1USER_NAMESPACES(7)         Linux Programmer's Manual        USER_NAMESPACES(7)
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NAME

6       user_namespaces - overview of Linux user namespaces
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DESCRIPTION

9       For an overview of namespaces, see namespaces(7).
10
11       User namespaces isolate security-related identifiers and attributes, in
12       particular, user IDs and group IDs (see credentials(7)), the  root  di‐
13       rectory,  keys  (see  keyrings(7)),  and  capabilities  (see  capabili‐
14       ties(7)).  A process's user and group IDs can be different  inside  and
15       outside  a  user namespace.  In particular, a process can have a normal
16       unprivileged user ID outside a user namespace while at  the  same  time
17       having a user ID of 0 inside the namespace; in other words, the process
18       has full privileges for operations inside the user  namespace,  but  is
19       unprivileged for operations outside the namespace.
20
21   Nested namespaces, namespace membership
22       User  namespaces can be nested; that is, each user namespace—except the
23       initial ("root") namespace—has a parent user namespace,  and  can  have
24       zero  or  more child user namespaces.  The parent user namespace is the
25       user namespace of the process that creates the  user  namespace  via  a
26       call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.
27
28       The  kernel imposes (since version 3.11) a limit of 32 nested levels of
29       user namespaces.  Calls to unshare(2) or clone(2) that would cause this
30       limit to be exceeded fail with the error EUSERS.
31
32       Each process is a member of exactly one user namespace.  A process cre‐
33       ated via fork(2) or clone(2) without the CLONE_NEWUSER flag is a member
34       of  the  same  user namespace as its parent.  A single-threaded process
35       can join another user namespace with setns(2) if it has the CAP_SYS_AD‐
36       MIN  in that namespace; upon doing so, it gains a full set of capabili‐
37       ties in that namespace.
38
39       A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag makes  the
40       new  child process (for clone(2)) or the caller (for unshare(2)) a mem‐
41       ber of the new user namespace created by the call.
42
43       The NS_GET_PARENT ioctl(2)  operation  can  be  used  to  discover  the
44       parental relationship between user namespaces; see ioctl_ns(2).
45
46   Capabilities
47       The  child  process  created  by  clone(2)  with the CLONE_NEWUSER flag
48       starts out with a complete set of capabilities in the  new  user  name‐
49       space.  Likewise, a process that creates a new user namespace using un‐
50       share(2) or joins an existing user namespace  using  setns(2)  gains  a
51       full  set  of  capabilities in that namespace.  On the other hand, that
52       process has no capabilities in the parent (in the case of clone(2))  or
53       previous  (in the case of unshare(2) and setns(2)) user namespace, even
54       if the new namespace is created or joined by the  root  user  (i.e.,  a
55       process with user ID 0 in the root namespace).
56
57       Note that a call to execve(2) will cause a process's capabilities to be
58       recalculated in the usual way (see capabilities(7)).  Consequently, un‐
59       less  the  process has a user ID of 0 within the namespace, or the exe‐
60       cutable file has a nonempty inheritable capabilities mask, the  process
61       will  lose  all  capabilities.  See the discussion of user and group ID
62       mappings, below.
63
64       A call to clone(2) or unshare(2) using the CLONE_NEWUSER flag or a call
65       to  setns(2) that moves the caller into another user namespace sets the
66       "securebits" flags (see capabilities(7)) to their default  values  (all
67       flags  disabled)  in the child (for clone(2)) or caller (for unshare(2)
68       or setns(2)).  Note that because the caller no longer has  capabilities
69       in its original user namespace after a call to setns(2), it is not pos‐
70       sible for a process to reset its "securebits" flags while retaining its
71       user  namespace membership by using a pair of setns(2) calls to move to
72       another user namespace and then return to its original user namespace.
73
74       The rules for determining whether or not a process has a capability  in
75       a particular user namespace are as follows:
76
77       1. A process has a capability inside a user namespace if it is a member
78          of that namespace and it has the capability in its  effective  capa‐
79          bility  set.  A process can gain capabilities in its effective capa‐
80          bility set in various ways.  For example, it may execute a set-user-
81          ID  program  or an executable with associated file capabilities.  In
82          addition,  a  process  may  gain  capabilities  via  the  effect  of
83          clone(2), unshare(2), or setns(2), as already described.
84
85       2. If  a process has a capability in a user namespace, then it has that
86          capability in all child (and further removed descendant)  namespaces
87          as well.
88
89       3. When  a  user namespace is created, the kernel records the effective
90          user ID of the creating process as being the "owner"  of  the  name‐
91          space.   A  process that resides in the parent of the user namespace
92          and whose effective user ID matches the owner of the  namespace  has
93          all  capabilities in the namespace.  By virtue of the previous rule,
94          this means that the process has all capabilities in all further  re‐
95          moved  descendant  user  namespaces  as  well.  The NS_GET_OWNER_UID
96          ioctl(2) operation can be used to discover the user ID of the  owner
97          of the namespace; see ioctl_ns(2).
98
99   Effect of capabilities within a user namespace
100       Having  a  capability inside a user namespace permits a process to per‐
101       form operations (that require privilege) only on resources governed  by
102       that  namespace.   In  other words, having a capability in a user name‐
103       space permits a process to perform privileged operations  on  resources
104       that  are  governed  by (nonuser) namespaces owned by (associated with)
105       the user namespace (see the next subsection).
106
107       On the other hand, there are many privileged operations that affect re‐
108       sources  that  are not associated with any namespace type, for example,
109       changing the system (i.e., calendar) time (governed  by  CAP_SYS_TIME),
110       loading  a  kernel  module (governed by CAP_SYS_MODULE), and creating a
111       device (governed by CAP_MKNOD).  Only a process with privileges in  the
112       initial user namespace can perform such operations.
113
114       Holding  CAP_SYS_ADMIN  within the user namespace that owns a process's
115       mount namespace allows that process to create bind mounts and mount the
116       following types of filesystems:
117
118           * /proc (since Linux 3.8)
119           * /sys (since Linux 3.8)
120           * devpts (since Linux 3.9)
121           * tmpfs(5) (since Linux 3.9)
122           * ramfs (since Linux 3.9)
123           * mqueue (since Linux 3.9)
124           * bpf (since Linux 4.4)
125
126       Holding  CAP_SYS_ADMIN  within the user namespace that owns a process's
127       cgroup namespace allows (since Linux 4.6) that process to the mount the
128       cgroup  version  2  filesystem  and  cgroup version 1 named hierarchies
129       (i.e., cgroup filesystems mounted with the "none,name=" option).
130
131       Holding CAP_SYS_ADMIN within the user namespace that owns  a  process's
132       PID  namespace  allows  (since  Linux  3.8) that process to mount /proc
133       filesystems.
134
135       Note however, that mounting block-based filesystems can be done only by
136       a process that holds CAP_SYS_ADMIN in the initial user namespace.
137
138   Interaction of user namespaces and other types of namespaces
139       Starting  in  Linux  3.8,  unprivileged processes can create user name‐
140       spaces, and the other types of namespaces can be created with just  the
141       CAP_SYS_ADMIN capability in the caller's user namespace.
142
143       When  a nonuser namespace is created, it is owned by the user namespace
144       in which the creating process was a member at the time of the  creation
145       of  the  namespace.  Privileged operations on resources governed by the
146       nonuser namespace require that the process has the necessary  capabili‐
147       ties in the user namespace that owns the nonuser namespace.
148
149       If  CLONE_NEWUSER  is  specified along with other CLONE_NEW* flags in a
150       single clone(2) or unshare(2) call, the user namespace is guaranteed to
151       be  created  first,  giving the child (clone(2)) or caller (unshare(2))
152       privileges over the remaining namespaces created by the call.  Thus, it
153       is  possible  for an unprivileged caller to specify this combination of
154       flags.
155
156       When a new namespace (other than  a  user  namespace)  is  created  via
157       clone(2)  or  unshare(2),  the kernel records the user namespace of the
158       creating process as the owner of the new namespace.  (This  association
159       can't  be  changed.)   When a process in the new namespace subsequently
160       performs privileged operations that operate on  global  resources  iso‐
161       lated  by  the namespace, the permission checks are performed according
162       to the process's capabilities in the user namespace that the kernel as‐
163       sociated  with  the new namespace.  For example, suppose that a process
164       attempts to change the hostname (sethostname(2)), a  resource  governed
165       by  the  UTS  namespace.  In this case, the kernel will determine which
166       user namespace owns the process's UTS namespace, and check whether  the
167       process  has the required capability (CAP_SYS_ADMIN) in that user name‐
168       space.
169
170       The NS_GET_USERNS ioctl(2) operation can be used to discover  the  user
171       namespace that owns a nonuser namespace; see ioctl_ns(2).
172
173   User and group ID mappings: uid_map and gid_map
174       When  a  user  namespace is created, it starts out without a mapping of
175       user  IDs  (group   IDs)   to   the   parent   user   namespace.    The
176       /proc/[pid]/uid_map  and  /proc/[pid]/gid_map  files  (available  since
177       Linux 3.5) expose the mappings for user and group IDs inside  the  user
178       namespace  for  the  process  pid.  These files can be read to view the
179       mappings in a user namespace and written to (once) to define  the  map‐
180       pings.
181
182       The  description  in  the following paragraphs explains the details for
183       uid_map; gid_map is exactly the same, but each instance of "user ID" is
184       replaced by "group ID".
185
186       The  uid_map  file  exposes the mapping of user IDs from the user name‐
187       space of the process pid to the user  namespace  of  the  process  that
188       opened uid_map (but see a qualification to this point below).  In other
189       words, processes that are in different user namespaces will potentially
190       see  different  values when reading from a particular uid_map file, de‐
191       pending on the user ID mappings for the user namespaces of the  reading
192       processes.
193
194       Each  line in the uid_map file specifies a 1-to-1 mapping of a range of
195       contiguous user IDs between two user namespaces.  (When  a  user  name‐
196       space is first created, this file is empty.)  The specification in each
197       line takes the form of three numbers delimited  by  white  space.   The
198       first  two numbers specify the starting user ID in each of the two user
199       namespaces.  The third number specifies the length of the mapped range.
200       In detail, the fields are interpreted as follows:
201
202       (1) The  start  of  the  range of user IDs in the user namespace of the
203           process pid.
204
205       (2) The start of the range of user IDs to which the user IDs  specified
206           by  field one map.  How field two is interpreted depends on whether
207           the process that opened uid_map and the process pid are in the same
208           user namespace, as follows:
209
210           a) If the two processes are in different user namespaces: field two
211              is the start of a range of user IDs in the user namespace of the
212              process that opened uid_map.
213
214           b) If  the  two processes are in the same user namespace: field two
215              is the start of the range of user IDs in the parent  user  name‐
216              space  of  the  process  pid.   This  case enables the opener of
217              uid_map (the common case here is opening /proc/self/uid_map)  to
218              see  the  mapping  of  user  IDs  into the user namespace of the
219              process that created this user namespace.
220
221       (3) The length of the range of user IDs that is mapped between the  two
222           user namespaces.
223
224       System  calls  that return user IDs (group IDs)—for example, getuid(2),
225       getgid(2), and the credential  fields  in  the  structure  returned  by
226       stat(2)—return  the  user  ID  (group ID) mapped into the caller's user
227       namespace.
228
229       When a process accesses a file, its user and group IDs are mapped  into
230       the  initial  user namespace for the purpose of permission checking and
231       assigning IDs when creating a file.  When a process retrieves file user
232       and  group  IDs  via stat(2), the IDs are mapped in the opposite direc‐
233       tion, to produce values relative to the process user and group ID  map‐
234       pings.
235
236       The  initial  user  namespace has no parent namespace, but, for consis‐
237       tency, the kernel provides dummy user and group ID  mapping  files  for
238       this namespace.  Looking at the uid_map file (gid_map is the same) from
239       a shell in the initial namespace shows:
240
241           $ cat /proc/$$/uid_map
242                    0          0 4294967295
243
244       This mapping tells us that the range starting at  user  ID  0  in  this
245       namespace  maps  to  a  range starting at 0 in the (nonexistent) parent
246       namespace, and the length of the range is the largest  32-bit  unsigned
247       integer.  This leaves 4294967295 (the 32-bit signed -1 value) unmapped.
248       This is deliberate: (uid_t) -1 is used in several interfaces (e.g., se‐
249       treuid(2))  as  a  way to specify "no user ID".  Leaving (uid_t) -1 un‐
250       mapped and unusable guarantees that there will be no confusion when us‐
251       ing these interfaces.
252
253   Defining user and group ID mappings: writing to uid_map and gid_map
254       After  the creation of a new user namespace, the uid_map file of one of
255       the processes in the namespace may be written to  once  to  define  the
256       mapping  of  user  IDs  in the new user namespace.  An attempt to write
257       more than once to a uid_map file in a user namespace fails with the er‐
258       ror EPERM.  Similar rules apply for gid_map files.
259
260       The  lines  written  to uid_map (gid_map) must conform to the following
261       rules:
262
263       *  The three fields must be valid numbers, and the last field  must  be
264          greater than 0.
265
266       *  Lines are terminated by newline characters.
267
268       *  There  is a limit on the number of lines in the file.  In Linux 4.14
269          and earlier, this limit was (arbitrarily) set  at  5  lines.   Since
270          Linux  4.15,  the  limit  is  340 lines.  In addition, the number of
271          bytes written to the file must be less than the  system  page  size,
272          and  the  write  must  be  performed at the start of the file (i.e.,
273          lseek(2) and pwrite(2) can't be used to write to nonzero offsets  in
274          the file).
275
276       *  The  range  of  user  IDs  (group IDs) specified in each line cannot
277          overlap with the ranges in any other lines.  In the  initial  imple‐
278          mentation  (Linux 3.8), this requirement was satisfied by a simplis‐
279          tic implementation that imposed the  further  requirement  that  the
280          values  in  both  field 1 and field 2 of successive lines must be in
281          ascending numerical order, which prevented some otherwise valid maps
282          from being created.  Linux 3.9 and later fix this limitation, allow‐
283          ing any valid set of nonoverlapping maps.
284
285       *  At least one line must be written to the file.
286
287       Writes that violate the above rules fail with the error EINVAL.
288
289       In  order  for  a  process  to   write   to   the   /proc/[pid]/uid_map
290       (/proc/[pid]/gid_map)  file,  all of the following requirements must be
291       met:
292
293       1. The writing process must have the CAP_SETUID (CAP_SETGID) capability
294          in the user namespace of the process pid.
295
296       2. The  writing  process  must  either  be in the user namespace of the
297          process pid or be in the parent user namespace of the process pid.
298
299       3. The mapped user IDs (group IDs) must in turn have a mapping  in  the
300          parent user namespace.
301
302       4. One of the following two cases applies:
303
304          *  Either  the writing process has the CAP_SETUID (CAP_SETGID) capa‐
305             bility in the parent user namespace.
306
307             +  No further restrictions apply: the process can  make  mappings
308                to  arbitrary  user  IDs  (group IDs) in the parent user name‐
309                space.
310
311          *  Or otherwise all of the following restrictions apply:
312
313             +  The data written to uid_map (gid_map) must consist of a single
314                line  that maps the writing process's effective user ID (group
315                ID) in the parent user namespace to a user ID  (group  ID)  in
316                the user namespace.
317
318             +  The  writing  process  must have the same effective user ID as
319                the process that created the user namespace.
320
321             +  In the case of gid_map, use of the  setgroups(2)  system  call
322                must first be denied by writing "deny" to the /proc/[pid]/set‐
323                groups file (see below) before writing to gid_map.
324
325       Writes that violate the above rules fail with the error EPERM.
326
327   Interaction with system calls that change process UIDs or GIDs
328       In a user namespace where the uid_map file has not  been  written,  the
329       system calls that change user IDs will fail.  Similarly, if the gid_map
330       file has not been written, the system calls that change group IDs  will
331       fail.   After the uid_map and gid_map files have been written, only the
332       mapped values may be used in system calls that change  user  and  group
333       IDs.
334
335       For user IDs, the relevant system calls include setuid(2), setfsuid(2),
336       setreuid(2), and setresuid(2).  For  group  IDs,  the  relevant  system
337       calls  include  setgid(2),  setfsgid(2), setregid(2), setresgid(2), and
338       setgroups(2).
339
340       Writing "deny" to the  /proc/[pid]/setgroups  file  before  writing  to
341       /proc/[pid]/gid_map  will  permanently  disable  setgroups(2) in a user
342       namespace and allow writing to /proc/[pid]/gid_map without  having  the
343       CAP_SETGID capability in the parent user namespace.
344
345   The /proc/[pid]/setgroups file
346       The /proc/[pid]/setgroups file displays the string "allow" if processes
347       in the user namespace that contains the process pid  are  permitted  to
348       employ the setgroups(2) system call; it displays "deny" if setgroups(2)
349       is not permitted in that user namespace.  Note that regardless  of  the
350       value   in  the  /proc/[pid]/setgroups  file  (and  regardless  of  the
351       process's capabilities), calls to setgroups(2) are also  not  permitted
352       if /proc/[pid]/gid_map has not yet been set.
353
354       A  privileged  process  (one  with  the CAP_SYS_ADMIN capability in the
355       namespace) may write either of the strings "allow" or  "deny"  to  this
356       file  before  writing a group ID mapping for this user namespace to the
357       file /proc/[pid]/gid_map.   Writing  the  string  "deny"  prevents  any
358       process in the user namespace from employing setgroups(2).
359
360       The essence of the restrictions described in the preceding paragraph is
361       that it is permitted to write to /proc/[pid]/setgroups only so long  as
362       calling  setgroups(2) is disallowed because /proc/[pid]/gid_map has not
363       been set.  This ensures that a process cannot transition from  a  state
364       where  setgroups(2) is allowed to a state where setgroups(2) is denied;
365       a process can transition only from  setgroups(2)  being  disallowed  to
366       setgroups(2) being allowed.
367
368       The  default  value  of this file in the initial user namespace is "al‐
369       low".
370
371       Once /proc/[pid]/gid_map has been written to (which has the  effect  of
372       enabling  setgroups(2) in the user namespace), it is no longer possible
373       to disallow setgroups(2) by  writing  "deny"  to  /proc/[pid]/setgroups
374       (the write fails with the error EPERM).
375
376       A  child user namespace inherits the /proc/[pid]/setgroups setting from
377       its parent.
378
379       If the setgroups file has the value "deny", then the setgroups(2)  sys‐
380       tem  call  can't  subsequently  be reenabled (by writing "allow" to the
381       file) in this user namespace.  (Attempts to do so fail with  the  error
382       EPERM.)   This restriction also propagates down to all child user name‐
383       spaces of this user namespace.
384
385       The /proc/[pid]/setgroups file was added in Linux 3.19, but  was  back‐
386       ported to many earlier stable kernel series, because it addresses a se‐
387       curity issue.  The issue  concerned  files  with  permissions  such  as
388       "rwx---rwx".  Such files give fewer permissions to "group" than they do
389       to "other".  This means that dropping groups using  setgroups(2)  might
390       allow  a process file access that it did not formerly have.  Before the
391       existence of user namespaces this was not a concern, since only a priv‐
392       ileged  process  (one  with  the CAP_SETGID capability) could call set‐
393       groups(2).  However, with the introduction of user namespaces,  it  be‐
394       came  possible for an unprivileged process to create a new namespace in
395       which the user had all privileges.  This then allowed formerly unprivi‐
396       leged  users to drop groups and thus gain file access that they did not
397       previously have.  The /proc/[pid]/setgroups file was added  to  address
398       this security issue, by denying any pathway for an unprivileged process
399       to drop groups with setgroups(2).
400
401   Unmapped user and group IDs
402       There are various places where an unmapped user ID (group  ID)  may  be
403       exposed  to  user  space.  For example, the first process in a new user
404       namespace may call getuid(2) before a user ID mapping has been  defined
405       for  the  namespace.   In  most such cases, an unmapped user ID is con‐
406       verted to the overflow user ID (group ID); the default  value  for  the
407       overflow  user  ID  (group  ID)  is  65534.   See  the  descriptions of
408       /proc/sys/kernel/overflowuid   and   /proc/sys/kernel/overflowgid    in
409       proc(5).
410
411       The  cases where unmapped IDs are mapped in this fashion include system
412       calls that return user IDs (getuid(2), getgid(2), and similar), creden‐
413       tials  passed  over  a  UNIX  domain  socket,  credentials  returned by
414       stat(2), waitid(2), and the System V  IPC  "ctl"  IPC_STAT  operations,
415       credentials   exposed   by   /proc/[pid]/status   and   the   files  in
416       /proc/sysvipc/*, credentials returned via the si_uid field in the  sig‐
417       info_t  received  with a signal (see sigaction(2)), credentials written
418       to the process accounting file (see acct(5)), and credentials  returned
419       with POSIX message queue notifications (see mq_notify(3)).
420
421       There  is  one  notable  case where unmapped user and group IDs are not
422       converted to the corresponding  overflow  ID  value.   When  viewing  a
423       uid_map  or  gid_map  file  in which there is no mapping for the second
424       field, that field is displayed as 4294967295 (-1 as an  unsigned  inte‐
425       ger).
426
427   Accessing files
428       In order to determine permissions when an unprivileged process accesses
429       a file, the process credentials (UID, GID) and the file credentials are
430       in  effect  mapped back to what they would be in the initial user name‐
431       space and then compared to determine the permissions that  the  process
432       has  on  the  file.   The same is also of other objects that employ the
433       credentials plus permissions mask accessibility model, such as System V
434       IPC objects
435
436   Operation of file-related capabilities
437       Certain  capabilities allow a process to bypass various kernel-enforced
438       restrictions when performing operations on files owned by  other  users
439       or   groups.   These  capabilities  are:  CAP_CHOWN,  CAP_DAC_OVERRIDE,
440       CAP_DAC_READ_SEARCH, CAP_FOWNER, and CAP_FSETID.
441
442       Within a user namespace, these capabilities allow a process  to  bypass
443       the  rules  if  the  process has the relevant capability over the file,
444       meaning that:
445
446       *  the process has the relevant effective capability in its user  name‐
447          space; and
448
449       *  the file's user ID and group ID both have valid mappings in the user
450          namespace.
451
452       The CAP_FOWNER capability is treated somewhat exceptionally: it  allows
453       a  process  to  bypass  the corresponding rules so long as at least the
454       file's user ID has a mapping in the user namespace  (i.e.,  the  file's
455       group ID does not need to have a valid mapping).
456
457   Set-user-ID and set-group-ID programs
458       When  a  process  inside  a user namespace executes a set-user-ID (set-
459       group-ID) program, the process's effective user (group) ID  inside  the
460       namespace  is  changed to whatever value is mapped for the user (group)
461       ID of the file.  However, if either the user or the  group  ID  of  the
462       file  has  no mapping inside the namespace, the set-user-ID (set-group-
463       ID) bit is silently ignored: the  new  program  is  executed,  but  the
464       process's  effective  user (group) ID is left unchanged.  (This mirrors
465       the semantics of executing a set-user-ID or set-group-ID  program  that
466       resides  on  a  filesystem that was mounted with the MS_NOSUID flag, as
467       described in mount(2).)
468
469   Miscellaneous
470       When a process's user and group IDs  are  passed  over  a  UNIX  domain
471       socket  to a process in a different user namespace (see the description
472       of SCM_CREDENTIALS in unix(7)), they are  translated  into  the  corre‐
473       sponding  values  as per the receiving process's user and group ID map‐
474       pings.
475

CONFORMING TO

477       Namespaces are a Linux-specific feature.
478

NOTES

480       Over the years, there have been a lot of features that have been  added
481       to  the  Linux  kernel that have been made available only to privileged
482       users because of their potential to confuse  set-user-ID-root  applica‐
483       tions.   In  general,  it becomes safe to allow the root user in a user
484       namespace to use those features because it is impossible,  while  in  a
485       user  namespace,  to  gain  more privilege than the root user of a user
486       namespace has.
487
488   Availability
489       Use of user namespaces requires a kernel that is  configured  with  the
490       CONFIG_USER_NS  option.   User namespaces require support in a range of
491       subsystems across the kernel.  When an unsupported subsystem is config‐
492       ured  into  the kernel, it is not possible to configure user namespaces
493       support.
494
495       As at Linux 3.8, most relevant subsystems  supported  user  namespaces,
496       but  a  number of filesystems did not have the infrastructure needed to
497       map user and group IDs between user namespaces.  Linux  3.9  added  the
498       required  infrastructure  support for many of the remaining unsupported
499       filesystems (Plan 9 (9P), Andrew File System (AFS), Ceph,  CIFS,  CODA,
500       NFS,  and  OCFS2).  Linux 3.12 added support for the last of the unsup‐
501       ported major filesystems, XFS.
502

EXAMPLES

504       The program below is designed to allow experimenting  with  user  name‐
505       spaces, as well as other types of namespaces.  It creates namespaces as
506       specified by command-line options and then executes  a  command  inside
507       those namespaces.  The comments and usage() function inside the program
508       provide a full explanation of the program.  The following shell session
509       demonstrates its use.
510
511       First, we look at the run-time environment:
512
513           $ uname -rs     # Need Linux 3.8 or later
514           Linux 3.8.0
515           $ id -u         # Running as unprivileged user
516           1000
517           $ id -g
518           1000
519
520       Now  start a new shell in new user (-U), mount (-m), and PID (-p) name‐
521       spaces, with user ID (-M) and group ID (-G) 1000 mapped to 0 inside the
522       user namespace:
523
524           $ ./userns_child_exec -p -m -U -M '0 1000 1' -G '0 1000 1' bash
525
526       The  shell  has  PID  1, because it is the first process in the new PID
527       namespace:
528
529           bash$ echo $$
530           1
531
532       Mounting a new /proc filesystem and listing all of the processes  visi‐
533       ble  in  the  new PID namespace shows that the shell can't see any pro‐
534       cesses outside the PID namespace:
535
536           bash$ mount -t proc proc /proc
537           bash$ ps ax
538             PID TTY      STAT   TIME COMMAND
539               1 pts/3    S      0:00 bash
540              22 pts/3    R+     0:00 ps ax
541
542       Inside the user namespace, the shell has user and group  ID  0,  and  a
543       full set of permitted and effective capabilities:
544
545           bash$ cat /proc/$$/status | egrep '^[UG]id'
546           Uid: 0    0    0    0
547           Gid: 0    0    0    0
548           bash$ cat /proc/$$/status | egrep '^Cap(Prm|Inh|Eff)'
549           CapInh:   0000000000000000
550           CapPrm:   0000001fffffffff
551           CapEff:   0000001fffffffff
552
553   Program source
554
555       /* userns_child_exec.c
556
557          Licensed under GNU General Public License v2 or later
558
559          Create a child process that executes a shell command in new
560          namespace(s); allow UID and GID mappings to be specified when
561          creating a user namespace.
562       */
563       #define _GNU_SOURCE
564       #include <sched.h>
565       #include <unistd.h>
566       #include <stdint.h>
567       #include <stdlib.h>
568       #include <sys/wait.h>
569       #include <signal.h>
570       #include <fcntl.h>
571       #include <stdio.h>
572       #include <string.h>
573       #include <limits.h>
574       #include <errno.h>
575
576       /* A simple error-handling function: print an error message based
577          on the value in 'errno' and terminate the calling process */
578
579       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
580                               } while (0)
581
582       struct child_args {
583           char **argv;        /* Command to be executed by child, with args */
584           int    pipe_fd[2];  /* Pipe used to synchronize parent and child */
585       };
586
587       static int verbose;
588
589       static void
590       usage(char *pname)
591       {
592           fprintf(stderr, "Usage: %s [options] cmd [arg...]\n\n", pname);
593           fprintf(stderr, "Create a child process that executes a shell "
594                   "command in a new user namespace,\n"
595                   "and possibly also other new namespace(s).\n\n");
596           fprintf(stderr, "Options can be:\n\n");
597       #define fpe(str) fprintf(stderr, "    %s", str);
598           fpe("-i          New IPC namespace\n");
599           fpe("-m          New mount namespace\n");
600           fpe("-n          New network namespace\n");
601           fpe("-p          New PID namespace\n");
602           fpe("-u          New UTS namespace\n");
603           fpe("-U          New user namespace\n");
604           fpe("-M uid_map  Specify UID map for user namespace\n");
605           fpe("-G gid_map  Specify GID map for user namespace\n");
606           fpe("-z          Map user's UID and GID to 0 in user namespace\n");
607           fpe("            (equivalent to: -M '0 <uid> 1' -G '0 <gid> 1')\n");
608           fpe("-v          Display verbose messages\n");
609           fpe("\n");
610           fpe("If -z, -M, or -G is specified, -U is required.\n");
611           fpe("It is not permitted to specify both -z and either -M or -G.\n");
612           fpe("\n");
613           fpe("Map strings for -M and -G consist of records of the form:\n");
614           fpe("\n");
615           fpe("    ID-inside-ns   ID-outside-ns   len\n");
616           fpe("\n");
617           fpe("A map string can contain multiple records, separated"
618               " by commas;\n");
619           fpe("the commas are replaced by newlines before writing"
620               " to map files.\n");
621
622           exit(EXIT_FAILURE);
623       }
624
625       /* Update the mapping file 'map_file', with the value provided in
626          'mapping', a string that defines a UID or GID mapping. A UID or
627          GID mapping consists of one or more newline-delimited records
628          of the form:
629
630              ID_inside-ns    ID-outside-ns   length
631
632          Requiring the user to supply a string that contains newlines is
633          of course inconvenient for command-line use. Thus, we permit the
634          use of commas to delimit records in this string, and replace them
635          with newlines before writing the string to the file. */
636
637       static void
638       update_map(char *mapping, char *map_file)
639       {
640           int fd;
641           size_t map_len;     /* Length of 'mapping' */
642
643           /* Replace commas in mapping string with newlines */
644
645           map_len = strlen(mapping);
646           for (int j = 0; j < map_len; j++)
647               if (mapping[j] == ',')
648                   mapping[j] = '\n';
649
650           fd = open(map_file, O_RDWR);
651           if (fd == -1) {
652               fprintf(stderr, "ERROR: open %s: %s\n", map_file,
653                       strerror(errno));
654               exit(EXIT_FAILURE);
655           }
656
657           if (write(fd, mapping, map_len) != map_len) {
658               fprintf(stderr, "ERROR: write %s: %s\n", map_file,
659                       strerror(errno));
660               exit(EXIT_FAILURE);
661           }
662
663           close(fd);
664       }
665
666       /* Linux 3.19 made a change in the handling of setgroups(2) and the
667          'gid_map' file to address a security issue. The issue allowed
668          *unprivileged* users to employ user namespaces in order to drop
669          The upshot of the 3.19 changes is that in order to update the
670          'gid_maps' file, use of the setgroups() system call in this
671          user namespace must first be disabled by writing "deny" to one of
672          the /proc/PID/setgroups files for this namespace.  That is the
673          purpose of the following function. */
674
675       static void
676       proc_setgroups_write(pid_t child_pid, char *str)
677       {
678           char setgroups_path[PATH_MAX];
679           int fd;
680
681           snprintf(setgroups_path, PATH_MAX, "/proc/%jd/setgroups",
682                   (intmax_t) child_pid);
683
684           fd = open(setgroups_path, O_RDWR);
685           if (fd == -1) {
686
687               /* We may be on a system that doesn't support
688                  /proc/PID/setgroups. In that case, the file won't exist,
689                  and the system won't impose the restrictions that Linux 3.19
690                  added. That's fine: we don't need to do anything in order
691                  to permit 'gid_map' to be updated.
692
693                  However, if the error from open() was something other than
694                  the ENOENT error that is expected for that case,  let the
695                  user know. */
696
697               if (errno != ENOENT)
698                   fprintf(stderr, "ERROR: open %s: %s\n", setgroups_path,
699                       strerror(errno));
700               return;
701           }
702
703           if (write(fd, str, strlen(str)) == -1)
704               fprintf(stderr, "ERROR: write %s: %s\n", setgroups_path,
705                   strerror(errno));
706
707           close(fd);
708       }
709
710       static int              /* Start function for cloned child */
711       childFunc(void *arg)
712       {
713           struct child_args *args = arg;
714           char ch;
715
716           /* Wait until the parent has updated the UID and GID mappings.
717              See the comment in main(). We wait for end of file on a
718              pipe that will be closed by the parent process once it has
719              updated the mappings. */
720
721           close(args->pipe_fd[1]);    /* Close our descriptor for the write
722                                          end of the pipe so that we see EOF
723                                          when parent closes its descriptor */
724           if (read(args->pipe_fd[0], &ch, 1) != 0) {
725               fprintf(stderr,
726                       "Failure in child: read from pipe returned != 0\n");
727               exit(EXIT_FAILURE);
728           }
729
730           close(args->pipe_fd[0]);
731
732           /* Execute a shell command */
733
734           printf("About to exec %s\n", args->argv[0]);
735           execvp(args->argv[0], args->argv);
736           errExit("execvp");
737       }
738
739       #define STACK_SIZE (1024 * 1024)
740
741       static char child_stack[STACK_SIZE];    /* Space for child's stack */
742
743       int
744       main(int argc, char *argv[])
745       {
746           int flags, opt, map_zero;
747           pid_t child_pid;
748           struct child_args args;
749           char *uid_map, *gid_map;
750           const int MAP_BUF_SIZE = 100;
751           char map_buf[MAP_BUF_SIZE];
752           char map_path[PATH_MAX];
753
754           /* Parse command-line options. The initial '+' character in
755              the final getopt() argument prevents GNU-style permutation
756              of command-line options. That's useful, since sometimes
757              the 'command' to be executed by this program itself
758              has command-line options. We don't want getopt() to treat
759              those as options to this program. */
760
761           flags = 0;
762           verbose = 0;
763           gid_map = NULL;
764           uid_map = NULL;
765           map_zero = 0;
766           while ((opt = getopt(argc, argv, "+imnpuUM:G:zv")) != -1) {
767               switch (opt) {
768               case 'i': flags |= CLONE_NEWIPC;        break;
769               case 'm': flags |= CLONE_NEWNS;         break;
770               case 'n': flags |= CLONE_NEWNET;        break;
771               case 'p': flags |= CLONE_NEWPID;        break;
772               case 'u': flags |= CLONE_NEWUTS;        break;
773               case 'v': verbose = 1;                  break;
774               case 'z': map_zero = 1;                 break;
775               case 'M': uid_map = optarg;             break;
776               case 'G': gid_map = optarg;             break;
777               case 'U': flags |= CLONE_NEWUSER;       break;
778               default:  usage(argv[0]);
779               }
780           }
781
782           /* -M or -G without -U is nonsensical */
783
784           if (((uid_map != NULL || gid_map != NULL || map_zero) &&
785                       !(flags & CLONE_NEWUSER)) ||
786                   (map_zero && (uid_map != NULL || gid_map != NULL)))
787               usage(argv[0]);
788
789           args.argv = &argv[optind];
790
791           /* We use a pipe to synchronize the parent and child, in order to
792              ensure that the parent sets the UID and GID maps before the child
793              calls execve(). This ensures that the child maintains its
794              capabilities during the execve() in the common case where we
795              want to map the child's effective user ID to 0 in the new user
796              namespace. Without this synchronization, the child would lose
797              its capabilities if it performed an execve() with nonzero
798              user IDs (see the capabilities(7) man page for details of the
799              transformation of a process's capabilities during execve()). */
800
801           if (pipe(args.pipe_fd) == -1)
802               errExit("pipe");
803
804           /* Create the child in new namespace(s) */
805
806           child_pid = clone(childFunc, child_stack + STACK_SIZE,
807                             flags | SIGCHLD, &args);
808           if (child_pid == -1)
809               errExit("clone");
810
811           /* Parent falls through to here */
812
813           if (verbose)
814               printf("%s: PID of child created by clone() is %jd\n",
815                       argv[0], (intmax_t) child_pid);
816
817           /* Update the UID and GID maps in the child */
818
819           if (uid_map != NULL || map_zero) {
820               snprintf(map_path, PATH_MAX, "/proc/%jd/uid_map",
821                       (intmax_t) child_pid);
822               if (map_zero) {
823                   snprintf(map_buf, MAP_BUF_SIZE, "0 %jd 1",
824                           (intmax_t) getuid());
825                   uid_map = map_buf;
826               }
827               update_map(uid_map, map_path);
828           }
829
830           if (gid_map != NULL || map_zero) {
831               proc_setgroups_write(child_pid, "deny");
832
833               snprintf(map_path, PATH_MAX, "/proc/%jd/gid_map",
834                       (intmax_t) child_pid);
835               if (map_zero) {
836                   snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1",
837                           (intmax_t) getgid());
838                   gid_map = map_buf;
839               }
840               update_map(gid_map, map_path);
841           }
842
843           /* Close the write end of the pipe, to signal to the child that we
844              have updated the UID and GID maps */
845
846           close(args.pipe_fd[1]);
847
848           if (waitpid(child_pid, NULL, 0) == -1)      /* Wait for child */
849               errExit("waitpid");
850
851           if (verbose)
852               printf("%s: terminating\n", argv[0]);
853
854           exit(EXIT_SUCCESS);
855       }
856

SEE ALSO

858       newgidmap(1),  newuidmap(1), clone(2), ptrace(2), setns(2), unshare(2),
859       proc(5), subgid(5), subuid(5),  capabilities(7),  cgroup_namespaces(7),
860       credentials(7), namespaces(7), pid_namespaces(7)
861
862       The kernel source file Documentation/namespaces/resource-control.txt.
863

COLOPHON

865       This  page  is  part of release 5.10 of the Linux man-pages project.  A
866       description of the project, information about reporting bugs,  and  the
867       latest     version     of     this    page,    can    be    found    at
868       https://www.kernel.org/doc/man-pages/.
869
870
871
872Linux                             2020-11-01                USER_NAMESPACES(7)
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