For review: user_namespace(7) man page
Michael Kerrisk (man-pages)
mtk.manpages at gmail.com
Wed Aug 20 23:36:42 UTC 2014
Hello Eric et al.,
For various reasons, my work on the namespaces man pages
fell off the table a while back. Nevertheless, the pages have
been close to completion for a while now, and I recently restarted,
in an effort to finish them. As you also noted to me f2f, there have
been recently been some small namespace changes that you may affect
the content of the pages. Therefore, I'll take the opportunity to
send the namespace-related pages out for further (final?) review.
So, here, I start with the user_namespaces(7) page, which is shown
in rendered form below, with source attached to this mail. I'll
send various other pages in follow-on mails.
Review comments/suggestions for improvements / bug fixes welcome.
Cheers,
Michael
==
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 creden‐
tials(7), the root directory, keys (see keyctl(2)), and capabili‐
ties (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 names‐
pace, and can have zero or more child user namespaces. The par‐
ent user namespace is the user namespace of the process that cre‐
ates 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 lev‐
els 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
process can join another user namespace with setns(2) if it has
the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a
full set of capabilities in that namespace.
A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag
makes the new child process (for clone(2)) or the caller (for
unshare(2)) a member of the new user namespace created by the
call.
Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag
starts out with a 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 cre‐
ated 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 to lose any
capabilities that it has, unless it has a user ID of 0 within the
namespace. 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 capabili‐
ties(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 retain‐
ing its user namespace membership by using a pair of setns(2)
calls to move to another user namespace and then return to its
original user namespace.
Having a capability inside a user namespace permits a process to
perform operations (that require privilege) only on resources
governed by that namespace. The rules for 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 asso‐
ciated 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 names‐
paces as well.
Interaction of user namespaces and other types of namespaces
Starting in Linux 3.8, unprivileged processes can create user
namespaces, and mount, PID, IPC, network, and UTS namespaces can
be created with just the CAP_SYS_ADMIN capability in the caller's
user namespace.
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 cre‐
ated by the call. Thus, it is possible for an unprivileged call‐
er 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 associ‐
ation 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 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 delim‐
ited 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 con‐
sistency, 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 fol‐
lowing 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 can‐
not overlap with the ranges in any other lines. In the ini‐
tial implementation (Linux 3.8), this requirement was satis‐
fied by a simplistic implementation that imposed the further
requirement that the values in both field 1 and field 2 of
successive lines must be in ascending numerical order, which
prevented some otherwise valid maps from being created. Linux
3.9 and later fix this limitation, allowing any valid set of
nonoverlapping maps.
* At least one line must be written to the file.
Writes that violate the above rules fail with the error EINVAL.
In order for a process to write to the /proc/[pid]/uid_map
(/proc/[pid]/gid_map) file, all of the following requirements
must be met:
1. The writing process must have the CAP_SETUID (CAP_SETGID)
capability in the user 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 filesystem user ID
(group ID) in the parent user namespace to a user ID (group
ID) in the user namespace. The usual case here is that
this single line provides a mapping for user ID of the
process that created the namespace.
* The process has the CAP_SETUID (CAP_SETGID) capability in
the parent user namespace. Thus, a privileged process can
make mappings to arbitrary user IDs (group IDs) in the par‐
ent user namespace.
Writes that violate the above rules fail with the error EPERM.
Unmapped user and group IDs
There are various places where an unmapped user ID (group ID) may
be exposed to user space. For example, the first process in a
new user namespace may call getuid() before a user ID mapping has
been defined for the namespace. In most such cases, an unmapped
user ID is converted to the overflow user ID (group ID); the
default value for the overflow user ID (group ID) is 65534. See
the descriptions of /proc/sys/kernel/overflowuid and
/proc/sys/kernel/overflowgid in proc(5).
The cases where unmapped IDs are mapped in this fashion include
system calls that return user IDs (getuid(2) getgid(2), and simi‐
lar), 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 view‐
ing a uid_map or gid_map file in which there is no mapping for
the second field, that field is displayed as 4294967295 (-1 as an
unsigned integer);
Set-user-ID and set-group-ID programs
When a process inside a user namespace executes a set-user-ID
(set-group-ID) program, the process's effective user (group) ID
inside the namespace is changed to whatever value is mapped for
the user (group) ID of the file. However, if either the user or
the group ID of the file has no mapping inside the namespace, the
set-user-ID (set-group-ID) bit is silently ignored: the new pro‐
gram is executed, but the process's effective user (group) ID is
left unchanged. (This mirrors the semantics of executing a set-
user-ID or set-group-ID program that resides on a filesystem that
was mounted with the MS_NOSUID flag, as described in mount(2).)
Miscellaneous
When a process's user and group IDs are passed over a UNIX domain
socket to a process in a different user namespace (see the
description of SCM_CREDENTIALS in unix(7)), they are translated
into the corresponding values as per the receiving process's user
and group ID mappings.
CONFORMING TO
Namespaces are a Linux-specific feature.
NOTES
Over the years, there have been a lot of features that have been
added to the Linux kernel that have been made available only to
privileged users because of their potential to 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 sub‐
system is configured into the kernel, it is not possible to con‐
figure user namespaces support.
As at Linux 3.8, most relevant subsystems supported user names‐
paces, but a number of filesystems did not have the infrastruc‐
ture needed to map user and group IDs between user namespaces.
Linux 3.9 added the required infrastructure support for many of
the remaining unsupported filesystems (Plan 9 (9P), Andrew File
System (AFS), Ceph, CIFS, CODA, NFS, and OCFS2). Linux 3.11
added support the last of the unsupported major filesystems, XFS.
EXAMPLE
The program below is designed to allow experimenting with user
namespaces, as well as other types of namespaces. It creates
namespaces as specified by command-line options and then executes
a command inside those namespaces. The comments and usage()
function inside the program provide a full explanation of the
program. The following shell session 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
Inside the user namespace, the shell has user and group ID 0, and
a full set of permitted and effective capabilities:
bash$ cat /proc/$$/status | egrep '^[UG]id'
Uid: 0 0 0 0
Gid: 0 0 0 0
bash$ cat /proc/$$/status | egrep '^Cap(Prm|Inh|Eff)'
CapInh: 0000000000000000
CapPrm: 0000001fffffffff
CapEff: 0000001fffffffff
Mounting a new /proc filesystem and listing all of the processes
visible in the new PID namespace shows that the shell can't see
any processes outside the PID namespace:
bash$ mount -t proc proc /proc
bash$ ps ax
PID TTY STAT TIME COMMAND
1 pts/3 S 0:00 bash
22 pts/3 R+ 0:00 ps ax
Program source
/* userns_child_exec.c
Licensed under GNU General Public License v2 or later
Create a child process that executes a shell command in new
namespace(s); allow UID and GID mappings to be specified when
creating a user namespace.
*/
#define _GNU_SOURCE
#include <sched.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <signal.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <limits.h>
#include <errno.h>
/* A simple error-handling function: print an error message based
on the value in 'errno' and terminate the calling process */
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
struct child_args {
char **argv; /* Command to be executed by child, with args */
int pipe_fd[2]; /* Pipe used to synchronize parent and child */
};
static int verbose;
static void
usage(char *pname)
{
fprintf(stderr, "Usage: %s [options] cmd [arg...]\n\n", pname);
fprintf(stderr, "Create a child process that executes a shell "
"command in a new user namespace,\n"
"and possibly also other new namespace(s).\n\n");
fprintf(stderr, "Options can be:\n\n");
#define fpe(str) fprintf(stderr, " %s", str);
fpe("-i New IPC namespace\n");
fpe("-m New mount namespace\n");
fpe("-n New network namespace\n");
fpe("-p New PID namespace\n");
fpe("-u New UTS namespace\n");
fpe("-U New user namespace\n");
fpe("-M uid_map Specify UID map for user namespace\n");
fpe("-G gid_map Specify GID map for user namespace\n");
fpe("-z Map user's UID and GID to 0 in user namespace\n");
fpe(" (equivalent to: -M '0 <uid> 1' -G '0 <gid> 1')\n");
fpe("-v Display verbose messages\n");
fpe("\n");
fpe("If -z, -M, or -G is specified, -U is required.\n");
fpe("It is not permitted to specify both -z and either -M or -G.\n");
fpe("\n");
fpe("Map strings for -M and -G consist of records of the form:\n");
fpe("\n");
fpe(" ID-inside-ns ID-outside-ns len\n");
fpe("\n");
fpe("A map string can contain multiple records, separated"
" by commas;\n");
fpe("the commas are replaced by newlines before writing"
" to map files.\n");
exit(EXIT_FAILURE);
}
/* Update the mapping file 'map_file', with the value provided in
'mapping', a string that defines a UID or GID mapping. A UID or
GID mapping consists of one or more newline-delimited records
of the form:
ID_inside-ns ID-outside-ns length
Requiring the user to supply a string that contains newlines is
of course inconvenient for command-line use. Thus, we permit the
use of commas to delimit records in this string, and replace them
with newlines before writing the string to the file. */
static void
update_map(char *mapping, char *map_file)
{
int fd, j;
size_t map_len; /* Length of 'mapping' */
/* Replace commas in mapping string with newlines */
map_len = strlen(mapping);
for (j = 0; j < map_len; j++)
if (mapping[j] == ',')
mapping[j] = '\n';
fd = open(map_file, O_RDWR);
if (fd == -1) {
fprintf(stderr, "ERROR: open %s: %s\n", map_file,
strerror(errno));
exit(EXIT_FAILURE);
}
if (write(fd, mapping, map_len) != map_len) {
fprintf(stderr, "ERROR: write %s: %s\n", map_file,
strerror(errno));
exit(EXIT_FAILURE);
}
close(fd);
}
static int /* Start function for cloned child */
childFunc(void *arg)
{
struct child_args *args = (struct child_args *) arg;
char ch;
/* Wait until the parent has updated the UID and GID mappings.
See the comment in main(). We wait for end of file on a
pipe that will be closed by the parent process once it has
updated the mappings. */
close(args->pipe_fd[1]); /* Close our descriptor for the write
end of the pipe so that we see EOF
when parent closes its descriptor */
if (read(args->pipe_fd[0], &ch, 1) != 0) {
fprintf(stderr,
"Failure in child: read from pipe returned != 0\n");
exit(EXIT_FAILURE);
}
/* Execute a shell command */
printf("About to exec %s\n", args->argv[0]);
execvp(args->argv[0], args->argv);
errExit("execvp");
}
#define STACK_SIZE (1024 * 1024)
static char child_stack[STACK_SIZE]; /* Space for child's stack */
int
main(int argc, char *argv[])
{
int flags, opt, map_zero;
pid_t child_pid;
struct child_args args;
char *uid_map, *gid_map;
const int MAP_BUF_SIZE = 100;
char map_buf[MAP_BUF_SIZE];
char map_path[PATH_MAX];
/* Parse command-line options. The initial '+' character in
the final getopt() argument prevents GNU-style permutation
of command-line options. That's useful, since sometimes
the 'command' to be executed by this program itself
has command-line options. We don't want getopt() to treat
those as options to this program. */
flags = 0;
verbose = 0;
gid_map = NULL;
uid_map = NULL;
map_zero = 0;
while ((opt = getopt(argc, argv, "+imnpuUM:G:zv")) != -1) {
switch (opt) {
case 'i': flags |= CLONE_NEWIPC; break;
case 'm': flags |= CLONE_NEWNS; break;
case 'n': flags |= CLONE_NEWNET; break;
case 'p': flags |= CLONE_NEWPID; break;
case 'u': flags |= CLONE_NEWUTS; break;
case 'v': verbose = 1; break;
case 'z': map_zero = 1; break;
case 'M': uid_map = optarg; break;
case 'G': gid_map = optarg; break;
case 'U': flags |= CLONE_NEWUSER; break;
default: usage(argv[0]);
}
}
/* -M or -G without -U is nonsensical */
if (((uid_map != NULL || gid_map != NULL || map_zero) &&
!(flags & CLONE_NEWUSER)) ||
(map_zero && (uid_map != NULL || gid_map != NULL)))
usage(argv[0]);
args.argv = &argv[optind];
/* We use a pipe to synchronize the parent and child, in order to
ensure that the parent sets the UID and GID maps before the child
calls execve(). This ensures that the child maintains its
capabilities during the execve() in the common case where we
want to map the child's effective user ID to 0 in the new user
namespace. Without this synchronization, the child would lose
its capabilities if it performed an execve() with nonzero
user IDs (see the capabilities(7) man page for details of the
transformation of a process's capabilities during execve()). */
if (pipe(args.pipe_fd) == -1)
errExit("pipe");
/* Create the child in new namespace(s) */
child_pid = clone(childFunc, child_stack + STACK_SIZE,
flags | SIGCHLD, &args);
if (child_pid == -1)
errExit("clone");
/* Parent falls through to here */
if (verbose)
printf("%s: PID of child created by clone() is %ld\n",
argv[0], (long) child_pid);
/* Update the UID and GID maps in the child */
if (uid_map != NULL || map_zero) {
snprintf(map_path, PATH_MAX, "/proc/%ld/uid_map",
(long) child_pid);
if (map_zero) {
snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getuid());
uid_map = map_buf;
}
update_map(uid_map, map_path);
}
if (gid_map != NULL || map_zero) {
snprintf(map_path, PATH_MAX, "/proc/%ld/gid_map",
(long) child_pid);
if (map_zero) {
snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1", (long) getgid());
gid_map = map_buf;
}
update_map(gid_map, map_path);
}
/* Close the write end of the pipe, to signal to the child that we
have updated the UID and GID maps */
close(args.pipe_fd[1]);
if (waitpid(child_pid, NULL, 0) == -1) /* Wait for child */
errExit("waitpid");
if (verbose)
printf("%s: terminating\n", argv[0]);
exit(EXIT_SUCCESS);
}
SEE ALSO
newgidmap(1), newuidmap(1), clone(2), setns(2), unshare(2),
proc(5), subgid(5), subuid(5), credentials(7), capabilities(7),
namespaces(7), pid_namespaces(7)
The kernel source file Documentation/namespaces/resource-con‐
trol.txt.
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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