user_namespaces

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

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 capabili-
       ties(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 version 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 cre-
       ated 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 mem-
       ber 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_ns(2).

   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  names-
       pace.   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 exe-
       cutable 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), unshare(2), or  setns(2)  using  the  CLONE_NEWUSER
       flag 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:

       1. 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  capa-
          bility  set.  A process can gain capabilities in its effective capa-
          bility 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.

       2. 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.

       3. When  a  user namespace is created, the kernel records the effective
          user ID of the creating process as being the "owner" of  the  names-
          pace.   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 per-
       form operations (that require privilege) only on resources governed  by
       that  namespace.   In other words, having a capability in a user names-
       pace permits a process to perform privileged  operations  on  resources
       that  are  governed  by  (nonuser)  namespaces 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 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  names-
       pace can perform such operations.

       Holding  CAP_SYS_ADMIN  within  the  user  namespace  associated with 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)

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

       Holding CAP_SYS_ADMIN within  the  user  namespace  associated  with  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
       Starting  in  Linux  3.8, unprivileged processes can create user names-
       paces, and other the other types of namespaces can be created with just
       the CAP_SYS_ADMIN capability in the caller's user namespace.

       When a non-user-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.  Actions on the non-user-namespace require capabili-
       ties in the corresponding user 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 against 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  names-
       pace  is associated with 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  with  which  a  non-user  namespace   is   associated;   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  map-
       pings.

       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 names-
       pace 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  read-
       ing 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  names-
       pace  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  names-
              pace  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 direc-
       tion, to produce values relative to the process user and group ID  map-
       pings.

       The  initial  user  namespace has no parent namespace, but, for consis-
       tency, 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 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
       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  imple-
          mentation  (Linux 3.8), this requirement was satisfied by a simplis-
          tic 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, allow-
          ing 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 requirements must be
       met:

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

       2. 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.

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

       4. One of the following two cases applies:

          *  Either  the writing process has the CAP_SETUID (CAP_SETGID) capa-
             bility in the parent user namespace.

             +  No further restrictions apply: the process can  make  mappings
                to  arbitrary  user  IDs (group IDs) in the parent user names-
                pace.

          *  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]/set-
                groups file (see below) before writing to gid_map.

       Writes that violate the above rules 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)  sys-
       tem  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 names-
       paces of this user namespace.

       The /proc/[pid]/setgroups file was added in Linux 3.19, but  was  back-
       ported  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 priv-
       ileged  process  (one  with  the CAP_SETGID capability) could call set-
       groups(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 con-
       verted 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), creden-
       tials  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  sig-
       info_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  inte-
       ger).

   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  corre-
       sponding  values  as per the receiving process's user and group ID map-
       pings.

CONFORMING TO
       Namespaces are a Linux-specific feature.

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  applica-
       tions.   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.

   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 config-
       ured  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 the last of the unsupported
       major filesystems, XFS.

EXAMPLE
       The program below is designed to allow experimenting with  user  names-
       paces,  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) names-
       paces, 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  visi-
       ble  in  the  new PID namespace shows that the shell can't see any pro-
       cesses 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 <sched.h>
       #include <unistd.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>

       /* A simple error-handling function: print an error message based
          on the value in 'errno' and terminate the calling process */

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       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   len\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   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, j;
           size_t map_len;     /* Length of 'mapping' */

           /* Replace commas in mapping string with newlines */

           map_len = strlen(mapping);
           for (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
          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/%ld/setgroups",
                   (long) 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 = (struct child_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);
           errExit("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)
               errExit("pipe");

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

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

           /* Parent falls through to here */

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

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

           if (uid_map != NULL || map_zero) {
               snprintf(map_path, PATH_MAX, "/proc/%ld/uid_map",
                       (long) child_pid);
               if (map_zero) {
                   snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) 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/%ld/gid_map",
                       (long) child_pid);
               if (map_zero) {
                   snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) 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 */
               errExit("waitpid");

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

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       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), pid_namespaces(7)

       The kernel source file Documentation/namespaces/resource-control.txt.

COLOPHON
       This  page  is  part of release 4.15 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2018-02-02                USER_NAMESPACES(7)
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