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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       •  Lines are terminated by newline characters.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       •  One of the following two cases applies:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       A privileged process (one with the CAP_SYS_ADMIN capability in the namespace) may  write  either  of  the
       strings  "allow"  or "deny" to this file before writing a group ID mapping for this user namespace to the
       file /proc/pid/gid_map.  Writing the string "deny" prevents  any  process  in  the  user  namespace  from
       employing setgroups(2).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   Set-user-ID and set-group-ID programs
       When a process inside a user namespace executes  a  set-user-ID  (set-group-ID)  program,  the  process's
       effective  user  (group)  ID  inside  the  namespace  is changed to whatever value is mapped for the user
       (group) ID of the file.  However, if either the user or the group ID of the file has  no  mapping  inside
       the  namespace,  the set-user-ID (set-group-ID) bit is silently ignored: the new program is executed, but
       the process's effective user (group) ID is left unchanged.  (This mirrors the semantics  of  executing  a
       set-user-ID  or  set-group-ID  program  that  resides on a filesystem that was mounted with the MS_NOSUID
       flag, as described in mount(2).)

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

STANDARDS

       Linux.

NOTES

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

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

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

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

EXAMPLES

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

       First, we look at the run-time environment:

           $ uname -rs     # Need Linux 3.8 or later
           Linux 3.8.0
           $ id -u         # Running as unprivileged user
           1000
           $ id -g
           1000

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

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

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

           bash$ echo $$
           1

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

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

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

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

   Program source

       /* userns_child_exec.c

          Licensed under GNU General Public License v2 or later

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

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

       static int verbose;

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

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

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

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

           if (write(fd, mapping, map_len) != map_len) {
               fprintf(stderr, "ERROR: write %s: %s\n", map_file,
                       strerror(errno));
               exit(EXIT_FAILURE);
           }

           close(fd);
       }

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

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

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

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

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

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

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

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

           close(fd);
       }

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

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

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

           close(args->pipe_fd[0]);

           /* Execute a shell command. */

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

       #define STACK_SIZE (1024 * 1024)

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

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

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

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

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

           if (((uid_map != NULL || gid_map != NULL || map_zero) &&
                       !(flags & CLONE_NEWUSER)) ||
                   (map_zero && (uid_map != NULL || gid_map != NULL)))
               usage(argv[0]);

           args.argv = &argv[optind];

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

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

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

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

           /* Parent falls through to here. */

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

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

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

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

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

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

           close(args.pipe_fd[1]);

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

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

           exit(EXIT_SUCCESS);
       }

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/admin-guide/namespaces/resource-control.rst.