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27 .TH USER_NAMESPACES 7 2021-03-22 "Linux" "Linux Programmer's Manual"
29 user_namespaces \- overview of Linux user namespaces
31 For an overview of namespaces, see
34 User namespaces isolate security-related identifiers and attributes,
36 user IDs and group IDs (see
41 .\" FIXME: This page says very little about the interaction
42 .\" of user namespaces and keys. Add something on this topic.
44 .BR capabilities (7)).
45 A process's user and group IDs can be different
46 inside and outside a user namespace.
48 a process can have a normal unprivileged user ID outside a user namespace
49 while at the same time having a user ID of 0 inside the namespace;
51 the process has full privileges for operations inside the user namespace,
52 but is unprivileged for operations outside the namespace.
54 .\" ============================================================
56 .SS Nested namespaces, namespace membership
57 User namespaces can be nested;
58 that is, each user namespace\(emexcept the initial ("root")
59 namespace\(emhas a parent user namespace,
60 and can have zero or more child user namespaces.
61 The parent user namespace is the user namespace
62 of the process that creates the user namespace via a call to
70 The kernel imposes (since version 3.11) a limit of 32 nested levels of
71 .\" commit 8742f229b635bf1c1c84a3dfe5e47c814c20b5c8
73 .\" FIXME Explain the rationale for this limit. (What is the rationale?)
78 that would cause this limit to be exceeded fail with the error
81 Each process is a member of exactly one user namespace.
88 flag is a member of the same user namespace as its parent.
89 A single-threaded process can join another user namespace with
94 upon doing so, it gains a full set of capabilities in that namespace.
102 flag makes the new child process (for
106 a member of the new user namespace created by the call.
111 operation can be used to discover the parental relationship
112 between user namespaces; see
115 .\" ============================================================
118 The child process created by
122 flag starts out with a complete set
123 of capabilities in the new user namespace.
124 Likewise, a process that creates a new user namespace using
126 or joins an existing user namespace using
128 gains a full set of capabilities in that namespace.
130 that process has no capabilities in the parent (in the case of
132 or previous (in the case of
137 even if the new namespace is created or joined by the root user
138 (i.e., a process with user ID 0 in the root namespace).
142 will cause a process's capabilities to be recalculated in the usual way (see
143 .BR capabilities (7)).
145 unless the process has a user ID of 0 within the namespace,
146 or the executable file has a nonempty inheritable capabilities mask,
147 the process will lose all capabilities.
148 See the discussion of user and group ID mappings, below.
159 that moves the caller into another user namespace
160 sets the "securebits" flags
162 .BR capabilities (7))
163 to their default values (all flags disabled) in the child (for
169 Note that because the caller no longer has capabilities
170 in its original user namespace after a call to
172 it is not possible for a process to reset its "securebits" flags while
173 retaining its user namespace membership by using a pair of
175 calls to move to another user namespace and then return to
176 its original user namespace.
178 The rules for determining whether or not a process has a capability
179 in a particular user namespace are as follows:
181 A process has a capability inside a user namespace
182 if it is a member of that namespace and
183 it has the capability in its effective capability set.
184 A process can gain capabilities in its effective capability
186 For example, it may execute a set-user-ID program or an
187 executable with associated file capabilities.
189 a process may gain capabilities via the effect of
194 as already described.
195 .\" In the 3.8 sources, see security/commoncap.c::cap_capable():
197 If a process has a capability in a user namespace,
198 then it has that capability in all child (and further removed descendant)
201 .\" * The owner of the user namespace in the parent of the
202 .\" * user namespace has all caps.
203 When a user namespace is created, the kernel records the effective
204 user ID of the creating process as being the "owner" of the namespace.
205 .\" (and likewise associates the effective group ID of the creating process
206 .\" with the namespace).
207 A process that resides
208 in the parent of the user namespace
209 .\" See kernel commit 520d9eabce18edfef76a60b7b839d54facafe1f9 for a fix
211 and whose effective user ID matches the owner of the namespace
212 has all capabilities in the namespace.
213 .\" This includes the case where the process executes a set-user-ID
214 .\" program that confers the effective UID of the creator of the namespace.
215 By virtue of the previous rule,
216 this means that the process has all capabilities in all
217 further removed descendant user namespaces as well.
221 operation can be used to discover the user ID of the owner of the namespace;
225 .\" ============================================================
227 .SS Effect of capabilities within a user namespace
228 Having a capability inside a user namespace
229 permits a process to perform operations (that require privilege)
230 only on resources governed by that namespace.
231 In other words, having a capability in a user namespace permits a process
232 to perform privileged operations on resources that are governed by (nonuser)
233 namespaces owned by (associated with) the user namespace
234 (see the next subsection).
236 On the other hand, there are many privileged operations that affect
237 resources that are not associated with any namespace type,
238 for example, changing the system (i.e., calendar) time (governed by
240 loading a kernel module (governed by
241 .BR CAP_SYS_MODULE ),
242 and creating a device (governed by
244 Only a process with privileges in the
246 user namespace can perform such operations.
250 within the user namespace that owns a process's mount namespace
251 allows that process to create bind mounts
252 and mount the following types of filesystems:
253 .\" fs_flags = FS_USERNS_MOUNT in kernel sources
277 .\" commit b2197755b2633e164a439682fb05a9b5ea48f706
281 .\" commit 92dbc9dedccb9759c7f9f2f0ae6242396376988f
282 .\" commit 4cb2c00c43b3fe88b32f29df4f76da1b92c33224
289 within the user namespace that owns a process's cgroup namespace
290 allows (since Linux 4.6)
291 that process to the mount the cgroup version 2 filesystem and
292 cgroup version 1 named hierarchies
293 (i.e., cgroup filesystems mounted with the
299 within the user namespace that owns a process's PID namespace
300 allows (since Linux 3.8)
301 that process to mount
305 Note however, that mounting block-based filesystems can be done
306 only by a process that holds
308 in the initial user namespace.
310 .\" ============================================================
312 .SS Interaction of user namespaces and other types of namespaces
313 Starting in Linux 3.8, unprivileged processes can create user namespaces,
314 and the other types of namespaces can be created with just the
316 capability in the caller's user namespace.
318 When a nonuser namespace is created,
319 it is owned by the user namespace in which the creating process
320 was a member at the time of the creation of the namespace.
321 Privileged operations on resources governed by the nonuser namespace
322 require that the process has the necessary capabilities
323 in the user namespace that owns the nonuser namespace.
327 is specified along with other
333 call, the user namespace is guaranteed to be created first,
338 privileges over the remaining namespaces created by the call.
339 Thus, it is possible for an unprivileged caller to specify this combination
342 When a new namespace (other than a user namespace) is created via
346 the kernel records the user namespace of the creating process as the owner of
348 (This association can't be changed.)
349 When a process in the new namespace subsequently performs
350 privileged operations that operate on global
351 resources isolated by the namespace,
352 the permission checks are performed according to the process's capabilities
353 in the user namespace that the kernel associated with the new namespace.
354 For example, suppose that a process attempts to change the hostname
355 .RB ( sethostname (2)),
356 a resource governed by the UTS namespace.
358 the kernel will determine which user namespace owns
359 the process's UTS namespace, and check whether the process has the
361 .RB ( CAP_SYS_ADMIN )
362 in that user namespace.
367 operation can be used to discover the user namespace
368 that owns a nonuser namespace; see
371 .\" ============================================================
373 .SS User and group ID mappings: uid_map and gid_map
374 When a user namespace is created,
375 it starts out without a mapping of user IDs (group IDs)
376 to the parent user namespace.
378 .IR /proc/[pid]/uid_map
380 .IR /proc/[pid]/gid_map
381 files (available since Linux 3.5)
382 .\" commit 22d917d80e842829d0ca0a561967d728eb1d6303
383 expose the mappings for user and group IDs
384 inside the user namespace for the process
386 These files can be read to view the mappings in a user namespace and
387 written to (once) to define the mappings.
389 The description in the following paragraphs explains the details for
393 but each instance of "user ID" is replaced by "group ID".
397 file exposes the mapping of user IDs from the user namespace
400 to the user namespace of the process that opened
402 (but see a qualification to this point below).
403 In other words, processes that are in different user namespaces
404 will potentially see different values when reading from a particular
406 file, depending on the user ID mappings for the user namespaces
407 of the reading processes.
411 file specifies a 1-to-1 mapping of a range of contiguous
412 user IDs between two user namespaces.
413 (When a user namespace is first created, this file is empty.)
414 The specification in each line takes the form of
415 three numbers delimited by white space.
416 The first two numbers specify the starting user ID in
417 each of the two user namespaces.
418 The third number specifies the length of the mapped range.
419 In detail, the fields are interpreted as follows:
421 The start of the range of user IDs in
422 the user namespace of the process
425 The start of the range of user
426 IDs to which the user IDs specified by field one map.
427 How field two is interpreted depends on whether the process that opened
431 are in the same user namespace, as follows:
434 If the two processes are in different user namespaces:
435 field two is the start of a range of
436 user IDs in the user namespace of the process that opened
439 If the two processes are in the same user namespace:
440 field two is the start of the range of
441 user IDs in the parent user namespace of the process
443 This case enables the opener of
445 (the common case here is opening
446 .IR /proc/self/uid_map )
447 to see the mapping of user IDs into the user namespace of the process
448 that created this user namespace.
451 The length of the range of user IDs that is mapped between the two
454 System calls that return user IDs (group IDs)\(emfor example,
457 and the credential fields in the structure returned by
458 .BR stat (2)\(emreturn
459 the user ID (group ID) mapped into the caller's user namespace.
461 When a process accesses a file, its user and group IDs
462 are mapped into the initial user namespace for the purpose of permission
463 checking and assigning IDs when creating a file.
464 When a process retrieves file user and group IDs via
466 the IDs are mapped in the opposite direction,
467 to produce values relative to the process user and group ID mappings.
469 The initial user namespace has no parent namespace,
470 but, for consistency, the kernel provides dummy user and group
471 ID mapping files for this namespace.
476 is the same) from a shell in the initial namespace shows:
480 $ \fBcat /proc/$$/uid_map\fP
485 This mapping tells us
486 that the range starting at user ID 0 in this namespace
487 maps to a range starting at 0 in the (nonexistent) parent namespace,
488 and the length of the range is the largest 32-bit unsigned integer.
489 This leaves 4294967295 (the 32-bit signed \-1 value) unmapped.
492 is used in several interfaces (e.g.,
494 as a way to specify "no user ID".
497 unmapped and unusable guarantees that there will be no
498 confusion when using these interfaces.
500 .\" ============================================================
502 .SS Defining user and group ID mappings: writing to uid_map and gid_map
503 After the creation of a new user namespace, the
507 of the processes in the namespace may be written to
509 to define the mapping of user IDs in the new user namespace.
510 An attempt to write more than once to a
512 file in a user namespace fails with the error
514 Similar rules apply for
521 must conform to the following rules:
523 The three fields must be valid numbers,
524 and the last field must be greater than 0.
526 Lines are terminated by newline characters.
528 There is a limit on the number of lines in the file.
529 In Linux 4.14 and earlier, this limit was (arbitrarily)
530 .\" 5*12-byte records could fit in a 64B cache line
533 .\" commit 6397fac4915ab3002dc15aae751455da1a852f25
534 the limit is 340 lines.
535 In addition, the number of bytes written to
536 the file must be less than the system page size,
537 and the write must be performed at the start of the file (i.e.,
541 can't be used to write to nonzero offsets in the file).
543 The range of user IDs (group IDs)
544 specified in each line cannot overlap with the ranges
546 In the initial implementation (Linux 3.8), this requirement was
547 satisfied by a simplistic implementation that imposed the further
549 the values in both field 1 and field 2 of successive lines must be
550 in ascending numerical order,
551 which prevented some otherwise valid maps from being created.
553 .\" commit 0bd14b4fd72afd5df41e9fd59f356740f22fceba
554 fix this limitation, allowing any valid set of nonoverlapping maps.
556 At least one line must be written to the file.
558 Writes that violate the above rules fail with the error
561 In order for a process to write to the
562 .I /proc/[pid]/uid_map
563 .RI ( /proc/[pid]/gid_map )
564 file, all of the following requirements must be met:
566 The writing process must have the
569 capability in the user namespace of the process
572 The writing process must either be in the user namespace of the process
574 or be in the parent user namespace of the process
577 The mapped user IDs (group IDs) must in turn have a mapping
578 in the parent user namespace.
580 .\" commit db2e718a47984b9d71ed890eb2ea36ecf150de18
581 If a writing process is root (i.e. UID 0) trying to map host user ID 0,
584 capability (since Linux 5.12).
586 One of the following two cases applies:
590 the writing process has the
598 No further restrictions apply:
599 the process can make mappings to arbitrary user IDs (group IDs)
600 in the parent user namespace.
604 otherwise all of the following restrictions apply:
610 must consist of a single line that maps
611 the writing process's effective user ID
612 (group ID) in the parent user namespace to a user ID (group ID)
613 in the user namespace.
615 The writing process must have the same effective user ID as the process
616 that created the user namespace.
622 system call must first be denied by writing
625 .I /proc/[pid]/setgroups
626 file (see below) before writing to
631 Writes that violate the above rules fail with the error
634 .\" ============================================================
636 .SS Interaction with system calls that change process UIDs or GIDs
637 In a user namespace where the
639 file has not been written, the system calls that change user IDs will fail.
642 file has not been written, the system calls that change group IDs will fail.
647 files have been written, only the mapped values may be used in
648 system calls that change user and group IDs.
650 For user IDs, the relevant system calls include
656 For group IDs, the relevant system calls include
667 .I /proc/[pid]/setgroups
668 file before writing to
669 .I /proc/[pid]/gid_map
670 .\" Things changed in Linux 3.19
671 .\" commit 9cc46516ddf497ea16e8d7cb986ae03a0f6b92f8
672 .\" commit 66d2f338ee4c449396b6f99f5e75cd18eb6df272
673 .\" http://lwn.net/Articles/626665/
674 will permanently disable
676 in a user namespace and allow writing to
677 .I /proc/[pid]/gid_map
680 capability in the parent user namespace.
682 .\" ============================================================
684 .SS The /proc/[pid]/setgroups file
686 .\" commit 9cc46516ddf497ea16e8d7cb986ae03a0f6b92f8
687 .\" commit 66d2f338ee4c449396b6f99f5e75cd18eb6df272
688 .\" http://lwn.net/Articles/626665/
689 .\" http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2014-8989
692 .I /proc/[pid]/setgroups
693 file displays the string
695 if processes in the user namespace that contains the process
697 are permitted to employ the
699 system call; it displays
703 is not permitted in that user namespace.
704 Note that regardless of the value in the
705 .I /proc/[pid]/setgroups
706 file (and regardless of the process's capabilities), calls to
708 are also not permitted if
709 .IR /proc/[pid]/gid_map
710 has not yet been set.
712 A privileged process (one with the
714 capability in the namespace) may write either of the strings
720 writing a group ID mapping
721 for this user namespace to the file
722 .IR /proc/[pid]/gid_map .
725 prevents any process in the user namespace from employing
728 The essence of the restrictions described in the preceding
729 paragraph is that it is permitted to write to
730 .I /proc/[pid]/setgroups
731 only so long as calling
733 is disallowed because
734 .I /proc/[pid]/gid_map
736 This ensures that a process cannot transition from a state where
738 is allowed to a state where
741 a process can transition only from
747 The default value of this file in the initial user namespace is
751 .IR /proc/[pid]/gid_map
753 (which has the effect of enabling
755 in the user namespace),
756 it is no longer possible to disallow
761 .IR /proc/[pid]/setgroups
762 (the write fails with the error
765 A child user namespace inherits the
766 .IR /proc/[pid]/setgroups
767 setting from its parent.
775 system call can't subsequently be reenabled (by writing
777 to the file) in this user namespace.
778 (Attempts to do so fail with the error
780 This restriction also propagates down to all child user namespaces of
784 .I /proc/[pid]/setgroups
785 file was added in Linux 3.19,
786 but was backported to many earlier stable kernel series,
787 because it addresses a security issue.
788 The issue concerned files with permissions such as "rwx\-\-\-rwx".
789 Such files give fewer permissions to "group" than they do to "other".
790 This means that dropping groups using
792 might allow a process file access that it did not formerly have.
793 Before the existence of user namespaces this was not a concern,
794 since only a privileged process (one with the
796 capability) could call
798 However, with the introduction of user namespaces,
799 it became possible for an unprivileged process to create
800 a new namespace in which the user had all privileges.
801 This then allowed formerly unprivileged
802 users to drop groups and thus gain file access
803 that they did not previously have.
805 .I /proc/[pid]/setgroups
806 file was added to address this security issue,
807 by denying any pathway for an unprivileged process to drop groups with
810 .\" /proc/PID/setgroups
811 .\" [allow == setgroups() is allowed, "deny" == setgroups() is disallowed]
812 .\" * Can write if have CAP_SYS_ADMIN in NS
813 .\" * Must write BEFORE writing to /proc/PID/gid_map
816 .\" * Must already have written to gid_map
817 .\" * /proc/PID/setgroups must be "allow"
819 .\" /proc/PID/gid_map -- writing
820 .\" * Must already have written "deny" to /proc/PID/setgroups
822 .\" ============================================================
824 .SS Unmapped user and group IDs
825 There are various places where an unmapped user ID (group ID)
826 may be exposed to user space.
827 For example, the first process in a new user namespace may call
829 before a user ID mapping has been defined for the namespace.
830 In most such cases, an unmapped user ID is converted
831 .\" from_kuid_munged(), from_kgid_munged()
832 to the overflow user ID (group ID);
833 the default value for the overflow user ID (group ID) is 65534.
834 See the descriptions of
835 .IR /proc/sys/kernel/overflowuid
837 .IR /proc/sys/kernel/overflowgid
841 The cases where unmapped IDs are mapped in this fashion include
842 system calls that return user IDs
846 credentials passed over a UNIX domain socket,
848 credentials returned by
851 and the System V IPC "ctl"
854 credentials exposed by
855 .IR /proc/[pid]/status
857 .IR /proc/sysvipc/* ,
858 credentials returned via the
862 received with a signal (see
864 credentials written to the process accounting file (see
866 and credentials returned with POSIX message queue notifications (see
869 There is one notable case where unmapped user and group IDs are
871 .\" from_kuid(), from_kgid()
872 .\" Also F_GETOWNER_UIDS is an exception
873 converted to the corresponding overflow ID value.
878 file in which there is no mapping for the second field,
879 that field is displayed as 4294967295 (\-1 as an unsigned integer).
881 .\" ============================================================
884 In order to determine permissions when an unprivileged process accesses a file,
885 the process credentials (UID, GID) and the file credentials
886 are in effect mapped back to what they would be in
887 the initial user namespace and then compared to determine
888 the permissions that the process has on the file.
889 The same is also of other objects that employ the credentials plus
890 permissions mask accessibility model, such as System V IPC objects
892 .\" ============================================================
894 .SS Operation of file-related capabilities
895 Certain capabilities allow a process to bypass various
896 kernel-enforced restrictions when performing operations on
897 files owned by other users or groups.
898 These capabilities are:
900 .BR CAP_DAC_OVERRIDE ,
901 .BR CAP_DAC_READ_SEARCH ,
906 Within a user namespace,
907 these capabilities allow a process to bypass the rules
908 if the process has the relevant capability over the file,
911 the process has the relevant effective capability in its user namespace; and
913 the file's user ID and group ID both have valid mappings
914 in the user namespace.
918 capability is treated somewhat exceptionally:
919 .\" These are the checks performed by the kernel function
920 .\" inode_owner_or_capable(). There is one exception to the exception:
921 .\" overriding the directory sticky permission bit requires that
922 .\" the file has a valid mapping for both its UID and GID.
923 it allows a process to bypass the corresponding rules so long as
924 at least the file's user ID has a mapping in the user namespace
925 (i.e., the file's group ID does not need to have a valid mapping).
927 .\" ============================================================
929 .SS Set-user-ID and set-group-ID programs
930 When a process inside a user namespace executes
931 a set-user-ID (set-group-ID) program,
932 the process's effective user (group) ID inside the namespace is changed
933 to whatever value is mapped for the user (group) ID of the file.
934 However, if either the user
936 the group ID of the file has no mapping inside the namespace,
937 the set-user-ID (set-group-ID) bit is silently ignored:
938 the new program is executed,
939 but the process's effective user (group) ID is left unchanged.
940 (This mirrors the semantics of executing a set-user-ID or set-group-ID
941 program that resides on a filesystem that was mounted with the
943 flag, as described in
946 .\" ============================================================
949 When a process's user and group IDs are passed over a UNIX domain socket
950 to a process in a different user namespace (see the description of
954 they are translated into the corresponding values as per the
955 receiving process's user and group ID mappings.
958 Namespaces are a Linux-specific feature.
961 Over the years, there have been a lot of features that have been added
962 to the Linux kernel that have been made available only to privileged users
963 because of their potential to confuse set-user-ID-root applications.
964 In general, it becomes safe to allow the root user in a user namespace to
965 use those features because it is impossible, while in a user namespace,
966 to gain more privilege than the root user of a user namespace has.
968 .\" ============================================================
971 Use of user namespaces requires a kernel that is configured with the
974 User namespaces require support in a range of subsystems across
976 When an unsupported subsystem is configured into the kernel,
977 it is not possible to configure user namespaces support.
979 As at Linux 3.8, most relevant subsystems supported user namespaces,
980 but a number of filesystems did not have the infrastructure needed
981 to map user and group IDs between user namespaces.
982 Linux 3.9 added the required infrastructure support for many of
983 the remaining unsupported filesystems
984 (Plan 9 (9P), Andrew File System (AFS), Ceph, CIFS, CODA, NFS, and OCFS2).
985 Linux 3.12 added support for the last of the unsupported major filesystems,
986 .\" commit d6970d4b726cea6d7a9bc4120814f95c09571fc3
990 The program below is designed to allow experimenting with
991 user namespaces, as well as other types of namespaces.
992 It creates namespaces as specified by command-line options and then executes
993 a command inside those namespaces.
996 function inside the program provide a full explanation of the program.
997 The following shell session demonstrates its use.
999 First, we look at the run-time environment:
1003 $ \fBuname \-rs\fP # Need Linux 3.8 or later
1005 $ \fBid \-u\fP # Running as unprivileged user
1012 Now start a new shell in new user
1018 namespaces, with user ID
1022 1000 mapped to 0 inside the user namespace:
1026 $ \fB./userns_child_exec \-p \-m \-U \-M \(aq0 1000 1\(aq \-G \(aq0 1000 1\(aq bash\fP
1030 The shell has PID 1, because it is the first process in the new
1042 filesystem and listing all of the processes visible
1043 in the new PID namespace shows that the shell can't see
1044 any processes outside the PID namespace:
1048 bash$ \fBmount \-t proc proc /proc\fP
1050 PID TTY STAT TIME COMMAND
1052 22 pts/3 R+ 0:00 ps ax
1056 Inside the user namespace, the shell has user and group ID 0,
1057 and a full set of permitted and effective capabilities:
1061 bash$ \fBcat /proc/$$/status | egrep \(aq\(ha[UG]id\(aq\fP
1064 bash$ \fBcat /proc/$$/status | egrep \(aq\(haCap(Prm|Inh|Eff)\(aq\fP
1065 CapInh: 0000000000000000
1066 CapPrm: 0000001fffffffff
1067 CapEff: 0000001fffffffff
1073 /* userns_child_exec.c
1075 Licensed under GNU General Public License v2 or later
1077 Create a child process that executes a shell command in new
1078 namespace(s); allow UID and GID mappings to be specified when
1079 creating a user namespace.
1086 #include <sys/wait.h>
1094 /* A simple error\-handling function: print an error message based
1095 on the value in \(aqerrno\(aq and terminate the calling process. */
1097 #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \e
1101 char **argv; /* Command to be executed by child, with args */
1102 int pipe_fd[2]; /* Pipe used to synchronize parent and child */
1110 fprintf(stderr, "Usage: %s [options] cmd [arg...]\en\en", pname);
1111 fprintf(stderr, "Create a child process that executes a shell "
1112 "command in a new user namespace,\en"
1113 "and possibly also other new namespace(s).\en\en");
1114 fprintf(stderr, "Options can be:\en\en");
1115 #define fpe(str) fprintf(stderr, " %s", str);
1116 fpe("\-i New IPC namespace\en");
1117 fpe("\-m New mount namespace\en");
1118 fpe("\-n New network namespace\en");
1119 fpe("\-p New PID namespace\en");
1120 fpe("\-u New UTS namespace\en");
1121 fpe("\-U New user namespace\en");
1122 fpe("\-M uid_map Specify UID map for user namespace\en");
1123 fpe("\-G gid_map Specify GID map for user namespace\en");
1124 fpe("\-z Map user\(aqs UID and GID to 0 in user namespace\en");
1125 fpe(" (equivalent to: \-M \(aq0 <uid> 1\(aq \-G \(aq0 <gid> 1\(aq)\en");
1126 fpe("\-v Display verbose messages\en");
1128 fpe("If \-z, \-M, or \-G is specified, \-U is required.\en");
1129 fpe("It is not permitted to specify both \-z and either \-M or \-G.\en");
1131 fpe("Map strings for \-M and \-G consist of records of the form:\en");
1133 fpe(" ID\-inside\-ns ID\-outside\-ns len\en");
1135 fpe("A map string can contain multiple records, separated"
1137 fpe("the commas are replaced by newlines before writing"
1138 " to map files.\en");
1143 /* Update the mapping file \(aqmap_file\(aq, with the value provided in
1144 \(aqmapping\(aq, a string that defines a UID or GID mapping. A UID or
1145 GID mapping consists of one or more newline\-delimited records
1148 ID_inside\-ns ID\-outside\-ns length
1150 Requiring the user to supply a string that contains newlines is
1151 of course inconvenient for command\-line use. Thus, we permit the
1152 use of commas to delimit records in this string, and replace them
1153 with newlines before writing the string to the file. */
1156 update_map(char *mapping, char *map_file)
1159 size_t map_len; /* Length of \(aqmapping\(aq */
1161 /* Replace commas in mapping string with newlines. */
1163 map_len = strlen(mapping);
1164 for (int j = 0; j < map_len; j++)
1165 if (mapping[j] == \(aq,\(aq)
1166 mapping[j] = \(aq\en\(aq;
1168 fd = open(map_file, O_RDWR);
1170 fprintf(stderr, "ERROR: open %s: %s\en", map_file,
1175 if (write(fd, mapping, map_len) != map_len) {
1176 fprintf(stderr, "ERROR: write %s: %s\en", map_file,
1184 /* Linux 3.19 made a change in the handling of setgroups(2) and the
1185 \(aqgid_map\(aq file to address a security issue. The issue allowed
1186 *unprivileged* users to employ user namespaces in order to drop
1187 The upshot of the 3.19 changes is that in order to update the
1188 \(aqgid_maps\(aq file, use of the setgroups() system call in this
1189 user namespace must first be disabled by writing "deny" to one of
1190 the /proc/PID/setgroups files for this namespace. That is the
1191 purpose of the following function. */
1194 proc_setgroups_write(pid_t child_pid, char *str)
1196 char setgroups_path[PATH_MAX];
1199 snprintf(setgroups_path, PATH_MAX, "/proc/%jd/setgroups",
1200 (intmax_t) child_pid);
1202 fd = open(setgroups_path, O_RDWR);
1205 /* We may be on a system that doesn\(aqt support
1206 /proc/PID/setgroups. In that case, the file won\(aqt exist,
1207 and the system won\(aqt impose the restrictions that Linux 3.19
1208 added. That\(aqs fine: we don\(aqt need to do anything in order
1209 to permit \(aqgid_map\(aq to be updated.
1211 However, if the error from open() was something other than
1212 the ENOENT error that is expected for that case, let the
1215 if (errno != ENOENT)
1216 fprintf(stderr, "ERROR: open %s: %s\en", setgroups_path,
1221 if (write(fd, str, strlen(str)) == \-1)
1222 fprintf(stderr, "ERROR: write %s: %s\en", setgroups_path,
1228 static int /* Start function for cloned child */
1229 childFunc(void *arg)
1231 struct child_args *args = arg;
1234 /* Wait until the parent has updated the UID and GID mappings.
1235 See the comment in main(). We wait for end of file on a
1236 pipe that will be closed by the parent process once it has
1237 updated the mappings. */
1239 close(args\->pipe_fd[1]); /* Close our descriptor for the write
1240 end of the pipe so that we see EOF
1241 when parent closes its descriptor. */
1242 if (read(args\->pipe_fd[0], &ch, 1) != 0) {
1244 "Failure in child: read from pipe returned != 0\en");
1248 close(args\->pipe_fd[0]);
1250 /* Execute a shell command. */
1252 printf("About to exec %s\en", args\->argv[0]);
1253 execvp(args\->argv[0], args\->argv);
1257 #define STACK_SIZE (1024 * 1024)
1259 static char child_stack[STACK_SIZE]; /* Space for child\(aqs stack */
1262 main(int argc, char *argv[])
1264 int flags, opt, map_zero;
1266 struct child_args args;
1267 char *uid_map, *gid_map;
1268 const int MAP_BUF_SIZE = 100;
1269 char map_buf[MAP_BUF_SIZE];
1270 char map_path[PATH_MAX];
1272 /* Parse command\-line options. The initial \(aq+\(aq character in
1273 the final getopt() argument prevents GNU\-style permutation
1274 of command\-line options. That\(aqs useful, since sometimes
1275 the \(aqcommand\(aq to be executed by this program itself
1276 has command\-line options. We don\(aqt want getopt() to treat
1277 those as options to this program. */
1284 while ((opt = getopt(argc, argv, "+imnpuUM:G:zv")) != \-1) {
1286 case \(aqi\(aq: flags |= CLONE_NEWIPC; break;
1287 case \(aqm\(aq: flags |= CLONE_NEWNS; break;
1288 case \(aqn\(aq: flags |= CLONE_NEWNET; break;
1289 case \(aqp\(aq: flags |= CLONE_NEWPID; break;
1290 case \(aqu\(aq: flags |= CLONE_NEWUTS; break;
1291 case \(aqv\(aq: verbose = 1; break;
1292 case \(aqz\(aq: map_zero = 1; break;
1293 case \(aqM\(aq: uid_map = optarg; break;
1294 case \(aqG\(aq: gid_map = optarg; break;
1295 case \(aqU\(aq: flags |= CLONE_NEWUSER; break;
1296 default: usage(argv[0]);
1300 /* \-M or \-G without \-U is nonsensical */
1302 if (((uid_map != NULL || gid_map != NULL || map_zero) &&
1303 !(flags & CLONE_NEWUSER)) ||
1304 (map_zero && (uid_map != NULL || gid_map != NULL)))
1307 args.argv = &argv[optind];
1309 /* We use a pipe to synchronize the parent and child, in order to
1310 ensure that the parent sets the UID and GID maps before the child
1311 calls execve(). This ensures that the child maintains its
1312 capabilities during the execve() in the common case where we
1313 want to map the child\(aqs effective user ID to 0 in the new user
1314 namespace. Without this synchronization, the child would lose
1315 its capabilities if it performed an execve() with nonzero
1316 user IDs (see the capabilities(7) man page for details of the
1317 transformation of a process\(aqs capabilities during execve()). */
1319 if (pipe(args.pipe_fd) == \-1)
1322 /* Create the child in new namespace(s). */
1324 child_pid = clone(childFunc, child_stack + STACK_SIZE,
1325 flags | SIGCHLD, &args);
1326 if (child_pid == \-1)
1329 /* Parent falls through to here. */
1332 printf("%s: PID of child created by clone() is %jd\en",
1333 argv[0], (intmax_t) child_pid);
1335 /* Update the UID and GID maps in the child. */
1337 if (uid_map != NULL || map_zero) {
1338 snprintf(map_path, PATH_MAX, "/proc/%jd/uid_map",
1339 (intmax_t) child_pid);
1341 snprintf(map_buf, MAP_BUF_SIZE, "0 %jd 1",
1342 (intmax_t) getuid());
1345 update_map(uid_map, map_path);
1348 if (gid_map != NULL || map_zero) {
1349 proc_setgroups_write(child_pid, "deny");
1351 snprintf(map_path, PATH_MAX, "/proc/%jd/gid_map",
1352 (intmax_t) child_pid);
1354 snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1",
1355 (intmax_t) getgid());
1358 update_map(gid_map, map_path);
1361 /* Close the write end of the pipe, to signal to the child that we
1362 have updated the UID and GID maps. */
1364 close(args.pipe_fd[1]);
1366 if (waitpid(child_pid, NULL, 0) == \-1) /* Wait for child */
1370 printf("%s: terminating\en", argv[0]);
1376 .BR newgidmap (1), \" From the shadow package
1377 .BR newuidmap (1), \" From the shadow package
1383 .BR subgid (5), \" From the shadow package
1384 .BR subuid (5), \" From the shadow package
1385 .BR capabilities (7),
1386 .BR cgroup_namespaces (7),
1387 .BR credentials (7),
1389 .BR pid_namespaces (7)
1391 The kernel source file
1392 .IR Documentation/admin\-guide/namespaces/resource\-control.rst .