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man: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  the cgroup version 2 filesystem and cgroup version 1 named hier-
     archies  (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         Cap-
         Prm:   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 */ child-
     Func(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) get-
     gid());
                 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 4.16 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)

/data/webs/external/dokuwiki/data/pages/man/user_namespaces.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

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