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user_namespaces(7) - phpMan
USER_NAMESPACES(7) Linux Programmer's Manual USER_NAMESPACES(7)
NAME
user_namespaces - overview of Linux user_namespaces
DESCRIPTION
For an overview of namespaces, see namespaces(7).
User namespaces isolate security-related identifiers and attributes, in particular, user
IDs and group IDs (see credentials(7), the root directory, keys (see keyctl(2)), and capa‐
bilities (see capabilities(7)). A process's user and group IDs can be different inside
and outside a user namespace. In particular, a process can have a normal unprivileged
user ID outside a user namespace while at the same time having a user ID of 0 inside the
namespace; in other words, the process has full privileges for operations inside the user
namespace, but is unprivileged for operations outside the namespace.
Nested namespaces, namespace membership
User namespaces can be nested; that is, each user namespace—except the initial ("root")
namespace—has a parent user namespace, and can have zero or more child user namespaces.
The parent user namespace is the user namespace of the process that creates the user
namespace via a call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.
The kernel imposes (since version 3.11) a limit of 32 nested levels of user namespaces.
Calls to unshare(2) or clone(2) that would cause this limit to be exceeded fail with the
error EUSERS.
Each process is a member of exactly one user namespace. A process created via fork(2) or
clone(2) without the CLONE_NEWUSER flag is a member of the same user namespace as its par‐
ent. A single-threaded process can join another user namespace with setns(2) if it has
the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a full set of capabilities in
that namespace.
A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag makes the new child process
(for clone(2)) or the caller (for unshare(2)) a member of the new user namespace created
by the call.
Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag starts out with a com‐
plete set of capabilities in the new user namespace. Likewise, a process that creates a
new user namespace using unshare(2) or joins an existing user namespace using setns(2)
gains a full set of capabilities in that namespace. On the other hand, that process has
no capabilities in the parent (in the case of clone(2)) or previous (in the case of
unshare(2) and setns(2)) user namespace, even if the new namespace is created or joined by
the root user (i.e., a process with user ID 0 in the root namespace).
Note that a call to execve(2) will cause a process's capabilities to be recalculated in
the usual way (see capabilities(7)), so that usually, unless it has a user ID of 0 within
the namespace or the executable file has a nonempty inheritable capabilities mask, it will
lose all capabilities. See the discussion of user and group ID mappings, below.
A call to clone(2), unshare(2), or setns(2) using the CLONE_NEWUSER flag sets the
"securebits" flags (see capabilities(7)) to their default values (all flags disabled) in
the child (for clone(2)) or caller (for unshare(2), or setns(2)). Note that because the
caller no longer has capabilities in its original user namespace after a call to setns(2),
it is not possible for a process to reset its "securebits" flags while retaining its user
namespace membership by using a pair of setns(2) calls to move to another user namespace
and then return to its original user namespace.
Having a capability inside a user namespace permits a process to perform operations (that
require privilege) only on resources governed by that namespace. The rules for determin‐
ing whether or not a process has a capability in a particular user namespace are as fol‐
lows:
1. A process has a capability inside a user namespace if it is a member of that namespace
and it has the capability in its effective capability set. A process can gain capabil‐
ities in its effective capability set in various ways. For example, it may execute a
set-user-ID program or an executable with associated file capabilities. In addition, a
process may gain capabilities via the effect of clone(2), unshare(2), or setns(2), as
already described.
2. If a process has a capability in a user namespace, then it has that capability in all
child (and further removed descendant) namespaces as well.
3. When a user namespace is created, the kernel records the effective user ID of the cre‐
ating process as being the "owner" of the namespace. A process that resides in the
parent of the user namespace and whose effective user ID matches the owner of the
namespace has all capabilities in the namespace. By virtue of the previous rule, this
means that the process has all capabilities in all further removed descendant user
namespaces as well.
Interaction of user namespaces and other types of namespaces
Starting in Linux 3.8, unprivileged processes can create user namespaces, and mount, PID,
IPC, network, and UTS namespaces can be created with just the CAP_SYS_ADMIN capability in
the caller's user namespace.
When a non-user-namespace is created, it is owned by the user namespace in which the cre‐
ating process was a member at the time of the creation of the namespace. Actions on the
non-user-namespace require capabilities in the corresponding user namespace.
If CLONE_NEWUSER is specified along with other CLONE_NEW* flags in a single clone(2) or
unshare(2) call, the user namespace is guaranteed to be created first, giving the child
(clone(2)) or caller (unshare(2)) privileges over the remaining namespaces created by the
call. Thus, it is possible for an unprivileged caller to specify this combination of
flags.
When a new IPC, mount, network, PID, or UTS namespace is created via clone(2) or
unshare(2), the kernel records the user namespace of the creating process against the new
namespace. (This association can't be changed.) When a process in the new namespace sub‐
sequently performs privileged operations that operate on global resources isolated by the
namespace, the permission checks are performed according to the process's capabilities in
the user namespace that the kernel associated with the new namespace.
Restrictions on mount namespaces
Note the following points with respect to mount namespaces:
* A mount namespace has an owner user namespace. A mount namespace whose owner user
namespace is different from the owner user namespace of its parent mount namespace is
considered a less privileged mount namespace.
* When creating a less privileged mount namespace, shared mounts are reduced to slave
mounts. This ensures that mappings performed in less privileged mount namespaces will
not propagate to more privileged mount namespaces.
* Mounts that come as a single unit from more privileged mount are locked together and
may not be separated in a less privileged mount namespace. (The unshare(2) CLONE_NEWNS
operation brings across all of the mounts from the original mount namespace as a single
unit, and recursive mounts that propagate between mount namespaces propagate as a sin‐
gle unit.)
* The mount(2) flags MS_RDONLY, MS_NOSUID, MS_NOEXEC, and the "atime" flags (MS_NOATIME,
MS_NODIRATIME, MS_RELATIME) settings become locked when propagated from a more privi‐
leged to a less privileged mount namespace, and may not be changed in the less privi‐
leged mount namespace.
* A file or directory that is a mount point in one namespace that is not a mount point in
another namespace, may be renamed, unlinked, or removed (rmdir(2)) in the mount names‐
pace in which it is not a mount point (subject to the usual permission checks).
Previously, attempting to unlink, rename, or remove a file or directory that was a
mount point in another mount namespace would result in the error EBUSY. That behavior
had technical problems of enforcement (e.g., for NFS) and permitted denial-of-service
attacks against more privileged users. (i.e., preventing individual files from being
updated by bind mounting on top of them).
User and group ID mappings: uid_map and gid_map
When a user namespace is created, it starts out without a mapping of user IDs (group IDs)
to the parent user namespace. The /proc/[pid]/uid_map and /proc/[pid]/gid_map files
(available since Linux 3.5) expose the mappings for user and group IDs inside the user
namespace for the process pid. These files can be read to view the mappings in a user
namespace and written to (once) to define the mappings.
The description in the following paragraphs explains the details for uid_map; gid_map is
exactly the same, but each instance of "user ID" is replaced by "group ID".
The uid_map file exposes the mapping of user IDs from the user namespace of the process
pid to the user namespace of the process that opened uid_map (but see a qualification to
this point below). In other words, processes that are in different user namespaces will
potentially see different values when reading from a particular uid_map file, depending on
the user ID mappings for the user namespaces of the reading processes.
Each line in the uid_map file specifies a 1-to-1 mapping of a range of contiguous user IDs
between two user namespaces. (When a user namespace is first created, this file is
empty.) The specification in each line takes the form of three numbers delimited by white
space. The first two numbers specify the starting user ID in each of the two user names‐
paces. The third number specifies the length of the mapped range. In detail, the fields
are interpreted as follows:
(1) The start of the range of user IDs in the user namespace of the process pid.
(2) The start of the range of user IDs to which the user IDs specified by field one map.
How field two is interpreted depends on whether the process that opened uid_map and
the process pid are in the same user namespace, as follows:
a) If the two processes are in different user namespaces: field two is the start of a
range of user IDs in the user namespace of the process that opened uid_map.
b) If the two processes are in the same user namespace: field two is the start of the
range of user IDs in the parent user namespace of the process pid. This case
enables the opener of uid_map (the common case here is opening /proc/self/uid_map)
to see the mapping of user IDs into the user namespace of the process that created
this user namespace.
(3) The length of the range of user IDs that is mapped between the two user namespaces.
System calls that return user IDs (group IDs)—for example, getuid(2), getgid(2), and the
credential fields in the structure returned by stat(2)—return the user ID (group ID)
mapped into the caller's user namespace.
When a process accesses a file, its user and group IDs are mapped into the initial user
namespace for the purpose of permission checking and assigning IDs when creating a file.
When a process retrieves file user and group IDs via stat(2), the IDs are mapped in the
opposite direction, to produce values relative to the process user and group ID mappings.
The initial user namespace has no parent namespace, but, for consistency, the kernel pro‐
vides dummy user and group ID mapping files for this namespace. Looking at the uid_map
file (gid_map is the same) from a shell in the initial namespace shows:
$ cat /proc/$$/uid_map
0 0 4294967295
This mapping tells us that the range starting at user ID 0 in this namespace maps to a
range starting at 0 in the (nonexistent) parent namespace, and the length of the range is
the largest 32-bit unsigned integer. (This deliberately leaves 4294967295 (the 32-bit
signed -1 value) unmapped. This is deliberate: (uid_t) -1 is used in several interfaces
(e.g., setreuid(2)) as a way to specify "no user ID". Leaving (uid_t) -1 unmapped and
unusable guarantees that there will be no confusion when using these interfaces.
Defining user and group ID mappings: writing to uid_map and gid_map
After the creation of a new user namespace, the uid_map file of one of the processes in
the namespace may be written to once to define the mapping of user IDs in the new user
namespace. An attempt to write more than once to a uid_map file in a user namespace fails
with the error EPERM. Similar rules apply for gid_map files.
The lines written to uid_map (gid_map) must conform to the following rules:
* The three fields must be valid numbers, and the last field must be greater than 0.
* Lines are terminated by newline characters.
* There is an (arbitrary) limit on the number of lines in the file. As at Linux 3.8, the
limit is five lines. In addition, the number of bytes written to the file must be less
than the system page size, and the write must be performed at the start of the file
(i.e., lseek(2) and pwrite(2) can't be used to write to nonzero offsets in the file).
* The range of user IDs (group IDs) specified in each line cannot overlap with the ranges
in any other lines. In the initial implementation (Linux 3.8), this requirement was
satisfied by a simplistic implementation that imposed the further requirement that the
values in both field 1 and field 2 of successive lines must be in ascending numerical
order, which prevented some otherwise valid maps from being created. Linux 3.9 and
later fix this limitation, allowing any valid set of nonoverlapping maps.
* At least one line must be written to the file.
Writes that violate the above rules fail with the error EINVAL.
In order for a process to write to the /proc/[pid]/uid_map (/proc/[pid]/gid_map) file, all
of the following requirements must be met:
1. The writing process must have the CAP_SETUID (CAP_SETGID) capability in the user names‐
pace of the process pid.
2. The writing process must be in either the user namespace of the process pid or inside
the parent user namespace of the process pid.
3. The mapped user IDs (group IDs) must in turn have a mapping in the parent user names‐
pace.
4. One of the following is true:
* The data written to uid_map (gid_map) consists of a single line that maps the writ‐
ing process's filesystem user ID (group ID) in the parent user namespace to a user
ID (group ID) in the user namespace. The usual case here is that this single line
provides a mapping for user ID of the process that created the namespace.
* The opening process has the CAP_SETUID (CAP_SETGID) capability in the parent user
namespace. Thus, a privileged process can make mappings to arbitrary user IDs
(group IDs) in the parent user namespace.
Writes that violate the above rules fail with the error EPERM.
Unmapped user and group IDs
There are various places where an unmapped user ID (group ID) may be exposed to user
space. For example, the first process in a new user namespace may call getuid() before a
user ID mapping has been defined for the namespace. In most such cases, an unmapped user
ID is converted to the overflow user ID (group ID); the default value for the overflow
user ID (group ID) is 65534. See the descriptions of /proc/sys/kernel/overflowuid and
/proc/sys/kernel/overflowgid in proc(5).
The cases where unmapped IDs are mapped in this fashion include system calls that return
user IDs (getuid(2) getgid(2), and similar), credentials passed over a UNIX domain socket,
credentials returned by stat(2), waitid(2), and the System V IPC "ctl" IPC_STAT opera‐
tions, credentials exposed by /proc/PID/status and the files in /proc/sysvipc/*, creden‐
tials returned via the si_uid field in the siginfo_t received with a signal (see sigac‐
tion(2)), credentials written to the process accounting file (see acct(5)), and creden‐
tials returned with POSIX message queue notifications (see mq_notify(3)).
There is one notable case where unmapped user and group IDs are not converted to the cor‐
responding overflow ID value. When viewing a uid_map or gid_map file in which there is no
mapping for the second field, that field is displayed as 4294967295 (-1 as an unsigned
integer);
Set-user-ID and set-group-ID programs
When a process inside a user namespace executes a set-user-ID (set-group-ID) program, the
process's effective user (group) ID inside the namespace is changed to whatever value is
mapped for the user (group) ID of the file. However, if either the user or the group ID
of the file has no mapping inside the namespace, the set-user-ID (set-group-ID) bit is
silently ignored: the new program is executed, but the process's effective user (group) ID
is left unchanged. (This mirrors the semantics of executing a set-user-ID or set-group-ID
program that resides on a filesystem that was mounted with the MS_NOSUID flag, as
described in mount(2).)
Miscellaneous
When a process's user and group IDs are passed over a UNIX domain socket to a process in a
different user namespace (see the description of SCM_CREDENTIALS in unix(7)), they are
translated into the corresponding values as per the receiving process's user and group ID
mappings.
CONFORMING TO
Namespaces are a Linux-specific feature.
NOTES
Over the years, there have been a lot of features that have been added to the Linux kernel
that have been made available only to privileged users because of their potential to con‐
fuse set-user-ID-root applications. In general, it becomes safe to allow the root user in
a user namespace to use those features because it is impossible, while in a user names‐
pace, to gain more privilege than the root user of a user namespace has.
Availability
Use of user namespaces requires a kernel that is configured with the CONFIG_USER_NS
option. User namespaces require support in a range of subsystems across the kernel. When
an unsupported subsystem is configured into the kernel, it is not possible to configure
user namespaces support.
As at Linux 3.8, most relevant subsystems supported user namespaces, but a number of
filesystems did not have the infrastructure needed to map user and group IDs between user
namespaces. Linux 3.9 added the required infrastructure support for many of the remaining
unsupported filesystems (Plan 9 (9P), Andrew File System (AFS), Ceph, CIFS, CODA, NFS, and
OCFS2). Linux 3.11 added support the last of the unsupported major filesystems, XFS.
EXAMPLE
The program below is designed to allow experimenting with user namespaces, as well as
other types of namespaces. It creates namespaces as specified by command-line options and
then executes a command inside those namespaces. The comments and usage() function inside
the program provide a full explanation of the program. The following shell session demon‐
strates its use.
First, we look at the run-time environment:
$ uname -rs # Need Linux 3.8 or later
Linux 3.8.0
$ id -u # Running as unprivileged user
1000
$ id -g
1000
Now start a new shell in new user (-U), mount (-m), and PID (-p) namespaces, with user ID
(-M) and group ID (-G) 1000 mapped to 0 inside the user namespace:
$ ./userns_child_exec -p -m -U -M '0 1000 1' -G '0 1000 1' bash
The shell has PID 1, because it is the first process in the new PID namespace:
bash$ echo $$
1
Inside the user namespace, the shell has user and group ID 0, and a full set of permitted
and effective capabilities:
bash$ cat /proc/$$/status | egrep '^[UG]id'
Uid: 0 0 0 0
Gid: 0 0 0 0
bash$ cat /proc/$$/status | egrep '^Cap(Prm|Inh|Eff)'
CapInh: 0000000000000000
CapPrm: 0000001fffffffff
CapEff: 0000001fffffffff
Mounting a new /proc filesystem and listing all of the processes visible in the new PID
namespace shows that the shell can't see any processes outside the PID namespace:
bash$ mount -t proc proc /proc
bash$ ps ax
PID TTY STAT TIME COMMAND
1 pts/3 S 0:00 bash
22 pts/3 R+ 0:00 ps ax
Program source
/* userns_child_exec.c
Licensed under GNU General Public License v2 or later
Create a child process that executes a shell command in new
namespace(s); allow UID and GID mappings to be specified when
creating a user namespace.
*/
#define _GNU_SOURCE
#include <sched.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <signal.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <limits.h>
#include <errno.h>
/* A simple error-handling function: print an error message based
on the value in 'errno' and terminate the calling process */
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
struct child_args {
char **argv; /* Command to be executed by child, with args */
int pipe_fd[2]; /* Pipe used to synchronize parent and child */
};
static int verbose;
static void
usage(char *pname)
{
fprintf(stderr, "Usage: %s [options] cmd [arg...]\n\n", pname);
fprintf(stderr, "Create a child process that executes a shell "
"command in a new user namespace,\n"
"and possibly also other new namespace(s).\n\n");
fprintf(stderr, "Options can be:\n\n");
#define fpe(str) fprintf(stderr, " %s", str);
fpe("-i New IPC namespace\n");
fpe("-m New mount namespace\n");
fpe("-n New network namespace\n");
fpe("-p New PID namespace\n");
fpe("-u New UTS namespace\n");
fpe("-U New user namespace\n");
fpe("-M uid_map Specify UID map for user namespace\n");
fpe("-G gid_map Specify GID map for user namespace\n");
fpe("-z Map user's UID and GID to 0 in user namespace\n");
fpe(" (equivalent to: -M '0 <uid> 1' -G '0 <gid> 1')\n");
fpe("-v Display verbose messages\n");
fpe("\n");
fpe("If -z, -M, or -G is specified, -U is required.\n");
fpe("It is not permitted to specify both -z and either -M or -G.\n");
fpe("\n");
fpe("Map strings for -M and -G consist of records of the form:\n");
fpe("\n");
fpe(" ID-inside-ns ID-outside-ns len\n");
fpe("\n");
fpe("A map string can contain multiple records, separated"
" by commas;\n");
fpe("the commas are replaced by newlines before writing"
" to map files.\n");
exit(EXIT_FAILURE);
}
/* Update the mapping file 'map_file', with the value provided in
'mapping', a string that defines a UID or GID mapping. A UID or
GID mapping consists of one or more newline-delimited records
of the form:
ID_inside-ns ID-outside-ns length
Requiring the user to supply a string that contains newlines is
of course inconvenient for command-line use. Thus, we permit the
use of commas to delimit records in this string, and replace them
with newlines before writing the string to the file. */
static void
update_map(char *mapping, char *map_file)
{
int fd, j;
size_t map_len; /* Length of 'mapping' */
/* Replace commas in mapping string with newlines */
map_len = strlen(mapping);
for (j = 0; j < map_len; j++)
if (mapping[j] == ',')
mapping[j] = '\n';
fd = open(map_file, O_RDWR);
if (fd == -1) {
fprintf(stderr, "ERROR: open %s: %s\n", map_file,
strerror(errno));
exit(EXIT_FAILURE);
}
if (write(fd, mapping, map_len) != map_len) {
fprintf(stderr, "ERROR: write %s: %s\n", map_file,
strerror(errno));
exit(EXIT_FAILURE);
}
close(fd);
}
static int /* Start function for cloned child */
childFunc(void *arg)
{
struct child_args *args = (struct child_args *) arg;
char ch;
/* Wait until the parent has updated the UID and GID mappings.
See the comment in main(). We wait for end of file on a
pipe that will be closed by the parent process once it has
updated the mappings. */
close(args->pipe_fd[1]); /* Close our descriptor for the write
end of the pipe so that we see EOF
when parent closes its descriptor */
if (read(args->pipe_fd[0], &ch, 1) != 0) {
fprintf(stderr,
"Failure in child: read from pipe returned != 0\n");
exit(EXIT_FAILURE);
}
/* Execute a shell command */
printf("About to exec %s\n", args->argv[0]);
execvp(args->argv[0], args->argv);
errExit("execvp");
}
#define STACK_SIZE (1024 * 1024)
static char child_stack[STACK_SIZE]; /* Space for child's stack */
int
main(int argc, char *argv[])
{
int flags, opt, map_zero;
pid_t child_pid;
struct child_args args;
char *uid_map, *gid_map;
const int MAP_BUF_SIZE = 100;
char map_buf[MAP_BUF_SIZE];
char map_path[PATH_MAX];
/* Parse command-line options. The initial '+' character in
the final getopt() argument prevents GNU-style permutation
of command-line options. That's useful, since sometimes
the 'command' to be executed by this program itself
has command-line options. We don't want getopt() to treat
those as options to this program. */
flags = 0;
verbose = 0;
gid_map = NULL;
uid_map = NULL;
map_zero = 0;
while ((opt = getopt(argc, argv, "+imnpuUM:G:zv")) != -1) {
switch (opt) {
case 'i': flags |= CLONE_NEWIPC; break;
case 'm': flags |= CLONE_NEWNS; break;
case 'n': flags |= CLONE_NEWNET; break;
case 'p': flags |= CLONE_NEWPID; break;
case 'u': flags |= CLONE_NEWUTS; break;
case 'v': verbose = 1; break;
case 'z': map_zero = 1; break;
case 'M': uid_map = optarg; break;
case 'G': gid_map = optarg; break;
case 'U': flags |= CLONE_NEWUSER; break;
default: usage(argv[0]);
}
}
/* -M or -G without -U is nonsensical */
if (((uid_map != NULL || gid_map != NULL || map_zero) &&
!(flags & CLONE_NEWUSER)) ||
(map_zero && (uid_map != NULL || gid_map != NULL)))
usage(argv[0]);
args.argv = &argv[optind];
/* We use a pipe to synchronize the parent and child, in order to
ensure that the parent sets the UID and GID maps before the child
calls execve(). This ensures that the child maintains its
capabilities during the execve() in the common case where we
want to map the child's effective user ID to 0 in the new user
namespace. Without this synchronization, the child would lose
its capabilities if it performed an execve() with nonzero
user IDs (see the capabilities(7) man page for details of the
transformation of a process's capabilities during execve()). */
if (pipe(args.pipe_fd) == -1)
errExit("pipe");
/* Create the child in new namespace(s) */
child_pid = clone(childFunc, child_stack + STACK_SIZE,
flags | SIGCHLD, &args);
if (child_pid == -1)
errExit("clone");
/* Parent falls through to here */
if (verbose)
printf("%s: PID of child created by clone() is %ld\n",
argv[0], (long) child_pid);
/* Update the UID and GID maps in the child */
if (uid_map != NULL || map_zero) {
snprintf(map_path, PATH_MAX, "/proc/%ld/uid_map",
(long) child_pid);
if (map_zero) {
snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getuid());
uid_map = map_buf;
}
update_map(uid_map, map_path);
}
if (gid_map != NULL || map_zero) {
snprintf(map_path, PATH_MAX, "/proc/%ld/gid_map",
(long) child_pid);
if (map_zero) {
snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getgid());
gid_map = map_buf;
}
update_map(gid_map, map_path);
}
/* Close the write end of the pipe, to signal to the child that we
have updated the UID and GID maps */
close(args.pipe_fd[1]);
if (waitpid(child_pid, NULL, 0) == -1) /* Wait for child */
errExit("waitpid");
if (verbose)
printf("%s: terminating\n", argv[0]);
exit(EXIT_SUCCESS);
}
SEE ALSO
newgidmap(1), newuidmap(1), clone(2), setns(2), unshare(2), proc(5), subgid(5), subuid(5),
credentials(7), capabilities(7), namespaces(7), pid_namespaces(7)
The kernel source file Documentation/namespaces/resource-control.txt.
COLOPHON
This page is part of release 3.74 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at http://www.kernel.org/doc/man-pages/.
Linux 2014-09-21 USER_NAMESPACES(7)
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