Hi Eric et al.,
All: The attached page aims to provide a fairly complete overview of
user namespaces. I'm looking for review comments (corrections,
improvements, additions, etc.) on this man page. I've provided it in
two forms inline below, and reviewers can comment on whichever form
they are most comfortable with:
1) The rendered page as plain text
2) The *roff source (also attached); rendering that source will enable
readers to see proper formatting for the page.
Note that the namespaces(7) page referred to in this page is not yet
finished; I'll send it out for review at a future time.
Eric: Various pieces of the page are shifted out of other pages
(clone(2), setns(2), etc.) and are derived from comments you've
emailed me off list, so you are (jointly) in the copyright of the
page. I've chosen the common license for man-pages (the same as used
for the pid_namespaces(7) page). I assume that license will be okay
for you, since you gae it a pass for pid_namespaces(7), but please
confirm.
Cheers,
Michael
==========
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, in particular,
user IDs and group IDs (see credentials(7), keys (see keyctl(2)), 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 names‐
pace.
Use of user namespaces requires a kernel that is configured with the
CONFIG_USER_NS option.
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.
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 process can join another
user namespace with setns(2) if it has the CAP_SYS_ADMIN in that names‐
pace; upon doing so, it gains a full set of capabilities in that names‐
pace.
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.
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 names‐
pace. 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). (Nevertheless, a
process owned by the root user will be able to access resources such as
files that are owned by user ID 0, and will be able to do things such
as sending signals to processes belonging to user ID 0.)
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 per‐
form operations (that require privilege) only on resources governed by
that 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 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.
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 names‐
pace. 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 names‐
paces, and mount, PID, IPC, network, and UTS namespaces can be created
with just the CAP_SYS_ADMIN capability in the caller's 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 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.
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 map‐
pings.
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 names‐
pace 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 read‐
ing 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 names‐
pace 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 names‐
pace 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 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.
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 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 requirements must be
met:
1. The writing process must have the CAP_SETUID (CAP_SETGID) capability
in the user namespace 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 namespace.
4. One of the following is true:
* The data written to uid_map (gid_map) consists of a single line
that maps the writing process's file system 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 process has the CAP_SETUID (CAP_SETGID) capability in the
parent user namespace. Thus, a privileged process can make map‐
pings to arbitrary user IDs (group IDs) in the parent user names‐
pace.
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 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);
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 file system 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.
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 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.
EXAMPLE
The program below is designed to allow experimenting with user names‐
paces, 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) names‐
paces, 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 file system and listing all of the processes visi‐
ble 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
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(" If -M or -G is specified, -U is required\n");
fpe("-v Display verbose messages\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, ");
fpe("separated by commas;\n");
fpe("the commas are replaced by newlines before writing");
fpe(" 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, "open %s: %s\n", map_file, strerror(errno));
exit(EXIT_FAILURE);
}
if (write(fd, mapping, map_len) != map_len) {
fprintf(stderr, "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 */
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;
pid_t child_pid;
struct child_args args;
char *uid_map, *gid_map;
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;
while ((opt = getopt(argc, argv, "+imnpuUM:G:v")) != -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 '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) &&
!(flags & CLONE_NEWUSER))
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) {
snprintf(map_path, PATH_MAX, "/proc/%ld/uid_map",
(long) child_pid);
update_map(uid_map, map_path);
}
if (gid_map != NULL) {
snprintf(map_path, PATH_MAX, "/proc/%ld/gid_map",
(long) child_pid);
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
clone(2), setns(2), unshare(2), proc(5), credentials(7), capabili‐
ties(7), namespaces(7), pid_namespaces(7)
The kernel source file Documentation/namespaces/resource-control.txt.
Linux 2013-01-14 USER_NAMESPACES(7)
========== *roff source ==========
$ cat user_namespaces.7
.\" Copyright (c) 2013 by Michael Kerrisk <mtk.manpages@xxxxxxxxx>
.\" and Copyright (c) 2012 by Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\"
.TH USER_NAMESPACES 7 2013-01-14 "Linux" "Linux Programmer's Manual"
.SH NAME
user_namespaces \- overview of Linux user_namespaces
.SH DESCRIPTION
For an overview of namespaces, see
.BR namespaces (7).
User namespaces isolate security-related identifiers, in particular,
user IDs and group IDs (see
.BR credentials (7),
keys (see
.BR keyctl (2)),
and capabilities (see
.BR 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.
Use of user namespaces requires a kernel that is configured with the
.B CONFIG_USER_NS
option.
.\"
.\" ============================================================
.\"
.SS Nested namespaces, namespace membership
User namespaces can be nested;
that is, each user namespace\(emexcept the initial ("root")
namespace\(emhas 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
.BR unshare (2)
or
.BR clone (2)
with the
.BR CLONE_NEWUSER
flag.
Each process is a member of exactly one user namespace.
A process created via
.BR fork (2)
or
.BR clone (2)
without the
.BR CLONE_NEWUSER
flag is a member of the same user namespace as its parent.
A process can join another user namespace with
.BR setns (2)
if it has the
.BR CAP_SYS_ADMIN
in that namespace;
upon doing so, it gains a full set of capabilities in that namespace.
A call to
.BR clone (2)
or
.BR unshare (2)
with the
.BR CLONE_NEWUSER
flag makes the new child process (for
.BR clone (2))
or the caller (for
.BR unshare (2))
a member of the new user namespace created by the call.
.\"
.\" ============================================================
.\"
.SS Capabilities
The child process created by
.BR clone (2)
with the
.BR 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
.BR unshare (2)
or joins an existing user namespace using
.BR 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
.BR clone (2))
or previous (in the case of
.BR unshare (2)
and
.BR 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).
(Nevertheless, a process owned by the root user
will be able to access resources such as
files that are owned by user ID 0,
and will be able to do things such as sending signals
to processes belonging to user ID 0.)
A call to
.BR clone (2),
.BR unshare (2),
or
.BR setns (2)
using the
.BR CLONE_NEWUSER
flag sets the "securebits" flags
(see
.BR capabilities (7))
to their default values (all flags disabled) in the child (for
.BR clone (2))
or caller (for
.BR unshare (2),
or
.BR setns (2)).
Note that because the caller no longer has capabilities
in its original user namespace after a call to
.BR 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
.BR 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 determining whether or not a process has a capability
in a particular user namespace are as follows:
.IP 1. 3
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
.BR clone (2),
.BR unshare (2),
or
.BR setns (2),
as already described.
.\" In the 3.8 sources, see security/commoncap.c::cap_capable():
.IP 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.
.IP 3.
.\" * The owner of the user namespace in the parent of the
.\" * user namespace has all caps.
When a user namespace is created, the kernel records the effective
user ID of the creating process as being the "owner" of the namespace.
.\" (and likewise associates the effective group ID of the creating process
.\" with the namespace).
A process that resides
in the parent of the user namespace
.\" See kernel commit 520d9eabce18edfef76a60b7b839d54facafe1f9 for a fix
.\" on this point
and whose effective user ID matches the owner of the namespace
has all capabilities in the namespace.
.\" This includes the case where the process executes a set-user-ID
.\" program that confers the effective UID of the creator of 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.
.\"
.\" ============================================================
.\"
.SS 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
.B CAP_SYS_ADMIN
capability in the caller's user namespace.
If
.BR CLONE_NEWUSER
is specified along with other
.B CLONE_NEW*
flags in a single
.BR clone (2)
or
.BR unshare (2)
call, the user namespace is guaranteed to be created first,
giving the child
.RB ( clone (2))
or caller
.RB ( 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
.BR clone (2)
or
.BR 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.
.\"
.\" ============================================================
.\"
.SS 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
.IR /proc/[pid]/uid_map
and
.IR /proc/[pid]/gid_map
files (available since Linux 3.5)
.\" commit 22d917d80e842829d0ca0a561967d728eb1d6303
expose the mappings for user and group IDs
inside the user namespace for the process
.IR 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
.IR uid_map ;
.IR gid_map
is exactly the same,
but each instance of "user ID" is replaced by "group ID".
The
.I uid_map
file exposes the mapping of user IDs from the user namespace
of the process
.IR pid
to the user namespace of the process that opened
.IR 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
.I uid_map
file, depending on the user ID mappings for the user namespaces
of the reading processes.
Each line in the
.I 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:
.IP (1) 4
The start of the range of user IDs in
the user namespace of the process
.IR pid .
.IP (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
.I uid_map
and the process
.IR pid
are in the same user namespace, as follows:
.RS
.IP a) 3
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
.IR uid_map .
.IP 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
.IR pid .
This case enables the opener of
.I uid_map
(the common case here is opening
.IR /proc/self/uid_map )
to see the mapping of user IDs into the user namespace of the process
that created this user namespace.
.RE
.IP (3)
The length of the range of user IDs that is mapped between the two
user namespaces.
.PP
System calls that return user IDs (group IDs)\(emfor example,
.BR getuid (2),
.BR getgid (2),
and the credential fields in the structure returned by
.BR stat (2)\(emreturn
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
.BR 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
.I uid_map
file
.RI ( gid_map
is the same) from a shell in the initial namespace shows:
.in +4n
.nf
$ \fBcat /proc/$$/uid_map\fP
0 0 4294967295
.fi
.in
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.
.\"
.\" ============================================================
.\"
.SS Defining user and group ID mappings: writing to uid_map and gid_map
.PP
After the creation of a new user namespace, the
.I uid_map
file of
.I one
of the processes in the namespace may be written to
.I once
to define the mapping of user IDs in the new user namespace.
An attempt to write more than once to a
.I uid_map
file in a user namespace fails with the error
.BR EPERM .
Similar rules apply for
.I gid_map
files.
The lines written to
.IR uid_map
.RI ( gid_map )
must conform to the following rules:
.IP * 3
The three fields must be valid numbers,
and the last field must be greater than 0.
.IP *
Lines are terminated by newline characters.
.IP *
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,
.\" FIXME(Eric): the restriction "less than" rather than "less than or equal"
.\" seems strangely arbitrary. Furthermore, the comment does not agree
.\" with the code in kernel/user_namespace.c. Which is correct.
and the write must be performed at the start of the file (i.e.,
.BR lseek (2)
and
.BR pwrite (2)
can't be used to write to nonzero offsets in the file).
.IP *
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
.\" commit 0bd14b4fd72afd5df41e9fd59f356740f22fceba
fix this limitation, allowing any valid set of nonoverlapping maps.
.IP *
At least one line must be written to the file.
.PP
Writes that violate the above rules fail with the error
.BR EINVAL .
In order for a process to write to the
.I /proc/[pid]/uid_map
.RI ( /proc/[pid]/gid_map )
file, all of the following requirements must be met:
.IP 1. 3
The writing process must have the
.BR CAP_SETUID
.RB ( CAP_SETGID )
capability in the user namespace of the process
.IR pid .
.IP 2.
The writing process must be in either the user namespace of the process
.I pid
or inside the parent user namespace of the process
.IR pid .
.IP 3.
The mapped user IDs (group IDs) must in turn have a mapping
in the parent user namespace.
.IP 4.
One of the following is true:
.RS
.IP * 3
The data written to
.I uid_map
.RI ( gid_map )
consists of a single line that maps the writing process's file system 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.
.IP * 3
The process has the
.BR CAP_SETUID
.RB ( 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.
.RE
.PP
Writes that violate the above rules fail with the error
.BR EPERM .
.\"
.\" ============================================================
.\"
.SS Unmapped user and group IDs
.PP
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
.BR getuid ()
before a user ID mapping has been defined for the namespace.
In most such cases, an unmapped user ID is converted
.\" from_kuid_munged(), from_kgid_munged()
to the overflow user ID (group ID);
the default value for the overflow user ID (group ID) is 65534.
See the descriptions of
.IR /proc/sys/kernel/overflowuid
and
.IR /proc/sys/kernel/overflowgid
in
.BR proc (5).
The cases where unmapped IDs are mapped in this fashion include
system calls that return user IDs
.RB ( getuid (2)
.BR getgid (2),
and similar),
credentials passed over a UNIX domain socket,
.\" also SO_PEERCRED
credentials returned by
.BR stat (2),
.BR waitid (2),
and the System V IPC "ctl"
.B IPC_STAT
operations,
credentials exposed by
.IR /proc/PID/status
and the files in
.IR /proc/sysvipc/* ,
credentials returned via the
.I si_uid
field in the
.I siginfo_t
received with a signal (see
.BR sigaction (2)),
credentials written to the process accounting file (see
.BR acct (5)),
and credentials returned with POSIX message queue notifications (see
.BR mq_notify (3)).
There is one notable case where unmapped user and group IDs are
.I not
.\" from_kuid(), from_kgid()
.\" Also F_GETOWNER_UIDS is an exception
converted to the corresponding overflow ID value.
When viewing a
.I uid_map
or
.I gid_map
file in which there is no mapping for the second field,
that field is displayed as 4294967295 (\-1 as an unsigned integer);
.\"
.\" ============================================================
.\"
.SS Set-user-ID and set-group-ID programs
.PP
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
.I 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 file system that was mounted with the
.BR MS_NOSUID
flag, as described in
.BR mount (2).)
.\"
.\" ============================================================
.\"
.SS Miscellaneous
.PP
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
.B SCM_CREDENTIALS
in
.BR unix (7)),
they are translated into the corresponding values as per the
receiving process's user and group ID mappings.
.SH CONFORMING TO
Namespaces are a Linux-specific feature.
.SH 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 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.
.SH 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
.I 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:
.in +4n
.nf
$ \fBuname -rs\fP # Need Linux 3.8 or later
Linux 3.8.0
$ \fBid -u\fP # Running as unprivileged user
1000
$ \fBid -g\fP
1000
.fi
.in
Now start a new shell in new user
.RI ( \-U ),
mount
.RI ( \-m ),
and PID
.RI ( \-p )
namespaces, with user ID
.RI ( \-M )
and group ID
.RI ( \-G )
1000 mapped to 0 inside the user namespace:
.in +4n
.nf
$ \fB./userns_child_exec -p -m -U -M '0 1000 1' -G '0 1000 1' bash\fP
.fi
.in
The shell has PID 1, because it is the first process in the new
PID namespace:
.in +4n
.nf
bash$ \fBecho $$\fP
1
.fi
.in
Inside the user namespace, the shell has user and group ID 0,
and a full set of permitted and effective capabilities:
.in +4n
.nf
bash$ \fBcat /proc/$$/status | egrep '^[UG]id'\fP
Uid: 0 0 0 0
Gid: 0 0 0 0
bash$ \fBcat /proc/$$/status | egrep '^Cap(Prm|Inh|Eff)'\fP
CapInh: 0000000000000000
CapPrm: 0000001fffffffff
CapEff: 0000001fffffffff
.fi
.in
Mounting a new
.I /proc
file system 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:
.in +4n
.nf
bash$ \fBmount -t proc proc /proc\fP
bash$ \fBps ax\fP
PID TTY STAT TIME COMMAND
1 pts/3 S 0:00 bash
22 pts/3 R+ 0:00 ps ax
.fi
.in
.SS Program source
\&
.nf
/* 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 \(aqerrno\(aq 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(" If \-M or \-G is specified, \-U is required\\n");
fpe("\-v Display verbose messages\\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, ");
fpe("separated by commas;\\n");
fpe("the commas are replaced by newlines before writing");
fpe(" to map files.\\n");
exit(EXIT_FAILURE);
}
/* Update the mapping file \(aqmap_file\(aq, with the value provided in
\(aqmapping\(aq, 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 \(aqmapping\(aq */
/* Replace commas in mapping string with newlines */
map_len = strlen(mapping);
for (j = 0; j < map_len; j++)
if (mapping[j] == \(aq,\(aq)
mapping[j] = \(aq\\n\(aq;
fd = open(map_file, O_RDWR);
if (fd == \-1) {
fprintf(stderr, "open %s: %s\\n", map_file, strerror(errno));
exit(EXIT_FAILURE);
}
if (write(fd, mapping, map_len) != map_len) {
fprintf(stderr, "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 */
execvp(args\->argv[0], args\->argv);
errExit("execvp");
}
#define STACK_SIZE (1024 * 1024)
static char child_stack[STACK_SIZE]; /* Space for child\(aqs stack */
int
main(int argc, char *argv[])
{
int flags, opt;
pid_t child_pid;
struct child_args args;
char *uid_map, *gid_map;
char map_path[PATH_MAX];
/* Parse command\-line options. The initial \(aq+\(aq character in
the final getopt() argument prevents GNU\-style permutation
of command\-line options. That\(aqs useful, since sometimes
the \(aqcommand\(aq to be executed by this program itself
has command\-line options. We don\(aqt want getopt() to treat
those as options to this program. */
flags = 0;
verbose = 0;
gid_map = NULL;
uid_map = NULL;
while ((opt = getopt(argc, argv, "+imnpuUM:G:v")) != \-1) {
switch (opt) {
case \(aqi\(aq: flags |= CLONE_NEWIPC; break;
case \(aqm\(aq: flags |= CLONE_NEWNS; break;
case \(aqn\(aq: flags |= CLONE_NEWNET; break;
case \(aqp\(aq: flags |= CLONE_NEWPID; break;
case \(aqu\(aq: flags |= CLONE_NEWUTS; break;
case \(aqv\(aq: verbose = 1; break;
case \(aqM\(aq: uid_map = optarg; break;
case \(aqG\(aq: gid_map = optarg; break;
case \(aqU\(aq: flags |= CLONE_NEWUSER; break;
default: usage(argv[0]);
}
}
/* \-M or \-G without \-U is nonsensical */
if ((uid_map != NULL || gid_map != NULL) &&
!(flags & CLONE_NEWUSER))
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\(aqs 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\(aqs 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) {
snprintf(map_path, PATH_MAX, "/proc/%ld/uid_map",
(long) child_pid);
update_map(uid_map, map_path);
}
if (gid_map != NULL) {
snprintf(map_path, PATH_MAX, "/proc/%ld/gid_map",
(long) child_pid);
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);
}
.fi
.SH SEE ALSO
.BR clone (2),
.BR setns (2),
.BR unshare (2),
.BR proc (5),
.BR credentials (7),
.BR capabilities (7),
.BR namespaces (7),
.BR pid_namespaces (7)
.sp
The kernel source file
.IR Documentation/namespaces/resource-control.txt .
Attachment:
user_namespaces.7
Description: Binary data
