1USER_NAMESPACES(7) Linux Programmer's Manual USER_NAMESPACES(7)
2
6 user_namespaces - overview of Linux user namespaces
7
9 For an overview of namespaces, see namespaces(7).
10 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 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 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 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 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 The NS_GET_PARENT ioctl(2) operation can be used to discover the
44 parental relationship between user namespaces; see ioctl_ns(2).
45 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 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 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 The rules for determining whether or not a process has a capability in
75 a particular user namespace are as follows:
76 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 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 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 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 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 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 * /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 * overlayfs (since Linux 5.11)
126 Holding CAP_SYS_ADMIN within the user namespace that owns a process's
128 cgroup namespace allows (since Linux 4.6) that process to the mount the
129 cgroup version 2 filesystem and cgroup version 1 named hierarchies
130 (i.e., cgroup filesystems mounted with the "none,name=" option).
131 Holding CAP_SYS_ADMIN within the user namespace that owns a process's
133 PID namespace allows (since Linux 3.8) that process to mount /proc
134 filesystems.
135 Note however, that mounting block-based filesystems can be done only by
137 a process that holds CAP_SYS_ADMIN in the initial user namespace.
138 Interaction of user namespaces and other types of namespaces
140 Starting in Linux 3.8, unprivileged processes can create user name‐
141 spaces, and the other types of namespaces can be created with just the
142 CAP_SYS_ADMIN capability in the caller's user namespace.
143 When a nonuser namespace is created, it is owned by the user namespace
145 in which the creating process was a member at the time of the creation
146 of the namespace. Privileged operations on resources governed by the
147 nonuser namespace require that the process has the necessary capabili‐
148 ties in the user namespace that owns the nonuser namespace.
149 If CLONE_NEWUSER is specified along with other CLONE_NEW* flags in a
151 single clone(2) or unshare(2) call, the user namespace is guaranteed to
152 be created first, giving the child (clone(2)) or caller (unshare(2))
153 privileges over the remaining namespaces created by the call. Thus, it
154 is possible for an unprivileged caller to specify this combination of
155 flags.
156 When a new namespace (other than a user namespace) is created via
158 clone(2) or unshare(2), the kernel records the user namespace of the
159 creating process as the owner of the new namespace. (This association
160 can't be changed.) When a process in the new namespace subsequently
161 performs privileged operations that operate on global resources iso‐
162 lated by the namespace, the permission checks are performed according
163 to the process's capabilities in the user namespace that the kernel as‐
164 sociated with the new namespace. For example, suppose that a process
165 attempts to change the hostname (sethostname(2)), a resource governed
166 by the UTS namespace. In this case, the kernel will determine which
167 user namespace owns the process's UTS namespace, and check whether the
168 process has the required capability (CAP_SYS_ADMIN) in that user name‐
169 space.
170 The NS_GET_USERNS ioctl(2) operation can be used to discover the user
172 namespace that owns a nonuser namespace; see ioctl_ns(2).
173 User and group ID mappings: uid_map and gid_map
175 When a user namespace is created, it starts out without a mapping of
176 user IDs (group IDs) to the parent user namespace. The
177 /proc/[pid]/uid_map and /proc/[pid]/gid_map files (available since
178 Linux 3.5) expose the mappings for user and group IDs inside the user
179 namespace for the process pid. These files can be read to view the
180 mappings in a user namespace and written to (once) to define the map‐
181 pings.
182 The description in the following paragraphs explains the details for
184 uid_map; gid_map is exactly the same, but each instance of "user ID" is
185 replaced by "group ID".
186 The uid_map file exposes the mapping of user IDs from the user name‐
188 space of the process pid to the user namespace of the process that
189 opened uid_map (but see a qualification to this point below). In other
190 words, processes that are in different user namespaces will potentially
191 see different values when reading from a particular uid_map file, de‐
192 pending on the user ID mappings for the user namespaces of the reading
193 processes.
194 Each line in the uid_map file specifies a 1-to-1 mapping of a range of
196 contiguous user IDs between two user namespaces. (When a user name‐
197 space is first created, this file is empty.) The specification in each
198 line takes the form of three numbers delimited by white space. The
199 first two numbers specify the starting user ID in each of the two user
200 namespaces. The third number specifies the length of the mapped range.
201 In detail, the fields are interpreted as follows:
202 (1) The start of the range of user IDs in the user namespace of the
204 process pid.
205 (2) The start of the range of user IDs to which the user IDs specified
207 by field one map. How field two is interpreted depends on whether
208 the process that opened uid_map and the process pid are in the same
209 user namespace, as follows:
210 a) If the two processes are in different user namespaces: field two
212 is the start of a range of user IDs in the user namespace of the
213 process that opened uid_map.
214 b) If the two processes are in the same user namespace: field two
216 is the start of the range of user IDs in the parent user name‐
217 space of the process pid. This case enables the opener of
218 uid_map (the common case here is opening /proc/self/uid_map) to
219 see the mapping of user IDs into the user namespace of the
220 process that created this user namespace.
221 (3) The length of the range of user IDs that is mapped between the two
223 user namespaces.
224 System calls that return user IDs (group IDs)—for example, getuid(2),
226 getgid(2), and the credential fields in the structure returned by
227 stat(2)—return the user ID (group ID) mapped into the caller's user
228 namespace.
229 When a process accesses a file, its user and group IDs are mapped into
231 the initial user namespace for the purpose of permission checking and
232 assigning IDs when creating a file. When a process retrieves file user
233 and group IDs via stat(2), the IDs are mapped in the opposite direc‐
234 tion, to produce values relative to the process user and group ID map‐
235 pings.
236 The initial user namespace has no parent namespace, but, for consis‐
238 tency, the kernel provides dummy user and group ID mapping files for
239 this namespace. Looking at the uid_map file (gid_map is the same) from
240 a shell in the initial namespace shows:
241 $ cat /proc/$$/uid_map
243 0 0 4294967295
244 This mapping tells us that the range starting at user ID 0 in this
246 namespace maps to a range starting at 0 in the (nonexistent) parent
247 namespace, and the length of the range is the largest 32-bit unsigned
248 integer. This leaves 4294967295 (the 32-bit signed -1 value) unmapped.
249 This is deliberate: (uid_t) -1 is used in several interfaces (e.g., se‐
250 treuid(2)) as a way to specify "no user ID". Leaving (uid_t) -1 un‐
251 mapped and unusable guarantees that there will be no confusion when us‐
252 ing these interfaces.
253 Defining user and group ID mappings: writing to uid_map and gid_map
255 After the creation of a new user namespace, the uid_map file of one of
256 the processes in the namespace may be written to once to define the
257 mapping of user IDs in the new user namespace. An attempt to write
258 more than once to a uid_map file in a user namespace fails with the er‐
259 ror EPERM. Similar rules apply for gid_map files.
260 The lines written to uid_map (gid_map) must conform to the following
262 rules:
263 * The three fields must be valid numbers, and the last field must be
265 greater than 0.
266 * Lines are terminated by newline characters.
268 * There is a limit on the number of lines in the file. In Linux 4.14
270 and earlier, this limit was (arbitrarily) set at 5 lines. Since
271 Linux 4.15, the limit is 340 lines. In addition, the number of
272 bytes written to the file must be less than the system page size,
273 and the write must be performed at the start of the file (i.e.,
274 lseek(2) and pwrite(2) can't be used to write to nonzero offsets in
275 the file).
276 * The range of user IDs (group IDs) specified in each line cannot
278 overlap with the ranges in any other lines. In the initial imple‐
279 mentation (Linux 3.8), this requirement was satisfied by a simplis‐
280 tic implementation that imposed the further requirement that the
281 values in both field 1 and field 2 of successive lines must be in
282 ascending numerical order, which prevented some otherwise valid maps
283 from being created. Linux 3.9 and later fix this limitation, allow‐
284 ing any valid set of nonoverlapping maps.
285 * At least one line must be written to the file.
287 Writes that violate the above rules fail with the error EINVAL.
289 In order for a process to write to the /proc/[pid]/uid_map
291 (/proc/[pid]/gid_map) file, all of the following requirements must be
292 met:
293 1. The writing process must have the CAP_SETUID (CAP_SETGID) capability
295 in the user namespace of the process pid.
296 2. The writing process must either be in the user namespace of the
298 process pid or be in the parent user namespace of the process pid.
299 3. The mapped user IDs (group IDs) must in turn have a mapping in the
301 parent user namespace.
302 4. One of the following two cases applies:
304 * Either the writing process has the CAP_SETUID (CAP_SETGID) capa‐
306 bility in the parent user namespace.
307 + No further restrictions apply: the process can make mappings
309 to arbitrary user IDs (group IDs) in the parent user name‐
310 space.
311 * Or otherwise all of the following restrictions apply:
313 + The data written to uid_map (gid_map) must consist of a single
315 line that maps the writing process's effective user ID (group
316 ID) in the parent user namespace to a user ID (group ID) in
317 the user namespace.
318 + The writing process must have the same effective user ID as
320 the process that created the user namespace.
321 + In the case of gid_map, use of the setgroups(2) system call
323 must first be denied by writing "deny" to the /proc/[pid]/set‐
324 groups file (see below) before writing to gid_map.
325 Writes that violate the above rules fail with the error EPERM.
327 Interaction with system calls that change process UIDs or GIDs
329 In a user namespace where the uid_map file has not been written, the
330 system calls that change user IDs will fail. Similarly, if the gid_map
331 file has not been written, the system calls that change group IDs will
332 fail. After the uid_map and gid_map files have been written, only the
333 mapped values may be used in system calls that change user and group
334 IDs.
335 For user IDs, the relevant system calls include setuid(2), setfsuid(2),
337 setreuid(2), and setresuid(2). For group IDs, the relevant system
338 calls include setgid(2), setfsgid(2), setregid(2), setresgid(2), and
339 setgroups(2).
340 Writing "deny" to the /proc/[pid]/setgroups file before writing to
342 /proc/[pid]/gid_map will permanently disable setgroups(2) in a user
343 namespace and allow writing to /proc/[pid]/gid_map without having the
344 CAP_SETGID capability in the parent user namespace.
345 The /proc/[pid]/setgroups file
347 The /proc/[pid]/setgroups file displays the string "allow" if processes
348 in the user namespace that contains the process pid are permitted to
349 employ the setgroups(2) system call; it displays "deny" if setgroups(2)
350 is not permitted in that user namespace. Note that regardless of the
351 value in the /proc/[pid]/setgroups file (and regardless of the
352 process's capabilities), calls to setgroups(2) are also not permitted
353 if /proc/[pid]/gid_map has not yet been set.
354 A privileged process (one with the CAP_SYS_ADMIN capability in the
356 namespace) may write either of the strings "allow" or "deny" to this
357 file before writing a group ID mapping for this user namespace to the
358 file /proc/[pid]/gid_map. Writing the string "deny" prevents any
359 process in the user namespace from employing setgroups(2).
360 The essence of the restrictions described in the preceding paragraph is
362 that it is permitted to write to /proc/[pid]/setgroups only so long as
363 calling setgroups(2) is disallowed because /proc/[pid]/gid_map has not
364 been set. This ensures that a process cannot transition from a state
365 where setgroups(2) is allowed to a state where setgroups(2) is denied;
366 a process can transition only from setgroups(2) being disallowed to
367 setgroups(2) being allowed.
368 The default value of this file in the initial user namespace is "al‐
370 low".
371 Once /proc/[pid]/gid_map has been written to (which has the effect of
373 enabling setgroups(2) in the user namespace), it is no longer possible
374 to disallow setgroups(2) by writing "deny" to /proc/[pid]/setgroups
375 (the write fails with the error EPERM).
376 A child user namespace inherits the /proc/[pid]/setgroups setting from
378 its parent.
379 If the setgroups file has the value "deny", then the setgroups(2) sys‐
381 tem call can't subsequently be reenabled (by writing "allow" to the
382 file) in this user namespace. (Attempts to do so fail with the error
383 EPERM.) This restriction also propagates down to all child user name‐
384 spaces of this user namespace.
385 The /proc/[pid]/setgroups file was added in Linux 3.19, but was back‐
387 ported to many earlier stable kernel series, because it addresses a se‐
388 curity issue. The issue concerned files with permissions such as
389 "rwx---rwx". Such files give fewer permissions to "group" than they do
390 to "other". This means that dropping groups using setgroups(2) might
391 allow a process file access that it did not formerly have. Before the
392 existence of user namespaces this was not a concern, since only a priv‐
393 ileged process (one with the CAP_SETGID capability) could call set‐
394 groups(2). However, with the introduction of user namespaces, it be‐
395 came possible for an unprivileged process to create a new namespace in
396 which the user had all privileges. This then allowed formerly unprivi‐
397 leged users to drop groups and thus gain file access that they did not
398 previously have. The /proc/[pid]/setgroups file was added to address
399 this security issue, by denying any pathway for an unprivileged process
400 to drop groups with setgroups(2).
401 Unmapped user and group IDs
403 There are various places where an unmapped user ID (group ID) may be
404 exposed to user space. For example, the first process in a new user
405 namespace may call getuid(2) before a user ID mapping has been defined
406 for the namespace. In most such cases, an unmapped user ID is con‐
407 verted to the overflow user ID (group ID); the default value for the
408 overflow user ID (group ID) is 65534. See the descriptions of
409 /proc/sys/kernel/overflowuid and /proc/sys/kernel/overflowgid in
410 proc(5).
411 The cases where unmapped IDs are mapped in this fashion include system
413 calls that return user IDs (getuid(2), getgid(2), and similar), creden‐
414 tials passed over a UNIX domain socket, credentials returned by
415 stat(2), waitid(2), and the System V IPC "ctl" IPC_STAT operations,
416 credentials exposed by /proc/[pid]/status and the files in
417 /proc/sysvipc/*, credentials returned via the si_uid field in the sig‐
418 info_t received with a signal (see sigaction(2)), credentials written
419 to the process accounting file (see acct(5)), and credentials returned
420 with POSIX message queue notifications (see mq_notify(3)).
421 There is one notable case where unmapped user and group IDs are not
423 converted to the corresponding overflow ID value. When viewing a
424 uid_map or gid_map file in which there is no mapping for the second
425 field, that field is displayed as 4294967295 (-1 as an unsigned inte‐
426 ger).
427 Accessing files
429 In order to determine permissions when an unprivileged process accesses
430 a file, the process credentials (UID, GID) and the file credentials are
431 in effect mapped back to what they would be in the initial user name‐
432 space and then compared to determine the permissions that the process
433 has on the file. The same is also of other objects that employ the
434 credentials plus permissions mask accessibility model, such as System V
435 IPC objects
436 Operation of file-related capabilities
438 Certain capabilities allow a process to bypass various kernel-enforced
439 restrictions when performing operations on files owned by other users
440 or groups. These capabilities are: CAP_CHOWN, CAP_DAC_OVERRIDE,
441 CAP_DAC_READ_SEARCH, CAP_FOWNER, and CAP_FSETID.
442 Within a user namespace, these capabilities allow a process to bypass
444 the rules if the process has the relevant capability over the file,
445 meaning that:
446 * the process has the relevant effective capability in its user name‐
448 space; and
449 * the file's user ID and group ID both have valid mappings in the user
451 namespace.
452 The CAP_FOWNER capability is treated somewhat exceptionally: it allows
454 a process to bypass the corresponding rules so long as at least the
455 file's user ID has a mapping in the user namespace (i.e., the file's
456 group ID does not need to have a valid mapping).
457 Set-user-ID and set-group-ID programs
459 When a process inside a user namespace executes a set-user-ID (set-
460 group-ID) program, the process's effective user (group) ID inside the
461 namespace is changed to whatever value is mapped for the user (group)
462 ID of the file. However, if either the user or the group ID of the
463 file has no mapping inside the namespace, the set-user-ID (set-group-
464 ID) bit is silently ignored: the new program is executed, but the
465 process's effective user (group) ID is left unchanged. (This mirrors
466 the semantics of executing a set-user-ID or set-group-ID program that
467 resides on a filesystem that was mounted with the MS_NOSUID flag, as
468 described in mount(2).)
469 Miscellaneous
471 When a process's user and group IDs are passed over a UNIX domain
472 socket to a process in a different user namespace (see the description
473 of SCM_CREDENTIALS in unix(7)), they are translated into the corre‐
474 sponding values as per the receiving process's user and group ID map‐
475 pings.
476
478 Namespaces are a Linux-specific feature.
479
481 Over the years, there have been a lot of features that have been added
482 to the Linux kernel that have been made available only to privileged
483 users because of their potential to confuse set-user-ID-root applica‐
484 tions. In general, it becomes safe to allow the root user in a user
485 namespace to use those features because it is impossible, while in a
486 user namespace, to gain more privilege than the root user of a user
487 namespace has.
488 Availability
490 Use of user namespaces requires a kernel that is configured with the
491 CONFIG_USER_NS option. User namespaces require support in a range of
492 subsystems across the kernel. When an unsupported subsystem is config‐
493 ured into the kernel, it is not possible to configure user namespaces
494 support.
495 As at Linux 3.8, most relevant subsystems supported user namespaces,
497 but a number of filesystems did not have the infrastructure needed to
498 map user and group IDs between user namespaces. Linux 3.9 added the
499 required infrastructure support for many of the remaining unsupported
500 filesystems (Plan 9 (9P), Andrew File System (AFS), Ceph, CIFS, CODA,
501 NFS, and OCFS2). Linux 3.12 added support for the last of the unsup‐
502 ported major filesystems, XFS.
503
505 The program below is designed to allow experimenting with user name‐
506 spaces, as well as other types of namespaces. It creates namespaces as
507 specified by command-line options and then executes a command inside
508 those namespaces. The comments and usage() function inside the program
509 provide a full explanation of the program. The following shell session
510 demonstrates its use.
511 First, we look at the run-time environment:
513 $ uname -rs # Need Linux 3.8 or later
515 Linux 3.8.0
516 $ id -u # Running as unprivileged user
517 1000
518 $ id -g
519 1000
520 Now start a new shell in new user (-U), mount (-m), and PID (-p) name‐
522 spaces, with user ID (-M) and group ID (-G) 1000 mapped to 0 inside the
523 user namespace:
524 $ ./userns_child_exec -p -m -U -M '0 1000 1' -G '0 1000 1' bash
526 The shell has PID 1, because it is the first process in the new PID
528 namespace:
529 bash$ echo $$
531 1
532 Mounting a new /proc filesystem and listing all of the processes visi‐
534 ble in the new PID namespace shows that the shell can't see any pro‐
535 cesses outside the PID namespace:
536 bash$ mount -t proc proc /proc
538 bash$ ps ax
539 PID TTY STAT TIME COMMAND
540 1 pts/3 S 0:00 bash
541 22 pts/3 R+ 0:00 ps ax
542 Inside the user namespace, the shell has user and group ID 0, and a
544 full set of permitted and effective capabilities:
545 bash$ cat /proc/$$/status | egrep '^[UG]id'
547 Uid: 0 0 0 0
548 Gid: 0 0 0 0
549 bash$ cat /proc/$$/status | egrep '^Cap(Prm|Inh|Eff)'
550 CapInh: 0000000000000000
551 CapPrm: 0000001fffffffff
552 CapEff: 0000001fffffffff
553 Program source
555 /* userns_child_exec.c
557 Licensed under GNU General Public License v2 or later
559 Create a child process that executes a shell command in new
561 namespace(s); allow UID and GID mappings to be specified when
562 creating a user namespace.
563 */
564 #define _GNU_SOURCE
565 #include <sched.h>
566 #include <unistd.h>
567 #include <stdint.h>
568 #include <stdlib.h>
569 #include <sys/wait.h>
570 #include <signal.h>
571 #include <fcntl.h>
572 #include <stdio.h>
573 #include <string.h>
574 #include <limits.h>
575 #include <errno.h>
576 /* A simple error-handling function: print an error message based
578 on the value in 'errno' and terminate the calling process. */
579 #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
581 } while (0)
582 struct child_args {
584 char **argv; /* Command to be executed by child, with args */
585 int pipe_fd[2]; /* Pipe used to synchronize parent and child */
586 };
587 static int verbose;
589 static void
591 usage(char *pname)
592 {
593 fprintf(stderr, "Usage: %s [options] cmd [arg...]\n\n", pname);
594 fprintf(stderr, "Create a child process that executes a shell "
595 "command in a new user namespace,\n"
596 "and possibly also other new namespace(s).\n\n");
597 fprintf(stderr, "Options can be:\n\n");
598 #define fpe(str) fprintf(stderr, " %s", str);
599 fpe("-i New IPC namespace\n");
600 fpe("-m New mount namespace\n");
601 fpe("-n New network namespace\n");
602 fpe("-p New PID namespace\n");
603 fpe("-u New UTS namespace\n");
604 fpe("-U New user namespace\n");
605 fpe("-M uid_map Specify UID map for user namespace\n");
606 fpe("-G gid_map Specify GID map for user namespace\n");
607 fpe("-z Map user's UID and GID to 0 in user namespace\n");
608 fpe(" (equivalent to: -M '0 <uid> 1' -G '0 <gid> 1')\n");
609 fpe("-v Display verbose messages\n");
610 fpe("\n");
611 fpe("If -z, -M, or -G is specified, -U is required.\n");
612 fpe("It is not permitted to specify both -z and either -M or -G.\n");
613 fpe("\n");
614 fpe("Map strings for -M and -G consist of records of the form:\n");
615 fpe("\n");
616 fpe(" ID-inside-ns ID-outside-ns len\n");
617 fpe("\n");
618 fpe("A map string can contain multiple records, separated"
619 " by commas;\n");
620 fpe("the commas are replaced by newlines before writing"
621 " to map files.\n");
622 exit(EXIT_FAILURE);
624 }
625 /* Update the mapping file 'map_file', with the value provided in
627 'mapping', a string that defines a UID or GID mapping. A UID or
628 GID mapping consists of one or more newline-delimited records
629 of the form:
630 ID_inside-ns ID-outside-ns length
632 Requiring the user to supply a string that contains newlines is
634 of course inconvenient for command-line use. Thus, we permit the
635 use of commas to delimit records in this string, and replace them
636 with newlines before writing the string to the file. */
637 static void
639 update_map(char *mapping, char *map_file)
640 {
641 int fd;
642 size_t map_len; /* Length of 'mapping' */
643 /* Replace commas in mapping string with newlines. */
645 map_len = strlen(mapping);
647 for (int j = 0; j < map_len; j++)
648 if (mapping[j] == ',')
649 mapping[j] = '\n';
650 fd = open(map_file, O_RDWR);
652 if (fd == -1) {
653 fprintf(stderr, "ERROR: open %s: %s\n", map_file,
654 strerror(errno));
655 exit(EXIT_FAILURE);
656 }
657 if (write(fd, mapping, map_len) != map_len) {
659 fprintf(stderr, "ERROR: write %s: %s\n", map_file,
660 strerror(errno));
661 exit(EXIT_FAILURE);
662 }
663 close(fd);
665 }
666 /* Linux 3.19 made a change in the handling of setgroups(2) and the
668 'gid_map' file to address a security issue. The issue allowed
669 *unprivileged* users to employ user namespaces in order to drop
670 The upshot of the 3.19 changes is that in order to update the
671 'gid_maps' file, use of the setgroups() system call in this
672 user namespace must first be disabled by writing "deny" to one of
673 the /proc/PID/setgroups files for this namespace. That is the
674 purpose of the following function. */
675 static void
677 proc_setgroups_write(pid_t child_pid, char *str)
678 {
679 char setgroups_path[PATH_MAX];
680 int fd;
681 snprintf(setgroups_path, PATH_MAX, "/proc/%jd/setgroups",
683 (intmax_t) child_pid);
684 fd = open(setgroups_path, O_RDWR);
686 if (fd == -1) {
687 /* We may be on a system that doesn't support
689 /proc/PID/setgroups. In that case, the file won't exist,
690 and the system won't impose the restrictions that Linux 3.19
691 added. That's fine: we don't need to do anything in order
692 to permit 'gid_map' to be updated.
693 However, if the error from open() was something other than
695 the ENOENT error that is expected for that case, let the
696 user know. */
697 if (errno != ENOENT)
699 fprintf(stderr, "ERROR: open %s: %s\n", setgroups_path,
700 strerror(errno));
701 return;
702 }
703 if (write(fd, str, strlen(str)) == -1)
705 fprintf(stderr, "ERROR: write %s: %s\n", setgroups_path,
706 strerror(errno));
707 close(fd);
709 }
710 static int /* Start function for cloned child */
712 childFunc(void *arg)
713 {
714 struct child_args *args = arg;
715 char ch;
716 /* Wait until the parent has updated the UID and GID mappings.
718 See the comment in main(). We wait for end of file on a
719 pipe that will be closed by the parent process once it has
720 updated the mappings. */
721 close(args->pipe_fd[1]); /* Close our descriptor for the write
723 end of the pipe so that we see EOF
724 when parent closes its descriptor. */
725 if (read(args->pipe_fd[0], &ch, 1) != 0) {
726 fprintf(stderr,
727 "Failure in child: read from pipe returned != 0\n");
728 exit(EXIT_FAILURE);
729 }
730 close(args->pipe_fd[0]);
732 /* Execute a shell command. */
734 printf("About to exec %s\n", args->argv[0]);
736 execvp(args->argv[0], args->argv);
737 errExit("execvp");
738 }
739 #define STACK_SIZE (1024 * 1024)
741 static char child_stack[STACK_SIZE]; /* Space for child's stack */
743 int
745 main(int argc, char *argv[])
746 {
747 int flags, opt, map_zero;
748 pid_t child_pid;
749 struct child_args args;
750 char *uid_map, *gid_map;
751 const int MAP_BUF_SIZE = 100;
752 char map_buf[MAP_BUF_SIZE];
753 char map_path[PATH_MAX];
754 /* Parse command-line options. The initial '+' character in
756 the final getopt() argument prevents GNU-style permutation
757 of command-line options. That's useful, since sometimes
758 the 'command' to be executed by this program itself
759 has command-line options. We don't want getopt() to treat
760 those as options to this program. */
761 flags = 0;
763 verbose = 0;
764 gid_map = NULL;
765 uid_map = NULL;
766 map_zero = 0;
767 while ((opt = getopt(argc, argv, "+imnpuUM:G:zv")) != -1) {
768 switch (opt) {
769 case 'i': flags |= CLONE_NEWIPC; break;
770 case 'm': flags |= CLONE_NEWNS; break;
771 case 'n': flags |= CLONE_NEWNET; break;
772 case 'p': flags |= CLONE_NEWPID; break;
773 case 'u': flags |= CLONE_NEWUTS; break;
774 case 'v': verbose = 1; break;
775 case 'z': map_zero = 1; break;
776 case 'M': uid_map = optarg; break;
777 case 'G': gid_map = optarg; break;
778 case 'U': flags |= CLONE_NEWUSER; break;
779 default: usage(argv[0]);
780 }
781 }
782 /* -M or -G without -U is nonsensical */
784 if (((uid_map != NULL || gid_map != NULL || map_zero) &&
786 !(flags & CLONE_NEWUSER)) ||
787 (map_zero && (uid_map != NULL || gid_map != NULL)))
788 usage(argv[0]);
789 args.argv = &argv[optind];
791 /* We use a pipe to synchronize the parent and child, in order to
793 ensure that the parent sets the UID and GID maps before the child
794 calls execve(). This ensures that the child maintains its
795 capabilities during the execve() in the common case where we
796 want to map the child's effective user ID to 0 in the new user
797 namespace. Without this synchronization, the child would lose
798 its capabilities if it performed an execve() with nonzero
799 user IDs (see the capabilities(7) man page for details of the
800 transformation of a process's capabilities during execve()). */
801 if (pipe(args.pipe_fd) == -1)
803 errExit("pipe");
804 /* Create the child in new namespace(s). */
806 child_pid = clone(childFunc, child_stack + STACK_SIZE,
808 flags | SIGCHLD, &args);
809 if (child_pid == -1)
810 errExit("clone");
811 /* Parent falls through to here. */
813 if (verbose)
815 printf("%s: PID of child created by clone() is %jd\n",
816 argv[0], (intmax_t) child_pid);
817 /* Update the UID and GID maps in the child. */
819 if (uid_map != NULL || map_zero) {
821 snprintf(map_path, PATH_MAX, "/proc/%jd/uid_map",
822 (intmax_t) child_pid);
823 if (map_zero) {
824 snprintf(map_buf, MAP_BUF_SIZE, "0 %jd 1",
825 (intmax_t) getuid());
826 uid_map = map_buf;
827 }
828 update_map(uid_map, map_path);
829 }
830 if (gid_map != NULL || map_zero) {
832 proc_setgroups_write(child_pid, "deny");
833 snprintf(map_path, PATH_MAX, "/proc/%jd/gid_map",
835 (intmax_t) child_pid);
836 if (map_zero) {
837 snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1",
838 (intmax_t) getgid());
839 gid_map = map_buf;
840 }
841 update_map(gid_map, map_path);
842 }
843 /* Close the write end of the pipe, to signal to the child that we
845 have updated the UID and GID maps. */
846 close(args.pipe_fd[1]);
848 if (waitpid(child_pid, NULL, 0) == -1) /* Wait for child */
850 errExit("waitpid");
851 if (verbose)
853 printf("%s: terminating\n", argv[0]);
854 exit(EXIT_SUCCESS);
856 }
857
859 newgidmap(1), newuidmap(1), clone(2), ptrace(2), setns(2), unshare(2),
860 proc(5), subgid(5), subuid(5), capabilities(7), cgroup_namespaces(7),
861 credentials(7), namespaces(7), pid_namespaces(7)
862 The kernel source file Documentation/namespaces/resource-control.txt.
864
866 This page is part of release 5.12 of the Linux man-pages project. A
867 description of the project, information about reporting bugs, and the
868 latest version of this page, can be found at
869 https://www.kernel.org/doc/man-pages/.
870Linux 2021-03-22 USER_NAMESPACES(7)