user_namespaces(7) Miscellaneous Information 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 di-
rectory, keys (see keyrings(7)), and capabilities (see capabili-
ties(7)). A process's user and group IDs can be different inside and
outside a user namespace. In particular, a process can have a normal
unprivileged user ID outside a user namespace while at the same time
having a user ID of 0 inside the namespace; in other words, the process
has full privileges for operations inside the user namespace, but is
unprivileged for operations outside the namespace.
Nested namespaces, namespace membership
User namespaces can be nested; that is, each user namespace—except the
initial ("root") namespace—has a parent user namespace, and can have
zero or more child user namespaces. The parent user namespace is the
user namespace of the process that creates the user namespace via a
call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.
The kernel imposes (since Linux 3.11) a limit of 32 nested levels of
user namespaces. Calls to unshare(2) or clone(2) that would cause this
limit to be exceeded fail with the error EUSERS.
Each process is a member of exactly one user namespace. A process cre-
ated via fork(2) or clone(2) without the CLONE_NEWUSER flag is a member
of the same user namespace as its parent. A single-threaded process
can join another user namespace with setns(2) if it has the CAP_SYS_AD-
MIN in that namespace; upon doing so, it gains a full set of capabili-
ties 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 mem-
ber of the new user namespace created by the call.
The NS_GET_PARENT ioctl(2) operation can be used to discover the
parental relationship between user namespaces; see ioctl_ns(2).
A task that changes one of its effective IDs will have its dumpability
reset to the value in /proc/sys/fs/suid_dumpable. This may affect the
ownership of proc files of child processes and may thus cause the par-
ent to lack the permissions to write to mapping files of child
processes running in a new user namespace. In such cases making the
parent process dumpable, using PR_SET_DUMPABLE in a call to prctl(2),
before creating a child process in a new user namespace may rectify
this problem. See prctl(2) and proc(5) for details on how ownership is
affected.
Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag
starts out with a complete set of capabilities in the new user name-
space. Likewise, a process that creates a new user namespace using un-
share(2) or joins an existing user namespace using setns(2) gains a
full set of capabilities in that namespace. On the other hand, that
process has no capabilities in the parent (in the case of clone(2)) or
previous (in the case of unshare(2) and setns(2)) user namespace, even
if the new namespace is created or joined by the root user (i.e., a
process with user ID 0 in the root namespace).
Note that a call to execve(2) will cause a process's capabilities to be
recalculated in the usual way (see capabilities(7)). Consequently, un-
less the process has a user ID of 0 within the namespace, or the exe-
cutable file has a nonempty inheritable capabilities mask, the process
will lose all capabilities. See the discussion of user and group ID
mappings, below.
A call to clone(2) or unshare(2) using the CLONE_NEWUSER flag or a call
to setns(2) that moves the caller into another user namespace sets the
"securebits" flags (see capabilities(7)) to their default values (all
flags disabled) in the child (for clone(2)) or caller (for unshare(2)
or setns(2)). Note that because the caller no longer has capabilities
in its original user namespace after a call to setns(2), it is not pos-
sible for a process to reset its "securebits" flags while retaining its
user namespace membership by using a pair of setns(2) calls to move to
another user namespace and then return to its original user namespace.
The rules for determining whether or not a process has a capability in
a particular user namespace are as follows:
• A process has a capability inside a user namespace if it is a member
of that namespace and it has the capability in its effective capa-
bility set. A process can gain capabilities in its effective capa-
bility set in various ways. For example, it may execute a set-user-
ID program or an executable with associated file capabilities. In
addition, a process may gain capabilities via the effect of
clone(2), unshare(2), or setns(2), as already described.
• If a process has a capability in a user namespace, then it has that
capability in all child (and further removed descendant) namespaces
as well.
• When a user namespace is created, the kernel records the effective
user ID of the creating process as being the "owner" of the name-
space. 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 re-
moved descendant user namespaces as well. The NS_GET_OWNER_UID
ioctl(2) operation can be used to discover the user ID of the owner
of the namespace; see ioctl_ns(2).
Effect of capabilities within a user namespace
Having a capability inside a user namespace permits a process to per-
form operations (that require privilege) only on resources governed by
that namespace. In other words, having a capability in a user name-
space permits a process to perform privileged operations on resources
that are governed by (nonuser) namespaces owned by (associated with)
the user namespace (see the next subsection).
On the other hand, there are many privileged operations that affect re-
sources that are not associated with any namespace type, for example,
changing the system (i.e., calendar) time (governed by CAP_SYS_TIME),
loading a kernel module (governed by CAP_SYS_MODULE), and creating a
device (governed by CAP_MKNOD). Only a process with privileges in the
initial user namespace can perform such operations.
Holding CAP_SYS_ADMIN within the user namespace that owns a process's
mount namespace allows that process to create bind mounts and mount the
following types of filesystems:
• /proc (since Linux 3.8)
• /sys (since Linux 3.8)
• devpts (since Linux 3.9)
• tmpfs(5) (since Linux 3.9)
• ramfs (since Linux 3.9)
• mqueue (since Linux 3.9)
• bpf (since Linux 4.4)
• overlayfs (since Linux 5.11)
Holding CAP_SYS_ADMIN within the user namespace that owns a process's
cgroup namespace allows (since Linux 4.6) that process to the mount the
cgroup version 2 filesystem and cgroup version 1 named hierarchies
(i.e., cgroup filesystems mounted with the "none,name=" option).
Holding CAP_SYS_ADMIN within the user namespace that owns a process's
PID namespace allows (since Linux 3.8) that process to mount /proc
filesystems.
Note, however, that mounting block-based filesystems can be done only
by a process that holds CAP_SYS_ADMIN in the initial user namespace.
Interaction of user namespaces and other types of namespaces
Starting in Linux 3.8, unprivileged processes can create user name-
spaces, and the other types of namespaces can be created with just the
CAP_SYS_ADMIN capability in the caller's user namespace.
When a nonuser namespace is created, it is owned by the user namespace
in which the creating process was a member at the time of the creation
of the namespace. Privileged operations on resources governed by the
nonuser namespace require that the process has the necessary capabili-
ties in the user namespace that owns the nonuser namespace.
If CLONE_NEWUSER is specified along with other CLONE_NEW* flags in a
single clone(2) or unshare(2) call, the user namespace is guaranteed to
be created first, giving the child (clone(2)) or caller (unshare(2))
privileges over the remaining namespaces created by the call. Thus, it
is possible for an unprivileged caller to specify this combination of
flags.
When a new namespace (other than a user namespace) is created via
clone(2) or unshare(2), the kernel records the user namespace of the
creating process as the owner of the new namespace. (This association
can't be changed.) When a process in the new namespace subsequently
performs privileged operations that operate on global resources iso-
lated by the namespace, the permission checks are performed according
to the process's capabilities in the user namespace that the kernel as-
sociated with the new namespace. For example, suppose that a process
attempts to change the hostname (sethostname(2)), a resource governed
by the UTS namespace. In this case, the kernel will determine which
user namespace owns the process's UTS namespace, and check whether the
process has the required capability (CAP_SYS_ADMIN) in that user name-
space.
The NS_GET_USERNS ioctl(2) operation can be used to discover the user
namespace that owns a nonuser namespace; see ioctl_ns(2).
User and group ID mappings: uid_map and gid_map
When a user namespace is created, it starts out without a mapping of
user IDs (group IDs) to the parent user namespace. The
/proc/pid/uid_map and /proc/pid/gid_map files (available since Linux
3.5) expose the mappings for user and group IDs inside the user name-
space for the process pid. These files can be read to view the map-
pings 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 name-
space 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, de-
pending 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 name-
space is first created, this file is empty.) The specification in each
line takes the form of three numbers delimited by white space. The
first two numbers specify the starting user ID in each of the two user
namespaces. The third number specifies the length of the mapped range.
In detail, the fields are interpreted as follows:
(1) The start of the range of user IDs in the user namespace of the
process pid.
(2) The start of the range of user IDs to which the user IDs specified
by field one map. How field two is interpreted depends on whether
the process that opened uid_map and the process pid are in the
same user namespace, as follows:
(a) If the two processes are in different user namespaces: field
two is the start of a range of user IDs in the user namespace
of the process that opened uid_map.
(b) If the two processes are in the same user namespace: field
two is the start of the range of user IDs in the parent user
namespace of the process pid. This case enables the opener
of uid_map (the common case here is opening
/proc/self/uid_map) to see the mapping of user IDs into the
user namespace of the process that created this user name-
space.
(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 direc-
tion, to produce values relative to the process user and group ID map-
pings.
The initial user namespace has no parent namespace, but, for consis-
tency, the kernel provides dummy user and group ID mapping files for
this namespace. Looking at the uid_map file (gid_map is the same) from
a shell in the initial namespace shows:
$ cat /proc/$$/uid_map
0 0 4294967295
This mapping tells us that the range starting at user ID 0 in this
namespace maps to a range starting at 0 in the (nonexistent) parent
namespace, and the length of the range is the largest 32-bit unsigned
integer. This leaves 4294967295 (the 32-bit signed -1 value) unmapped.
This is deliberate: (uid_t) -1 is used in several interfaces (e.g., se-
treuid(2)) as a way to specify "no user ID". Leaving (uid_t) -1 un-
mapped and unusable guarantees that there will be no confusion when us-
ing 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 er-
ror EPERM. Similar rules apply for gid_map files.
The lines written to uid_map (gid_map) must conform to the following
validity rules:
• The three fields must be valid numbers, and the last field must be
greater than 0.
• Lines are terminated by newline characters.
• There is a limit on the number of lines in the file. In Linux 4.14
and earlier, this limit was (arbitrarily) set at 5 lines. Since
Linux 4.15, the limit is 340 lines. In addition, the number of
bytes written to the file must be less than the system page size,
and the write must be performed at the start of the file (i.e.,
lseek(2) and pwrite(2) can't be used to write to nonzero offsets in
the file).
• The range of user IDs (group IDs) specified in each line cannot
overlap with the ranges in any other lines. In the initial imple-
mentation (Linux 3.8), this requirement was satisfied by a simplis-
tic 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, allow-
ing any valid set of nonoverlapping maps.
• At least one line must be written to the file.
Writes that violate the above rules fail with the error EINVAL.
In order for a process to write to the /proc/pid/uid_map
(/proc/pid/gid_map) file, all of the following permission requirements
must be met:
• The writing process must have the CAP_SETUID (CAP_SETGID) capability
in the user namespace of the process pid.
• The writing process must either be in the user namespace of the
process pid or be in the parent user namespace of the process pid.
• The mapped user IDs (group IDs) must in turn have a mapping in the
parent user namespace.
• If updating /proc/pid/uid_map to create a mapping that maps UID 0 in
the parent namespace, then one of the following must be true:
(a) if writing process is in the parent user namespace, then it
must have the CAP_SETFCAP capability in that user namespace; or
(b) if the writing process is in the child user namespace, then the
process that created the user namespace must have had the
CAP_SETFCAP capability when the namespace was created.
This rule has been in place since Linux 5.12. It eliminates an ear-
lier security bug whereby a UID 0 process that lacks the CAP_SETFCAP
capability, which is needed to create a binary with namespaced file
capabilities (as described in capabilities(7)), could nevertheless
create such a binary, by the following steps:
(1) Create a new user namespace with the identity mapping (i.e.,
UID 0 in the new user namespace maps to UID 0 in the parent
namespace), so that UID 0 in both namespaces is equivalent to
the same root user ID.
(2) Since the child process has the CAP_SETFCAP capability, it
could create a binary with namespaced file capabilities that
would then be effective in the parent user namespace (because
the root user IDs are the same in the two namespaces).
• One of the following two cases applies:
(a) Either the writing process has the CAP_SETUID (CAP_SETGID) ca-
pability in the parent user namespace.
• No further restrictions apply: the process can make mappings
to arbitrary user IDs (group IDs) in the parent user name-
space.
(b) Or otherwise all of the following restrictions apply:
• The data written to uid_map (gid_map) must consist of a sin-
gle line that maps the writing process's effective user ID
(group ID) in the parent user namespace to a user ID (group
ID) in the user namespace.
• The writing process must have the same effective user ID as
the process that created the user namespace.
• In the case of gid_map, use of the setgroups(2) system call
must first be denied by writing "deny" to the /proc/pid/set-
groups file (see below) before writing to gid_map.
Writes that violate the above rules fail with the error EPERM.
Project ID mappings: projid_map
Similarly to user and group ID mappings, it is possible to create
project ID mappings for a user namespace. (Project IDs are used for
disk quotas; see setquota(8) and quotactl(2).)
Project ID mappings are defined by writing to the /proc/pid/projid_map
file (present since Linux 3.7).
The validity rules for writing to the /proc/pid/projid_map file are as
for writing to the uid_map file; violation of these rules causes
write(2) to fail with the error EINVAL.
The permission rules for writing to the /proc/pid/projid_map file are
as follows:
• The writing process must either be in the user namespace of the
process pid or be in the parent user namespace of the process pid.
• The mapped project IDs must in turn have a mapping in the parent
user namespace.
Violation of these rules causes write(2) to fail with the error EPERM.
Interaction with system calls that change process UIDs or GIDs
In a user namespace where the uid_map file has not been written, the
system calls that change user IDs will fail. Similarly, if the gid_map
file has not been written, the system calls that change group IDs will
fail. After the uid_map and gid_map files have been written, only the
mapped values may be used in system calls that change user and group
IDs.
For user IDs, the relevant system calls include setuid(2), setfsuid(2),
setreuid(2), and setresuid(2). For group IDs, the relevant system
calls include setgid(2), setfsgid(2), setregid(2), setresgid(2), and
setgroups(2).
Writing "deny" to the /proc/pid/setgroups file before writing to
/proc/pid/gid_map will permanently disable setgroups(2) in a user name-
space and allow writing to /proc/pid/gid_map without having the
CAP_SETGID capability in the parent user namespace.
The /proc/pid/setgroups file
The /proc/pid/setgroups file displays the string "allow" if processes
in the user namespace that contains the process pid are permitted to
employ the setgroups(2) system call; it displays "deny" if setgroups(2)
is not permitted in that user namespace. Note that regardless of the
value in the /proc/pid/setgroups file (and regardless of the process's
capabilities), calls to setgroups(2) are also not permitted if
/proc/pid/gid_map has not yet been set.
A privileged process (one with the CAP_SYS_ADMIN capability in the
namespace) may write either of the strings "allow" or "deny" to this
file before writing a group ID mapping for this user namespace to the
file /proc/pid/gid_map. Writing the string "deny" prevents any process
in the user namespace from employing setgroups(2).
The essence of the restrictions described in the preceding paragraph is
that it is permitted to write to /proc/pid/setgroups only so long as
calling setgroups(2) is disallowed because /proc/pid/gid_map has not
been set. This ensures that a process cannot transition from a state
where setgroups(2) is allowed to a state where setgroups(2) is denied;
a process can transition only from setgroups(2) being disallowed to
setgroups(2) being allowed.
The default value of this file in the initial user namespace is "al-
low".
Once /proc/pid/gid_map has been written to (which has the effect of en-
abling setgroups(2) in the user namespace), it is no longer possible to
disallow setgroups(2) by writing "deny" to /proc/pid/setgroups (the
write fails with the error EPERM).
A child user namespace inherits the /proc/pid/setgroups setting from
its parent.
If the setgroups file has the value "deny", then the setgroups(2) sys-
tem call can't subsequently be reenabled (by writing "allow" to the
file) in this user namespace. (Attempts to do so fail with the error
EPERM.) This restriction also propagates down to all child user name-
spaces of this user namespace.
The /proc/pid/setgroups file was added in Linux 3.19, but was back-
ported to many earlier stable kernel series, because it addresses a se-
curity issue. The issue concerned files with permissions such as
"rwx---rwx". Such files give fewer permissions to "group" than they do
to "other". This means that dropping groups using setgroups(2) might
allow a process file access that it did not formerly have. Before the
existence of user namespaces this was not a concern, since only a priv-
ileged process (one with the CAP_SETGID capability) could call set-
groups(2). However, with the introduction of user namespaces, it be-
came possible for an unprivileged process to create a new namespace in
which the user had all privileges. This then allowed formerly unprivi-
leged users to drop groups and thus gain file access that they did not
previously have. The /proc/pid/setgroups file was added to address
this security issue, by denying any pathway for an unprivileged process
to drop groups with setgroups(2).
Unmapped user and group IDs
There are various places where an unmapped user ID (group ID) may be
exposed to user space. For example, the first process in a new user
namespace may call getuid(2) before a user ID mapping has been defined
for the namespace. In most such cases, an unmapped user ID is con-
verted 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), creden-
tials passed over a UNIX domain socket, credentials returned by
stat(2), waitid(2), and the System V IPC "ctl" IPC_STAT operations,
credentials exposed by /proc/pid/status and the files in
/proc/sysvipc/*, credentials returned via the si_uid field in the sig-
info_t received with a signal (see sigaction(2)), credentials written
to the process accounting file (see acct(5)), and credentials returned
with POSIX message queue notifications (see mq_notify(3)).
There is one notable case where unmapped user and group IDs are not
converted to the corresponding overflow ID value. When viewing a
uid_map or gid_map file in which there is no mapping for the second
field, that field is displayed as 4294967295 (-1 as an unsigned inte-
ger).
Accessing files
In order to determine permissions when an unprivileged process accesses
a file, the process credentials (UID, GID) and the file credentials are
in effect mapped back to what they would be in the initial user name-
space and then compared to determine the permissions that the process
has on the file. The same is also true of other objects that employ
the credentials plus permissions mask accessibility model, such as Sys-
tem V IPC objects.
Operation of file-related capabilities
Certain capabilities allow a process to bypass various kernel-enforced
restrictions when performing operations on files owned by other users
or groups. These capabilities are: CAP_CHOWN, CAP_DAC_OVERRIDE,
CAP_DAC_READ_SEARCH, CAP_FOWNER, and CAP_FSETID.
Within a user namespace, these capabilities allow a process to bypass
the rules if the process has the relevant capability over the file,
meaning that:
• the process has the relevant effective capability in its user name-
space; and
• the file's user ID and group ID both have valid mappings in the user
namespace.
The CAP_FOWNER capability is treated somewhat exceptionally: it allows
a process to bypass the corresponding rules so long as at least the
file's user ID has a mapping in the user namespace (i.e., the file's
group ID does not need to have a valid mapping).
Set-user-ID and set-group-ID programs
When a process inside a user namespace executes a set-user-ID (set-
group-ID) program, the process's effective user (group) ID inside the
namespace is changed to whatever value is mapped for the user (group)
ID of the file. However, if either the user or the group ID of the
file has no mapping inside the namespace, the set-user-ID (set-group-
ID) bit is silently ignored: the new program is executed, but the
process's effective user (group) ID is left unchanged. (This mirrors
the semantics of executing a set-user-ID or set-group-ID program that
resides on a filesystem that was mounted with the MS_NOSUID flag, as
described in mount(2).)
Miscellaneous
When a process's user and group IDs are passed over a UNIX domain
socket to a process in a different user namespace (see the description
of SCM_CREDENTIALS in unix(7)), they are translated into the corre-
sponding values as per the receiving process's user and group ID map-
pings.
STANDARDS
Linux.
NOTES
Over the years, there have been a lot of features that have been added
to the Linux kernel that have been made available only to privileged
users because of their potential to confuse set-user-ID-root applica-
tions. In general, it becomes safe to allow the root user in a user
namespace to use those features because it is impossible, while in a
user namespace, to gain more privilege than the root user of a user
namespace has.
Global root
The term "global root" is sometimes used as a shorthand for user ID 0
in the initial user namespace.
Availability
Use of user namespaces requires a kernel that is configured with the
CONFIG_USER_NS option. User namespaces require support in a range of
subsystems across the kernel. When an unsupported subsystem is config-
ured into the kernel, it is not possible to configure user namespaces
support.
As at Linux 3.8, most relevant subsystems supported user namespaces,
but a number of filesystems did not have the infrastructure needed to
map user and group IDs between user namespaces. Linux 3.9 added the
required infrastructure support for many of the remaining unsupported
filesystems (Plan 9 (9P), Andrew File System (AFS), Ceph, CIFS, CODA,
NFS, and OCFS2). Linux 3.12 added support for the last of the unsup-
ported major filesystems, XFS.
EXAMPLES
The program below is designed to allow experimenting with user name-
spaces, as well as other types of namespaces. It creates namespaces as
specified by command-line options and then executes a command inside
those namespaces. The comments and usage() function inside the program
provide a full explanation of the program. The following shell session
demonstrates its use.
First, we look at the run-time environment:
$ uname -rs # Need Linux 3.8 or later
Linux 3.8.0
$ id -u # Running as unprivileged user
1000
$ id -g
1000
Now start a new shell in new user (-U), mount (-m), and PID (-p) name-
spaces, with user ID (-M) and group ID (-G) 1000 mapped to 0 inside the
user namespace:
$ ./userns_child_exec -p -m -U -M '0 1000 1' -G '0 1000 1' bash
The shell has PID 1, because it is the first process in the new PID
namespace:
bash$ echo $$
1
Mounting a new /proc filesystem and listing all of the processes visi-
ble in the new PID namespace shows that the shell can't see any
processes outside the PID namespace:
bash$ mount -t proc proc /proc
bash$ ps ax
PID TTY STAT TIME COMMAND
1 pts/3 S 0:00 bash
22 pts/3 R+ 0:00 ps ax
Inside the user namespace, the shell has user and group ID 0, and a
full set of permitted and effective capabilities:
bash$ cat /proc/$$/status | egrep '^[UG]id'
Uid: 0 0 0 0
Gid: 0 0 0 0
bash$ cat /proc/$$/status | egrep '^Cap(Prm|Inh|Eff)'
CapInh: 0000000000000000
CapPrm: 0000001fffffffff
CapEff: 0000001fffffffff
Program source
/* userns_child_exec.c
Licensed under GNU General Public License v2 or later
Create a child process that executes a shell command in new
namespace(s); allow UID and GID mappings to be specified when
creating a user namespace.
*/
#define _GNU_SOURCE
#include <err.h>
#include <sched.h>
#include <unistd.h>
#include <stdint.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <signal.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <limits.h>
#include <errno.h>
struct child_args {
char **argv; /* Command to be executed by child, with args */
int pipe_fd[2]; /* Pipe used to synchronize parent and child */
};
static int verbose;
static void
usage(char *pname)
{
fprintf(stderr, "Usage: %s [options] cmd [arg...]\n\n", pname);
fprintf(stderr, "Create a child process that executes a shell "
"command in a new user namespace,\n"
"and possibly also other new namespace(s).\n\n");
fprintf(stderr, "Options can be:\n\n");
#define fpe(str) fprintf(stderr, " %s", str);
fpe("-i New IPC namespace\n");
fpe("-m New mount namespace\n");
fpe("-n New network namespace\n");
fpe("-p New PID namespace\n");
fpe("-u New UTS namespace\n");
fpe("-U New user namespace\n");
fpe("-M uid_map Specify UID map for user namespace\n");
fpe("-G gid_map Specify GID map for user namespace\n");
fpe("-z Map user's UID and GID to 0 in user namespace\n");
fpe(" (equivalent to: -M '0 <uid> 1' -G '0 <gid> 1')\n");
fpe("-v Display verbose messages\n");
fpe("\n");
fpe("If -z, -M, or -G is specified, -U is required.\n");
fpe("It is not permitted to specify both -z and either -M or -G.\n");
fpe("\n");
fpe("Map strings for -M and -G consist of records of the form:\n");
fpe("\n");
fpe(" ID-inside-ns ID-outside-ns len\n");
fpe("\n");
fpe("A map string can contain multiple records, separated"
" by commas;\n");
fpe("the commas are replaced by newlines before writing"
" to map files.\n");
exit(EXIT_FAILURE);
}
/* Update the mapping file 'map_file', with the value provided in
'mapping', a string that defines a UID or GID mapping. A UID or
GID mapping consists of one or more newline-delimited records
of the form:
ID_inside-ns ID-outside-ns length
Requiring the user to supply a string that contains newlines is
of course inconvenient for command-line use. Thus, we permit the
use of commas to delimit records in this string, and replace them
with newlines before writing the string to the file. */
static void
update_map(char *mapping, char *map_file)
{
int fd;
size_t map_len; /* Length of 'mapping' */
/* Replace commas in mapping string with newlines. */
map_len = strlen(mapping);
for (size_t j = 0; j < map_len; j++)
if (mapping[j] == ',')
mapping[j] = '\n';
fd = open(map_file, O_RDWR);
if (fd == -1) {
fprintf(stderr, "ERROR: open %s: %s\n", map_file,
strerror(errno));
exit(EXIT_FAILURE);
}
if (write(fd, mapping, map_len) != map_len) {
fprintf(stderr, "ERROR: write %s: %s\n", map_file,
strerror(errno));
exit(EXIT_FAILURE);
}
close(fd);
}
/* Linux 3.19 made a change in the handling of setgroups(2) and
the 'gid_map' file to address a security issue. The issue
allowed *unprivileged* users to employ user namespaces in
order to drop groups. The upshot of the 3.19 changes is that
in order to update the 'gid_maps' file, use of the setgroups()
system call in this user namespace must first be disabled by
writing "deny" to one of the /proc/PID/setgroups files for
this namespace. That is the purpose of the following function. */
static void
proc_setgroups_write(pid_t child_pid, char *str)
{
char setgroups_path[PATH_MAX];
int fd;
snprintf(setgroups_path, PATH_MAX, "/proc/%jd/setgroups",
(intmax_t) child_pid);
fd = open(setgroups_path, O_RDWR);
if (fd == -1) {
/* We may be on a system that doesn't support
/proc/PID/setgroups. In that case, the file won't exist,
and the system won't impose the restrictions that Linux 3.19
added. That's fine: we don't need to do anything in order
to permit 'gid_map' to be updated.
However, if the error from open() was something other than
the ENOENT error that is expected for that case, let the
user know. */
if (errno != ENOENT)
fprintf(stderr, "ERROR: open %s: %s\n", setgroups_path,
strerror(errno));
return;
}
if (write(fd, str, strlen(str)) == -1)
fprintf(stderr, "ERROR: write %s: %s\n", setgroups_path,
strerror(errno));
close(fd);
}
static int /* Start function for cloned child */
childFunc(void *arg)
{
struct child_args *args = arg;
char ch;
/* Wait until the parent has updated the UID and GID mappings.
See the comment in main(). We wait for end of file on a
pipe that will be closed by the parent process once it has
updated the mappings. */
close(args->pipe_fd[1]); /* Close our descriptor for the write
end of the pipe so that we see EOF
when parent closes its descriptor. */
if (read(args->pipe_fd[0], &ch, 1) != 0) {
fprintf(stderr,
"Failure in child: read from pipe returned != 0\n");
exit(EXIT_FAILURE);
}
close(args->pipe_fd[0]);
/* Execute a shell command. */
printf("About to exec %s\n", args->argv[0]);
execvp(args->argv[0], args->argv);
err(EXIT_FAILURE, "execvp");
}
#define STACK_SIZE (1024 * 1024)
static char child_stack[STACK_SIZE]; /* Space for child's stack */
int
main(int argc, char *argv[])
{
int flags, opt, map_zero;
pid_t child_pid;
struct child_args args;
char *uid_map, *gid_map;
const int MAP_BUF_SIZE = 100;
char map_buf[MAP_BUF_SIZE];
char map_path[PATH_MAX];
/* Parse command-line options. The initial '+' character in
the final getopt() argument prevents GNU-style permutation
of command-line options. That's useful, since sometimes
the 'command' to be executed by this program itself
has command-line options. We don't want getopt() to treat
those as options to this program. */
flags = 0;
verbose = 0;
gid_map = NULL;
uid_map = NULL;
map_zero = 0;
while ((opt = getopt(argc, argv, "+imnpuUM:G:zv")) != -1) {
switch (opt) {
case 'i': flags |= CLONE_NEWIPC; break;
case 'm': flags |= CLONE_NEWNS; break;
case 'n': flags |= CLONE_NEWNET; break;
case 'p': flags |= CLONE_NEWPID; break;
case 'u': flags |= CLONE_NEWUTS; break;
case 'v': verbose = 1; break;
case 'z': map_zero = 1; break;
case 'M': uid_map = optarg; break;
case 'G': gid_map = optarg; break;
case 'U': flags |= CLONE_NEWUSER; break;
default: usage(argv[0]);
}
}
/* -M or -G without -U is nonsensical */
if (((uid_map != NULL || gid_map != NULL || map_zero) &&
!(flags & CLONE_NEWUSER)) ||
(map_zero && (uid_map != NULL || gid_map != NULL)))
usage(argv[0]);
args.argv = &argv[optind];
/* We use a pipe to synchronize the parent and child, in order to
ensure that the parent sets the UID and GID maps before the child
calls execve(). This ensures that the child maintains its
capabilities during the execve() in the common case where we
want to map the child's effective user ID to 0 in the new user
namespace. Without this synchronization, the child would lose
its capabilities if it performed an execve() with nonzero
user IDs (see the capabilities(7) man page for details of the
transformation of a process's capabilities during execve()). */
if (pipe(args.pipe_fd) == -1)
err(EXIT_FAILURE, "pipe");
/* Create the child in new namespace(s). */
child_pid = clone(childFunc, child_stack + STACK_SIZE,
flags | SIGCHLD, &args);
if (child_pid == -1)
err(EXIT_FAILURE, "clone");
/* Parent falls through to here. */
if (verbose)
printf("%s: PID of child created by clone() is %jd\n",
argv[0], (intmax_t) child_pid);
/* Update the UID and GID maps in the child. */
if (uid_map != NULL || map_zero) {
snprintf(map_path, PATH_MAX, "/proc/%jd/uid_map",
(intmax_t) child_pid);
if (map_zero) {
snprintf(map_buf, MAP_BUF_SIZE, "0 %jd 1",
(intmax_t) getuid());
uid_map = map_buf;
}
update_map(uid_map, map_path);
}
if (gid_map != NULL || map_zero) {
proc_setgroups_write(child_pid, "deny");
snprintf(map_path, PATH_MAX, "/proc/%jd/gid_map",
(intmax_t) child_pid);
if (map_zero) {
snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1",
(intmax_t) getgid());
gid_map = map_buf;
}
update_map(gid_map, map_path);
}
/* Close the write end of the pipe, to signal to the child that we
have updated the UID and GID maps. */
close(args.pipe_fd[1]);
if (waitpid(child_pid, NULL, 0) == -1) /* Wait for child */
err(EXIT_FAILURE, "waitpid");
if (verbose)
printf("%s: terminating\n", argv[0]);
exit(EXIT_SUCCESS);
}
SEE ALSO
newgidmap(1), newuidmap(1), clone(2), ptrace(2), setns(2), unshare(2),
proc(5), subgid(5), subuid(5), capabilities(7), cgroup_namespaces(7),
credentials(7), namespaces(7), pid_namespaces(7)
The kernel source file Documentation/admin-guide/namespaces/re-
source-control.rst.
Linux man-pages 6.7 2024-02-25 user_namespaces(7)
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