xenial (7) user_namespaces.7.gz

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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   keyctl(2)),   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  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  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.

   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)), so that usually, unless it has a  user  ID  of  0  within  the  namespace  or  the
       executable  file  has  a  nonempty inheritable capabilities mask, it 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.

       Having  a  capability  inside  a  user  namespace  permits  a process to perform operations (that require
       privilege) only on resources governed by that 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 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.

       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 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.

   Interaction of user namespaces and other types of namespaces
       Starting in Linux 3.8, unprivileged processes can create user namespaces, and mount, PID,  IPC,  network,
       and UTS 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
       capabilities 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 IPC, mount, network, PID, or UTS 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.

   Restrictions on mount namespaces
       Note the following points with respect to mount namespaces:

       *  A mount namespace has an owner user namespace.  A  mount  namespace  whose  owner  user  namespace  is
          different  from the owner user namespace of its parent mount namespace is considered a less privileged
          mount namespace.

       *  When creating a less privileged mount namespace, shared mounts are  reduced  to  slave  mounts.   This
          ensures  that  mappings  performed  in  less  privileged  mount  namespaces will not propagate to more
          privileged mount namespaces.

       *  Mounts that come as a single unit from more privileged mount  are  locked  together  and  may  not  be
          separated  in  a less privileged mount namespace.  (The unshare(2) CLONE_NEWNS operation brings across
          all of the mounts from the original mount namespace as  a  single  unit,  and  recursive  mounts  that
          propagate between mount namespaces propagate as a single unit.)

       *  The  mount(2) flags MS_RDONLY, MS_NOSUID, MS_NOEXEC, and the "atime" flags (MS_NOATIME, MS_NODIRATIME,
          MS_RELATIME) settings become locked when propagated from a more privileged to a less privileged  mount
          namespace, and may not be changed in the less privileged mount namespace.

       *  A  file  or  directory  that  is  a  mount point in one namespace that is not a mount point in another
          namespace, may be renamed, unlinked, or removed (rmdir(2)) in the mount namespace in which it is not a
          mount point (subject to the usual permission checks).

          Previously,  attempting  to  unlink,  rename,  or remove a file or directory that was a mount point in
          another mount namespace would result in the error EBUSY.  That  behavior  had  technical  problems  of
          enforcement  (e.g.,  for  NFS)  and permitted denial-of-service attacks against more privileged users.
          (i.e., preventing individual files from being updated by bind mounting on top of them).

   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 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 an (arbitrary) limit on the number of lines in the file.  As at Linux 3.18, the limit is five
          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 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) 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.

          *  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.

   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 only transition 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 will 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() 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);

   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.

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
       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.

   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  3.11 added support the last of the
       unsupported major filesystems, XFS.

EXAMPLE

       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

       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

       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

   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);
           }

           /* 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),   setns(2),   unshare(2),   proc(5),   subgid(5),    subuid(5),
       credentials(7), capabilities(7), namespaces(7), pid_namespaces(7)

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

COLOPHON

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       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
       http://www.kernel.org/doc/man-pages/.