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USER_NAMESPACES(7) Linux Programmer's Manual USER_NAMESPACES(7)
user_namespaces - overview of Linux user namespaces
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 keyrings(7)), and capabilities (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 parent. 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.
The NS_GET_PARENT ioctl(2) operation can be used to discover the
parental relationship between user namespaces; see ioctl_ns(2).
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
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)).
Consequently, unless the process has a user ID of 0 within the
namespace, or the executable 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), 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.
The rules for determining whether or not a process has a capability
in a particular user namespace are as follows:
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 capabilities 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 creating 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. 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
perform operations (that require privilege) only on resources
governed by that namespace. In other words, having a capability in a
user namespace permits a process to perform privileged operations on
resources that are governed by (nonuser) namespaces associated with
the user namespace (see the next subsection).
On the other hand, there are many privileged operations that affect
resources that are not associated with any namespace type, for
example, changing the system 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 associated with 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)
Holding CAP_SYS_ADMIN within the user namespace associated with a
process's cgroup namespace allows (since Linux 4.6) that process to
the mount 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 associated with 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
namespaces, and other the other types of 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 creating 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 namespace (other than a user 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 subsequently
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. 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 is associated with the process's
UTS namespace, and check whether the process has the required
capability (CAP_SYS_ADMIN) in that user namespace.
The NS_GET_USERNS ioctl(2) operation can be used to discover the user
namespace with which a non-user namespace is associated; 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
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 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 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 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 inter‐
faces (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 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 sim‐
plistic 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) capabil‐
ity in the user namespace of the process pid.
2. 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.
3. The mapped user IDs (group IDs) must in turn have a mapping in the
parent user namespace.
4. One of the following two cases applies:
* Either the writing process has the CAP_SETUID (CAP_SETGID)
capability in the parent user namespace.
+ No further restrictions apply: the process can make mappings
to arbitrary user IDs (group IDs) in the parent user names‐
pace.
* 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]/setgroups file (see below) before writing to
gid_map.
Writes that violate the above rules 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 writ‐
ten, 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 rele‐
vant 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
namespace 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 pro‐
cesses in the user namespace that contains the process pid are per‐
mitted 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 dis‐
allowed to setgroups(2) being allowed.
The default value of this file in the initial user namespace is
"allow".
Once /proc/[pid]/gid_map has been written to (which has the effect of
enabling setgroups(2) in the user namespace), it is no longer possi‐
ble to disallow setgroups(2) by writing "deny" to /proc/[pid]/set‐
groups (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)
system 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 namespaces 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
security 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 privileged process (one with the CAP_SETGID capability) could
call setgroups(2). However, with the introduction of user names‐
paces, it became possible for an unprivileged process to create a new
namespace in which the user had all privileges. This then allowed
formerly unprivileged 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 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 sys‐
tem 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 operations,
credentials exposed by /proc/[pid]/status and the files in
/proc/sysvipc/*, credentials returned via the si_uid field in the
siginfo_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).
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 descrip‐
tion of SCM_CREDENTIALS in unix(7)), they are translated into the
corresponding values as per the receiving process's user and group ID
mappings.
Namespaces are a Linux-specific feature.
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 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 namespace, 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 3.12 added support the last of the
unsupported major filesystems, XFS.
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 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)
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
Mounting a new /proc filesystem and listing all of the processes vis‐
ible in the new PID namespace shows that the shell can't see any pro‐
cesses 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 <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);
}
/* 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
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/%ld/setgroups",
(long) 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 = (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);
}
close(args->pipe_fd[0]);
/* 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) {
proc_setgroups_write(child_pid, "deny");
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);
}
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/namespaces/resource-control.txt.
This page is part of release 4.15 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
https://www.kernel.org/doc/man-pages/.
Linux 2018-02-02 USER_NAMESPACES(7)
Pages that refer to this page: nsenter(1), systemd-detect-virt(1), unshare(1), clone(2), getgroups(2), ioctl_ns(2), keyctl(2), seteuid(2), setgid(2), setns(2), setresuid(2), setreuid(2), setuid(2), unshare(2), proc(5), subgid(5), subuid(5), capabilities(7), cgroup_namespaces(7), cgroups(7), credentials(7), mount_namespaces(7), namespaces(7), network_namespaces(7), pid_namespaces(7)
Copyright and license for this manual page
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HTML rendering created 2018-02-02 by Michael Kerrisk, author of The Linux Programming Interface, maintainer of the Linux man-pages project. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH.
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