user_namespaces (7) - Linux Manuals
user_namespaces: overview of Linux user namespaces
NAME
user_namespaces - overview of Linux user namespaces
DESCRIPTION
For an overview of namespaces, see namespaces(7).User namespaces isolate security-related identifiers and attributes, in particular, user IDs and group IDs (see credentials(7)), the root directory, keys (see keyrings(7)), and capabilities (see capabilities(7)). A process's user and group IDs can be different inside and outside a user namespace. In particular, a process can have a normal unprivileged user ID outside a user namespace while at the same time having a user ID of 0 inside the namespace; in other words, the process has full privileges for operations inside the user namespace, but is unprivileged for operations outside the namespace.
Nested namespaces, namespace membership
User namespaces can be nested; that is, each user namespace---except the initial ("root") namespace---has a parent user namespace, and can have zero or more child user namespaces. The parent user namespace is the user namespace of the process that creates the user namespace via a call to unshare(2) or clone(2) with the CLONE_NEWUSER flag.The kernel imposes (since version 3.11) a limit of 32 nested levels of user namespaces. Calls to unshare(2) or clone(2) that would cause this limit to be exceeded fail with the error EUSERS.
Each process is a member of exactly one user namespace. A process created via fork(2) or clone(2) without the CLONE_NEWUSER flag is a member of the same user namespace as its parent. A single-threaded process can join another user namespace with setns(2) if it has the CAP_SYS_ADMIN in that namespace; upon doing so, it gains a full set of capabilities in that namespace.
A call to clone(2) or unshare(2) with the CLONE_NEWUSER flag makes the new child process (for clone(2)) or the caller (for unshare(2)) a member of the new user namespace created by the call.
The NS_GET_PARENT ioctl(2) operation can be used to discover the parental relationship between user namespaces; see ioctl_ns(2).
Capabilities
The child process created by clone(2) with the CLONE_NEWUSER flag starts out with a complete set of capabilities in the new user namespace. Likewise, a process that creates a new user namespace using unshare(2) or joins an existing user namespace using setns(2) gains a full set of capabilities in that namespace. On the other hand, that process has no capabilities in the parent (in the case of clone(2)) or previous (in the case of unshare(2) and setns(2)) user namespace, even if the new namespace is created or joined by the root user (i.e., a process with user ID 0 in the root namespace).Note that a call to execve(2) will cause a process's capabilities to be recalculated in the usual way (see capabilities(7)). Consequently, unless the process has a user ID of 0 within the namespace, or the executable file has a nonempty inheritable capabilities mask, the process will lose all capabilities. See the discussion of user and group ID mappings, below.
A call to clone(2) or unshare(2) using the CLONE_NEWUSER flag or a call to setns(2) that moves the caller into another user namespace sets the "securebits" flags (see capabilities(7)) to their default values (all flags disabled) in the child (for clone(2)) or caller (for unshare(2) or setns(2)). Note that because the caller no longer has capabilities in its original user namespace after a call to setns(2), it is not possible for a process to reset its "securebits" flags while retaining its user namespace membership by using a pair of setns(2) calls to move to another user namespace and then return to its original user namespace.
The rules for determining whether or not a process has a capability in a particular user namespace are as follows:
- 1.
- A process has a capability inside a user namespace if it is a member of that namespace and it has the capability in its effective capability set. A process can gain capabilities in its effective capability set in various ways. For example, it may execute a set-user-ID program or an executable with associated file capabilities. In addition, a process may gain capabilities via the effect of clone(2), unshare(2), or setns(2), as already described.
- 2.
- If a process has a capability in a user namespace, then it has that capability in all child (and further removed descendant) namespaces as well.
- 3.
- When a user namespace is created, the kernel records the effective user ID of the creating process as being the "owner" of the namespace. A process that resides in the parent of the user namespace and whose effective user ID matches the owner of the namespace has all capabilities in the namespace. By virtue of the previous rule, this means that the process has all capabilities in all further removed descendant user namespaces as well. The NS_GET_OWNER_UID ioctl(2) operation can be used to discover the user ID of the owner of the namespace; see ioctl_ns(2).
Effect of capabilities within a user namespace
Having a capability inside a user namespace permits a process to perform operations (that require privilege) only on resources governed by that namespace. In other words, having a capability in a user namespace permits a process to perform privileged operations on resources that are governed by (nonuser) namespaces 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 (i.e., calendar) time (governed by CAP_SYS_TIME), loading a kernel module (governed by CAP_SYS_MODULE), and creating a device (governed by CAP_MKNOD). Only a process with privileges in the initial user namespace can perform such operations.
Holding CAP_SYS_ADMIN within the user namespace that owns a process's mount namespace allows that process to create bind mounts and mount the following types of filesystems:
-
- *
- /proc (since Linux 3.8)
- *
- /sys (since Linux 3.8)
- *
- devpts (since Linux 3.9)
- *
- tmpfs(5) (since Linux 3.9)
- *
- ramfs (since Linux 3.9)
- *
- mqueue (since Linux 3.9)
- *
- bpf (since Linux 4.4)
Holding CAP_SYS_ADMIN within the user namespace that owns a process's cgroup namespace allows (since Linux 4.6) that process to the mount the cgroup version 2 filesystem and cgroup version 1 named hierarchies (i.e., cgroup filesystems mounted with the "none,name=" option).
Holding CAP_SYS_ADMIN within the user namespace that owns a process's PID namespace allows (since Linux 3.8) that process to mount /proc filesystems.
Note however, that mounting block-based filesystems can be done only by a process that holds CAP_SYS_ADMIN in the initial user namespace.
Interaction of user namespaces and other types of namespaces
Starting in Linux 3.8, unprivileged processes can create user 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).
User and group ID mappings: uid_map and gid_map
When a user namespace is created, it starts out without a mapping of user IDs (group IDs) to the parent user namespace. The /proc/[pid]/uid_map and /proc/[pid]/gid_map files (available since Linux 3.5) expose the mappings for user and group IDs inside the user namespace for the process pid. These files can be read to view the mappings in a user namespace and written to (once) to define the mappings.The description in the following paragraphs explains the details for uid_map; gid_map is exactly the same, but each instance of "user ID" is replaced by "group ID".
The uid_map file exposes the mapping of user IDs from the user namespace of the process pid to the user namespace of the process that opened uid_map (but see a qualification to this point below). In other words, processes that are in different user namespaces will potentially see different values when reading from a particular uid_map file, depending on the user ID mappings for the user namespaces of the reading processes.
Each line in the uid_map file specifies a 1-to-1 mapping of a range of contiguous user IDs between two user namespaces. (When a user namespace is first created, this file is empty.) The specification in each line takes the form of three numbers delimited by white space. The first two numbers specify the starting user ID in each of the two user namespaces. The third number specifies the length of the mapped range. In detail, the fields are interpreted as follows:
- (1)
- The start of the range of user IDs in the user namespace of the process pid.
- (2)
-
The start of the range of user
IDs to which the user IDs specified by field one map.
How field two is interpreted depends on whether the process that opened
uid_map
and the process
pid
are in the same user namespace, as follows:
-
- a)
- If the two processes are in different user namespaces: field two is the start of a range of user IDs in the user namespace of the process that opened uid_map.
- b)
- If the two processes are in the same user namespace: field two is the start of the range of user IDs in the parent user namespace of the process pid. This case enables the opener of uid_map (the common case here is opening /proc/self/uid_map) to see the mapping of user IDs into the user namespace of the process that created this user namespace.
-
- (3)
- The length of the range of user IDs that is mapped between the two user namespaces.
System calls that return user IDs (group IDs)---for example, getuid(2), getgid(2), and the credential fields in the structure returned by stat(2)---return the user ID (group ID) mapped into the caller's user namespace.
When a process accesses a file, its user and group IDs are mapped into the initial user namespace for the purpose of permission checking and assigning IDs when creating a file. When a process retrieves file user and group IDs via stat(2), the IDs are mapped in the opposite direction, to produce values relative to the process user and group ID mappings.
The initial user namespace has no parent namespace, but, for consistency, the kernel provides dummy user and group ID mapping files for this namespace. Looking at the uid_map file (gid_map is the same) from a shell in the initial namespace shows:
$ cat /proc/$$/uid_map
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:
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:
Writes that violate the above rules fail with the error
EPERM.
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.
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
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.
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.
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
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 'ha[UG]id'
Uid:
/* A simple error-handling function: print an error message based
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
struct child_args {
static int verbose;
static void
usage(char *pname)
{
Defining user and group ID mappings: writing to uid_map and gid_map
Interaction with system calls that change process UIDs or GIDs
In a user namespace where the
uid_map
file has not been written, the system calls that change user IDs will fail.
Similarly, if the
gid_map
file has not been written, the system calls that change group IDs will fail.
After the
uid_map
and
gid_map
files have been written, only the mapped values may be used in
system calls that change user and group IDs.
The /proc/[pid]/setgroups file
The
/proc/[pid]/setgroups
file displays the string
"allow"
if processes in the user namespace that contains the process
pid
are permitted to employ the
setgroups(2)
system call; it displays
"deny"
if
setgroups(2)
is not permitted in that user namespace.
Note that regardless of the value in the
/proc/[pid]/setgroups
file (and regardless of the process's capabilities), calls to
setgroups(2)
are also not permitted if
/proc/[pid]/gid_map
has not yet been set.
Unmapped user and group IDs
Accessing files
Operation of file-related capabilities
Set-user-ID and set-group-ID programs
Miscellaneous
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 subsystem is configured into the kernel,
it is not possible to configure user namespaces support.
EXAMPLES
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.
Gid:
bash$ cat /proc/$$/status | egrep 'haCap(Prm|Inh|Eff)'
CapInh: 0000000000000000
CapPrm: 0000001fffffffff
CapEff: 0000001fffffffff
Program source
/* userns_child_exec.c