# user_namespaces

## NAME



           user_namespaces - overview of Linux user namespaces
    


## DESCRIPTION



           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
           call.
    
           The **NS_GET_PARENT ioctl**(2) operation can be used to discover the
           parental relationship between user namespaces; see ioctl_nsfs(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.
    
       **Capabilities**
           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 namespace.
    
           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_nsfs(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
           subsection).
    
           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 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
           namespace.
    
       **Interaction of user namespaces and other types of namespaces**
           Since 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_nsfs(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 size 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 size 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 size 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 interfaces.
    
       **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 namespace.
    
                   •  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
                      _gid_map_.
    
           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
           **EPERM**.
    
       **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
           setgroups(2).
    
           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
           "_allow_ ".
    
           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 objects.
    
       **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** ,
           **CAP_DAC_OVERRIDE** , **CAP_DAC_READ_SEARCH** , **CAP_FOWNER** , and **CAP_FSETID**.
    
           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 mapping).
    
       **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
           mount(2).)
    
       **Miscellaneous**
           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



           Linux.
    


## NOTES



           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.
    
       **Availability**
           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



           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
               1000
               $ **id -g** ;
               1000
    
           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 $$** ;
               1
    
           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("\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("\n");
               fpe("Map strings for -M and -G consist of records of the form:\n");
               fpe("\n");
               fpe("    ID-inside-ns   ID-outside-ns   size\n");
               fpe("\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");
    
               exit(EXIT_FAILURE);
           }
    
           /* 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   size
    
              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,
                           strerror(errno));
                   exit(EXIT_FAILURE);
               }
    
               if (write(fd, mapping, map_len) != map_len) {
                   fprintf(stderr, "ERROR: write %s: %s\n", map_file,
                           strerror(errno));
                   exit(EXIT_FAILURE);
               }
    
               close(fd);
           }
    
           /* 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,
                           strerror(errno));
                   return;
               }
    
               if (write(fd, str, strlen(str)) == -1)
                   fprintf(stderr, "ERROR: write %s: %s\n", setgroups_path,
                       strerror(errno));
    
               close(fd);
           }
    
           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) {
                   fprintf(stderr,
                           "Failure in child: read from pipe returned != 0\n");
                   exit(EXIT_FAILURE);
               }
    
               close(args->pipe_fd[0]);
    
               /* 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 */
    
           int
           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)))
                   usage(argv[0]);
    
               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. */
    
               close(args.pipe_fd[1]);
    
               if (waitpid(child_pid, NULL, 0) == -1)      /* Wait for child */
                   err(EXIT_FAILURE, "waitpid");
    
               if (verbose)
                   printf("%s: terminating\n", argv[0]);
    
               exit(EXIT_SUCCESS);
           }
    


* * *