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.
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).
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).
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 owned by (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 that owns a process's
mount namespace allows that process to create bind mounts and mount the
following types of filesystems:
- *
- /proc (since Linux 3.8)
- *
- /sys (since Linux 3.8)
- *
- devpts (since Linux 3.9)
- *
- tmpfs(5) (since Linux 3.9)
- *
- ramfs (since Linux 3.9)
- *
- mqueue (since Linux 3.9)
- *
- bpf (since Linux 4.4)
Holding
CAP_SYS_ADMIN within the user namespace that owns a process's
cgroup namespace allows (since Linux 4.6) that process to the mount the cgroup
version 2 filesystem and cgroup version 1 named hierarchies (i.e., cgroup
filesystems mounted with the
"none,name=" option).
Holding
CAP_SYS_ADMIN within the user namespace that owns a process's PID
namespace allows (since Linux 3.8) that process to mount
/proc
filesystems.
Note however, that mounting block-based filesystems can be done only by a
process that holds
CAP_SYS_ADMIN in the initial user namespace.
Starting in Linux 3.8, unprivileged processes can create user namespaces, and
the other types of namespaces can be created with just the
CAP_SYS_ADMIN capability in the caller's user namespace.
When a nonuser namespace is created, it is owned by the user namespace in which
the creating process was a member at the time of the creation of the
namespace. Privileged operations on resources governed by the nonuser
namespace require that the process has the necessary capabilities in the user
namespace that owns the nonuser namespace.
If
CLONE_NEWUSER is specified along with other
CLONE_NEW* flags in
a single
clone(2) or
unshare(2) call, the user namespace is
guaranteed to be created first, giving the child (
clone(2)) or caller
(
unshare(2)) privileges over the remaining namespaces created by the
call. Thus, it is possible for an unprivileged caller to specify this
combination of flags.
When a new namespace (other than a user namespace) is created via
clone(2) or
unshare(2), the kernel records the user namespace of
the creating process as the owner of the new namespace. (This association
can't be changed.) When a process in the new namespace subsequently performs
privileged operations that operate on global resources 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 owns 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 that owns a nonuser namespace; see
ioctl_ns(2).
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 interfaces (e.g.,
setreuid(2)) as a way to specify "no user ID". Leaving
(uid_t) -1 unmapped and unusable guarantees that there will be
no confusion when using these interfaces.
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 implementation (Linux
3.8), this requirement was satisfied by a simplistic implementation that
imposed the further requirement that the values in both field 1 and field
2 of successive lines must be in ascending numerical order, which
prevented some otherwise valid maps from being created. Linux 3.9 and
later fix this limitation, allowing any valid set of nonoverlapping
maps.
- *
- At least one line must be written to the file.
Writes that violate the above rules fail with the error
EINVAL.
In order for a process to write to the
/proc/[pid]/uid_map
(
/proc/[pid]/gid_map) file, all of the following requirements must be
met:
- 1.
- The writing process must have the CAP_SETUID (CAP_SETGID)
capability in the user 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 namespace.
- *
- Or otherwise all of the following restrictions apply:
- +
- The data written to uid_map (gid_map) must consist of a
single 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.
In a user namespace where the
uid_map file has not been written, the
system calls that change user IDs will fail. Similarly, if the
gid_map
file has not been written, the system calls that change group IDs will fail.
After the
uid_map and
gid_map files have been written, only the
mapped values may be used in system calls that change user and group IDs.
For user IDs, the relevant system calls include
setuid(2),
setfsuid(2),
setreuid(2), and
setresuid(2). For group
IDs, the relevant system calls include
setgid(2),
setfsgid(2),
setregid(2),
setresgid(2), and
setgroups(2).
Writing "
deny" to the
/proc/[pid]/setgroups file before
writing to
/proc/[pid]/gid_map will permanently disable
setgroups(2) in a user 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 displays the string
"
allow" if processes in the user namespace that contains the
process
pid are permitted to employ the
setgroups(2) system
call; it displays "
deny" if
setgroups(2) is not
permitted in that user namespace. Note that regardless of the value in the
/proc/[pid]/setgroups file (and regardless of the process's
capabilities), calls to
setgroups(2) are also not permitted if
/proc/[pid]/gid_map has not yet been set.
A privileged process (one with the
CAP_SYS_ADMIN capability in the
namespace) may write either of the strings "
allow" or
"
deny" to this file
before writing a group ID mapping
for this user namespace to the file
/proc/[pid]/gid_map. Writing the
string "
deny" prevents any process in the user namespace from
employing
setgroups(2).
The essence of the restrictions described in the preceding paragraph is that it
is permitted to write to
/proc/[pid]/setgroups only so long as calling
setgroups(2) is disallowed because
/proc/[pid]/gid_map has not
been set. This ensures that a process cannot transition from a state where
setgroups(2) is allowed to a state where
setgroups(2) is denied;
a process can transition only from
setgroups(2) being disallowed to
setgroups(2) being allowed.
The default value of this file in the initial user namespace is
"
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 possible
to disallow
setgroups(2) by writing "
deny" to
/proc/[pid]/setgroups (the write fails with the error
EPERM).
A child user namespace inherits the
/proc/[pid]/setgroups setting from
its parent.
If the
setgroups file has the value "
deny", then the
setgroups(2) 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
backported 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 namespaces,
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).
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 system calls
that return user IDs (
getuid(2),
getgid(2), and similar),
credentials passed over a UNIX domain socket, credentials returned by
stat(2),
waitid(2), and the System V IPC "ctl"
IPC_STAT 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
integer).
In order to determine permissions when an unprivileged process accesses a file,
the process credentials (UID, GID) and the file credentials are in effect
mapped back to what they would be in the initial user namespace and then
compared to determine the permissions that the process has on the file. The
same is also of other objects that employ the credentials plus permissions
mask accessibility model, such as System V IPC objects
Certain capabilities allow a process to bypass various kernel-enforced
restrictions when performing operations on files owned by other users or
groups. These capabilities are:
CAP_CHOWN,
CAP_DAC_OVERRIDE,
CAP_DAC_READ_SEARCH,
CAP_FOWNER, and
CAP_FSETID.
Within a user namespace, these capabilities allow a process to bypass the rules
if the process has the relevant capability over the file, meaning that:
- *
- the process has the relevant effective capability in its user namespace;
and
- *
- the file's user ID and group ID both have valid mappings in the user
namespace.
The
CAP_FOWNER capability is treated somewhat exceptionally: it allows a
process to bypass the corresponding rules so long as at least the file's user
ID has a mapping in the user namespace (i.e., the file's group ID does not
need to have a valid mapping).
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).)
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.
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.
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
for 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 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
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
/* 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.