroups(2)
       system call can't subsequently be reenabled (by writing "allow"
       to the file) in this user namespace.  (Attempts to do so fail
       with the error EPERM.)  This restriction also propagates down to
       all child user namespaces of this user namespace.

       The /proc/pid/setgroups file was added in Linux 3.19, but was
       backported to many earlier stable kernel series, because it
       addresses a security issue.  The issue concerned files with
       permissions such as "rwx---rwx".  Such files give fewer
       permissions to "group" than they do to "other".  This means that
       dropping groups using setgroups(2) might allow a process file
       access that it did not formerly have.  Before the existence of
       user namespaces this was not a concern, since only a privileged
       process (one with the CAP_SETGID capability) could call
       setgroups(2).  However, with the introduction of user namespaces,
       it became possible for an unprivileged process to create a new
       namespace in which the user had all privileges.  This then
       allowed formerly unprivileged users to drop groups and thus gain
       file access that they did not previously have.  The
       /proc/pid/setgroups file was added to address this security
       issue, by denying any pathway for an unprivileged process to drop
       groups with setgroups(2).

   Unmapped user and group IDs
       There are various places where an unmapped user ID (group ID) may
       be exposed to user space.  For example, the first process in a
       new user namespace may call getuid(2) before a user ID mapping
       has been defined for the namespace.  In most such cases, an
       unmapped user ID is converted to the overflow user ID (group ID);
       the default value for the overflow user ID (group ID) is 65534.
       See the descriptions of /proc/sys/kernel/overflowuid and
       /proc/sys/kernel/overflowgid in proc(5).

       The cases where unmapped IDs are mapped in this fashion include
       system calls that return user IDs (getuid(2), getgid(2), and
       similar), credentials passed over a UNIX domain socket,
       credentials returned by stat(2), waitid(2), and the System V IPC
       "ctl" IPC_STAT operations, credentials exposed by
       /proc/pid/status and the files in /proc/sysvipc/*, credentials
       returned via the si_uid field in the siginfo_t received with a
       signal (see sigaction(2)), credentials written to the process
       accounting file (see acct(5)), and credentials returned with
       POSIX message queue notifications (see mq_notify(3)).

       There is one notable case where unmapped user and group IDs are
       not converted to the corresponding overflow ID value.  When
       viewing a uid_map or gid_map file in which there is no mapping
       for the second field, that field is displayed as 4294967295 (-1
       as an unsigned integer).

   Accessing files
       In order to determine permissions when an unprivileged process
       accesses a file, the process credentials (UID, GID) and the file
       credentials are in effect mapped back to what they would be in
       the initial user namespace and then compared to determine the
       permissions that the process has on the file.  The same is also
       true of other objects that employ the credentials plus
       permissions mask accessibility model, such as System V IPC

   Operation of file-related capabilities
       Certain capabilities allow a process to bypass various kernel-
       enforced restrictions when performing operations on files owned
       by other users or groups.  These capabilities are: CAP_CHOWN,

       Within a user namespace, these capabilities allow a process to
       bypass the rules if the process has the relevant capability over
       the file, meaning that:

       •  the process has the relevant effective capability in its user
          namespace; and

       •  the file's user ID and group ID both have valid mappings in
          the user namespace.

       The CAP_FOWNER capability is treated somewhat exceptionally: it
       allows a process to bypass the corresponding rules so long as at
       least the file's user ID has a mapping in the user namespace
       (i.e., the file's group ID does not need to have a valid

   Set-user-ID and set-group-ID programs
       When a process inside a user namespace executes a set-user-ID
       (set-group-ID) program, the process's effective user (group) ID
       inside the namespace is changed to whatever value is mapped for
       the user (group) ID of the file.  However, if either the user or
       the group ID of the file has no mapping inside the namespace, the
       set-user-ID (set-group-ID) bit is silently ignored: the new
       program is executed, but the process's effective user (group) ID
       is left unchanged.  (This mirrors the semantics of executing a
       set-user-ID or set-group-ID program that resides on a filesystem
       that was mounted with the MS_NOSUID flag, as described in

       When a process's user and group IDs are passed over a UNIX domain
       socket to a process in a different user namespace (see the
       description of SCM_CREDENTIALS in unix(7)), they are translated
       into the corresponding values as per the receiving process's user
       and group ID mappings.

STANDARDS         top


NOTES         top

       Over the years, there have been a lot of features that have been
       added to the Linux kernel that have been made available only to
       privileged users because of their potential to confuse set-user-
       ID-root applications.  In general, it becomes safe to allow the
       root user in a user namespace to use those features because it is
       impossible, while in a user namespace, to gain more privilege
       than the root user of a user namespace has.

   Global root
       The term "global root" is sometimes used as a shorthand for user
       ID 0 in the initial user namespace.

       Use of user namespaces requires a kernel that is configured with
       the CONFIG_USER_NS option.  User namespaces require support in a
       range of subsystems across the kernel.  When an unsupported
       subsystem is configured into the kernel, it is not possible to
       configure user namespaces support.

       As at Linux 3.8, most relevant subsystems supported user
       namespaces, but a number of filesystems did not have the
       infrastructure needed to map user and group IDs between user
       namespaces.  Linux 3.9 added the required infrastructure support
       for many of the remaining unsupported filesystems (Plan 9 (9P),
       Andrew File System (AFS), Ceph, CIFS, CODA, NFS, and OCFS2).
       Linux 3.12 added support for the last of the unsupported major
       filesystems, XFS.

EXAMPLES         top

       The program below is designed to allow experimenting with user
       namespaces, as well as other types of namespaces.  It creates
       namespaces as specified by command-line options and then executes
       a command inside those namespaces.  The comments and usage()
       function inside the program provide a full explanation of the
       program.  The following shell session demonstrates its use.

       First, we look at the run-time environment:

           $ uname -rs     # Need Linux 3.8 or later
           Linux 3.8.0
           $ id -u         # Running as unprivileged user
           $ id -g

       Now start a new shell in new user (-U), mount (-m), and PID (-p)
       namespaces, with user ID (-M) and group ID (-G) 1000 mapped to 0
       inside the user namespace:

           $ ./userns_child_exec -p -m -U -M '0 1000 1' -G '0 1000 1' bash

       The shell has PID 1, because it is the first process in the new
       PID namespace:

           bash$ echo $$

       Mounting a new /proc filesystem and listing all of the processes
       visible in the new PID namespace shows that the shell can't see
       any processes outside the PID namespace:

           bash$ mount -t proc proc /proc
           bash$ ps ax
             PID TTY      STAT   TIME COMMAND
               1 pts/3    S      0:00 bash
              22 pts/3    R+     0:00 ps ax

       Inside the user namespace, the shell has user and group ID 0, and
       a full set of permitted and effective capabilities:

           bash$ cat /proc/$$/status | egrep '^[UG]id'
           Uid: 0    0    0    0
           Gid: 0    0    0    0
           bash$ cat /proc/$$/status | egrep '^Cap(Prm|Inh|Eff)'
           CapInh:   0000000000000000
           CapPrm:   0000001fffffffff
           CapEff:   0000001fffffffff

   Program source

       /* userns_child_exec.c

          Licensed under GNU General Public License v2 or later

          Create a child process that executes a shell command in new
          namespace(s); allow UID and GID mappings to be specified when
          creating a user namespace.
       #define _GNU_SOURCE
       #include <err.h>
       #include <sched.h>
       #include <unistd.h>
       #include <stdint.h>
       #include <stdlib.h>
       #include <sys/wait.h>
       #include <signal.h>
       #include <fcntl.h>
       #include <stdio.h>
       #include <string.h>
       #include <limits.h>
       #include <errno.h>

       struct child_args {
           char **argv;        /* Command to be executed by child, with args */
           int    pipe_fd[2];  /* Pipe used to synchronize parent and child */

       static int verbose;

       static void
       usage(char *pname)
           fprintf(stderr, "Usage: %s [options] cmd [arg...]\n\n", pname);
           fprintf(stderr, "Create a child process that executes a shell "
                   "command in a new user namespace,\n"
                   "and possibly also other new namespace(s).\n\n");
           fprintf(stderr, "Options can be:\n\n");
       #define fpe(str) fprintf(stderr, "    %s", str);
           fpe("-i          New IPC namespace\n");
           fpe("-m          New mount namespace\n");
           fpe("-n          New network namespace\n");
           fpe("-p          New PID namespace\n");
           fpe("-u          New UTS namespace\n");
           fpe("-U          New user namespace\n");
           fpe("-M uid_map  Specify UID map for user namespace\n");
           fpe("-G gid_map  Specify GID map for user namespace\n");
           fpe("-z          Map user's UID and GID to 0 in user namespace\n");
           fpe("            (equivalent to: -M '0 <uid> 1' -G '0 <gid> 1')\n");
           fpe("-v          Display verbose messages\n");
           fpe("If -z, -M, or -G is specified, -U is required.\n");
           fpe("It is not permitted to specify both -z and either -M or -G.\n");
           fpe("Map strings for -M and -G consist of records of the form:\n");
           fpe("    ID-inside-ns   ID-outside-ns   len\n");
           fpe("A map string can contain multiple records, separated"
               " by commas;\n");
           fpe("the commas are replaced by newlines before writing"
               " to map files.\n");


       /* Update the mapping file 'map_file', with the value provided in
          'mapping', a string that defines a UID or GID mapping. A UID or
          GID mapping consists of one or more newline-delimited records
          of the form:

              ID_inside-ns    ID-outside-ns   length

          Requiring the user to supply a string that contains newlines is
          of course inconvenient for command-line use. Thus, we permit the
          use of commas to delimit records in this string, and replace them
          with newlines before writing the string to the file. */

       static void
       update_map(char *mapping, char *map_file)
           int fd;
           size_t map_len;     /* Length of 'mapping' */

           /* Replace commas in mapping string with newlines. */

           map_len = strlen(mapping);
           for (size_t j = 0; j < map_len; j++)
               if (mapping[j] == ',')
                   mapping[j] = '\n';

           fd = open(map_file, O_RDWR);
           if (fd == -1) {
               fprintf(stderr, "ERROR: open %s: %s\n", map_file,

           if (write(fd, mapping, map_len) != map_len) {
               fprintf(stderr, "ERROR: write %s: %s\n", map_file,


       /* Linux 3.19 made a change in the handling of setgroups(2) and
          the 'gid_map' file to address a security issue.  The issue
          allowed *unprivileged* users to employ user namespaces in
          order to drop groups.  The upshot of the 3.19 changes is that
          in order to update the 'gid_maps' file, use of the setgroups()
          system call in this user namespace must first be disabled by
          writing "deny" to one of the /proc/PID/setgroups files for
          this namespace.  That is the purpose of the following function.  */

       static void
       proc_setgroups_write(pid_t child_pid, char *str)
           char setgroups_path[PATH_MAX];
           int fd;

           snprintf(setgroups_path, PATH_MAX, "/proc/%jd/setgroups",
                   (intmax_t) child_pid);

           fd = open(setgroups_path, O_RDWR);
           if (fd == -1) {

               /* We may be on a system that doesn't support
                  /proc/PID/setgroups. In that case, the file won't exist,
                  and the system won't impose the restrictions that Linux 3.19
                  added. That's fine: we don't need to do anything in order
                  to permit 'gid_map' to be updated.

                  However, if the error from open() was something other than
                  the ENOENT error that is expected for that case,  let the
                  user know. */

               if (errno != ENOENT)
                   fprintf(stderr, "ERROR: open %s: %s\n", setgroups_path,

           if (write(fd, str, strlen(str)) == -1)
               fprintf(stderr, "ERROR: write %s: %s\n", setgroups_path,


       static int              /* Start function for cloned child */
       childFunc(void *arg)
           struct child_args *args = arg;
           char ch;

           /* Wait until the parent has updated the UID and GID mappings.
              See the comment in main(). We wait for end of file on a
              pipe that will be closed by the parent process once it has
              updated the mappings. */

           close(args->pipe_fd[1]);    /* Close our descriptor for the write
                                          end of the pipe so that we see EOF
                                          when parent closes its descriptor. */
           if (read(args->pipe_fd[0], &ch, 1) != 0) {
                       "Failure in child: read from pipe returned != 0\n");


           /* Execute a shell command. */

           printf("About to exec %s\n", args->argv[0]);
           execvp(args->argv[0], args->argv);
           err(EXIT_FAILURE, "execvp");

       #define STACK_SIZE (1024 * 1024)

       static char child_stack[STACK_SIZE];    /* Space for child's stack */

       main(int argc, char *argv[])
           int flags, opt, map_zero;
           pid_t child_pid;
           struct child_args args;
           char *uid_map, *gid_map;
           const int MAP_BUF_SIZE = 100;
           char map_buf[MAP_BUF_SIZE];
           char map_path[PATH_MAX];

           /* Parse command-line options. The initial '+' character in
              the final getopt() argument prevents GNU-style permutation
              of command-line options. That's useful, since sometimes
              the 'command' to be executed by this program itself
              has command-line options. We don't want getopt() to treat
              those as options to this program. */

           flags = 0;
           verbose = 0;
           gid_map = NULL;
           uid_map = NULL;
           map_zero = 0;
           while ((opt = getopt(argc, argv, "+imnpuUM:G:zv")) != -1) {
               switch (opt) {
               case 'i': flags |= CLONE_NEWIPC;        break;
               case 'm': flags |= CLONE_NEWNS;         break;
               case 'n': flags |= CLONE_NEWNET;        break;
               case 'p': flags |= CLONE_NEWPID;        break;
               case 'u': flags |= CLONE_NEWUTS;        break;
               case 'v': verbose = 1;                  break;
               case 'z': map_zero = 1;                 break;
               case 'M': uid_map = optarg;             break;
               case 'G': gid_map = optarg;             break;
               case 'U': flags |= CLONE_NEWUSER;       break;
               default:  usage(argv[0]);

           /* -M or -G without -U is nonsensical */

           if (((uid_map != NULL || gid_map != NULL || map_zero) &&
                       !(flags & CLONE_NEWUSER)) ||
                   (map_zero && (uid_map != NULL || gid_map != NULL)))

           args.argv = &argv[optind];

           /* We use a pipe to synchronize the parent and child, in order to
              ensure that the parent sets the UID and GID maps before the child
              calls execve(). This ensures that the child maintains its
              capabilities during the execve() in the common case where we
              want to map the child's effective user ID to 0 in the new user
              namespace. Without this synchronization, the child would lose
              its capabilities if it performed an execve() with nonzero
              user IDs (see the capabilities(7) man page for details of the
              transformation of a process's capabilities during execve()). */

           if (pipe(args.pipe_fd) == -1)
               err(EXIT_FAILURE, "pipe");

           /* Create the child in new namespace(s). */

           child_pid = clone(childFunc, child_stack + STACK_SIZE,
                             flags | SIGCHLD, &args);
           if (child_pid == -1)
               err(EXIT_FAILURE, "clone");

           /* Parent falls through to here. */

           if (verbose)
               printf("%s: PID of child created by clone() is %jd\n",
                       argv[0], (intmax_t) child_pid);

           /* Update the UID and GID maps in the child. */

           if (uid_map != NULL || map_zero) {
               snprintf(map_path, PATH_MAX, "/proc/%jd/uid_map",
                       (intmax_t) child_pid);
               if (map_zero) {
                   snprintf(map_buf, MAP_BUF_SIZE, "0 %jd 1",
                           (intmax_t) getuid());
                   uid_map = map_buf;
               update_map(uid_map, map_path);

           if (gid_map != NULL || map_zero) {
               proc_setgroups_write(child_pid, "deny");

               snprintf(map_path, PATH_MAX, "/proc/%jd/gid_map",
                       (intmax_t) child_pid);
               if (map_zero) {
                   snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1",
                           (intmax_t) getgid());
                   gid_map = map_buf;
               update_map(gid_map, map_path);

           /* Close the write end of the pipe, to signal to the child that we
              have updated the UID and GID maps. */


           if (waitpid(child_pid, NULL, 0) == -1)      /* Wait for child */
               err(EXIT_FAILURE, "waitpid");

           if (verbose)
               printf("%s: terminating\n", argv[0]);


SEE ALSO         top

       newgidmap(1), newuidmap(1), clone(2), ptrace(2), setns(2),
       unshare(2), proc(5), subgid(5), subuid(5), capabilities(7),
       cgroup_namespaces(7), credentials(7), namespaces(7),

       The kernel source file

Linux man-pages (unreleased)     (date)               user_namespaces(7)

Pages that refer to this page: nsenter(1)systemd-detect-virt(1)unshare(1)clone(2)getgroups(2)ioctl_ns(2)keyctl(2)mount_setattr(2)seteuid(2)setgid(2)setns(2)setresuid(2)setreuid(2)setuid(2)unshare(2)cap_get_file(3)cap_get_proc(3)lttng-ust(3)proc(5)subgid(5)subuid(5)capabilities(7)cgroup_namespaces(7)cgroups(7)credentials(7)mount_namespaces(7)namespaces(7)network_namespaces(7)pid_namespaces(7)getcap(8)setcap(8)