user_namespaces
Section: Environments, Tables, and Troff Macros (7)
Updated: 202-0-08
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NAME
user_namespaces - overview of Linux user namespaces
DESCRIPTION
For an overview of namespaces, see
namespaces(7).
User namespaces isolate securit-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
[em]except the initial ("root") namespace[em]
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 Linux 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 singl-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_nsfs(2).
A task that changes one of its effective IDs
will have its dumpability reset to the value in
/proc/sys/fs/suid_dumpable.
This may affect the ownership of proc files of child processes
and may thus cause the parent to
lack the permissions
to write to mapping files of child processes running in a new user namespace.
In such cases,
making the parent process dumpable,
using
PR_SET_DUMPABLE
in a call to
prctl(2),
before creating a child process in a new user namespace
may rectify this problem.
See
prctl(2)
and
proc(5)
for details on how ownership is affected.
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:
- [bu]
-
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 se-use-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.
- [bu]
-
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.
- [bu]
-
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_nsfs(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:
-
- [bu]
-
/proc
(since Linux 3.8)
- [bu]
-
/sys
(since Linux 3.8)
- [bu]
-
devpts
(since Linux 3.9)
- [bu]
-
tmpfs(5)
(since Linux 3.9)
- [bu]
-
ramfs
(since Linux 3.9)
- [bu]
-
mqueue
(since Linux 3.9)
- [bu]
-
bpf
(since Linux 4.4)
- [bu]
-
overlayfs
(since Linux 5.11)
Holding
CAP_SYS_ADMIN
within the user namespace that owns a process's cgroup namespace
allows (since Linux 4.6)
that process to mount
the cgroup version 2 filesystem
and cgroup version 1 named hierarchies
(i.e., cgroup filesystems mounted with the
[dq]none,name=[dq]
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.
However,
mounting bloc-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
Since 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_nsfs(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 -t-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 size 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 size of the range of user IDs that is mapped between the two
user namespaces.
System calls that return user IDs (group IDs)[em]for example,
getuid(2),
getgid(2),
and the credential fields in the structure returned by
stat(2)[em]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 size of the range is the largest 3-bit unsigned integer.
This leaves 4294967295 (the 3-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.
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 validity rules:
- [bu]
-
The three fields must be valid numbers,
and the last field must be greater than 0.
- [bu]
-
Lines are terminated by newline characters.
- [bu]
-
There is a limit on the number of lines in the file.
Up to Linux 4.14,
this limit was (arbitrarily) set at 5 lines.
Since Linux 4.16,
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).
- [bu]
-
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.
Since Linux 3.9,
this limitation has been removed,
allowing any valid set of nonoverlapping maps.
- [bu]
-
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 permission requirements must be met:
- [bu]
-
The writing process must have the
CAP_SETUID
(CAP_SETGID)
capability in the user namespace of the process
pid.
- [bu]
-
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.
- [bu]
-
The mapped user IDs (group IDs) must in turn
have a mapping
in the parent user namespace.
- [bu]
-
If updating
/proc/pid/uid_map
to create a mapping that maps UID 0 in the parent namespace,
then one of the following must be true:
-
- (a)
-
if writing process is in the parent user namespace,
then it must have the
CAP_SETFCAP
capability in that user namespace;
or
- (b)
-
if the writing process is in the child user namespace,
then the process that created the user namespace must have had the
CAP_SETFCAP
capability when the namespace was created.
-
This rule has been in place since
Linux 5.12.
It eliminates an earlier security bug whereby
a UID 0 process that lacks the
CAP_SETFCAP
capability,
which is needed to create a binary with namespaced file capabilities
(as described in
capabilities(7)),
could nevertheless create such a binary,
by the following steps:
-
- (1)
-
Create a new user namespace with the identity mapping
(i.e., UID 0 in the new user namespace maps to UID 0 in the parent namespace),
so that UID 0 in both namespaces is equivalent to the same root user ID.
- (2)
-
Since the child process has the
CAP_SETFCAP
capability,
it could create a binary with namespaced file capabilities
that would then be effective in the parent user namespace
(because the root user IDs are the same in the two namespaces).
- [bu]
-
One of the following two cases applies:
-
- (a)
-
Either
the writing process has the
CAP_SETUID
(CAP_SETGID)
capability in the
parent
user namespace.
-
- [bu]
-
No further restrictions apply:
the process can make mappings to arbitrary user IDs (group IDs)
in the parent user namespace.
- (b)
-
Or
otherwise all of the following restrictions apply:
-
- [bu]
-
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.
- [bu]
-
The writing process must have the same effective user ID
as the process that created the user namespace.
- [bu]
-
In the case of
gid_map,
use of the
setgroups(2)
system call must first be denied by writing
[dq]deny[dq]
to the
/proc/pid/setgroups
file (see below)
before writing to
gid_map.
Writes that violate the above rules fail with the error
EPERM.
Project ID mappings: projid_map
Similarly to user and group ID mappings,
it is possible to create project ID mappings for a user namespace.
(Project IDs are used for disk quotas;
see
setquota(8)
and
quotactl(2).)
Project ID mappings are defined by writing to the
/proc/pid
/projid_map
file
(present since Linux 3.7).
The validity rules for writing to the
/proc/pid
/projid_map
file are as for writing to the
uid_map
file;
violation of these rules causes
write(2)
to fail with the error
EINVAL.
The permission rules for writing to the
/proc/pid
/projid_map
file are as follows:
- [bu]
-
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.
- [bu]
-
The mapped project IDs must in turn
have a mapping
in the parent user namespace.
Violation of these rules causes
write(2)
to fail with the error
EPERM.
Interaction with system calls that change process UIDs or GIDs
In a user namespace where the
uid_map
file has not been written,
the system calls that change user IDs will fail.
Similarly,
if the
gid_map
file has not been written,
the system calls that change group IDs will fail.
After the
uid_map
and
gid_map
files have been 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
[dq]
deny[dq]
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 I]/proc/]pidI]/setgroups] file
The
/proc/pid
/setgroups
file displays the string
[dq]
allow[dq]
if processes in the user namespace that contains the process
pid
are permitted to employ the
setgroups(2)
system call;
it displays
[dq]
deny[dq]
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
[dq]
allow[dq]
or
[dq]
deny[dq]
to this file
before
writing a group ID mapping
for this user namespace to the file
/proc/pid
/gid_map.
Writing the string
[dq]
deny[dq]
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
[dq]
allow[dq].
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
[dq]
deny[dq]
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
[dq]
deny[dq],
then the
setgroups(2)
system call can't subsequently be reenabled
(by writing
[dq]
allow[dq]
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).
Unmapped user and group IDs
There are various places where an unmapped user ID (group ID)
may be exposed to user space.
For example,
the first process in a new user namespace may call
getuid(2)
before a user ID mapping has been defined for the namespace.
In most such cases,
an unmapped user ID is converted
to the overflow user ID (group ID);
the default value for the overflow user ID (group ID) is 65534.
See the descriptions of
/proc/sys/kernel/overflowuid
and
/proc/sys/kernel/overflowgid
in
proc(5).
The cases where unmapped IDs are mapped in this fashion include
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).
Accessing files
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 true of other objects that
employ the credentials plus permissions mask accessibility model,
such as System V IPC objects.
Operation of fil-related capabilities
Certain capabilities allow a process to
bypass various kerne-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:
- [bu]
-
the process has the relevant effective capability in its user namespace;
and
- [bu]
-
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).
Se-use-ID and se-grou-ID programs
When a process inside a user namespace
executes a se-use-ID (se-grou-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 se-use-ID (se-grou-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 se-use-ID or se-grou-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.
STANDARDS
Linux.
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 se-use-I-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.
Global root
The term "global root" is sometimes used as a shorthand for
user ID 0 in the initial user namespace.
Availability
Use of user namespaces requires a kernel that is configured with the
CONFIG_USER_NS
option.
User namespaces require support
in a range of subsystems across the kernel.
When an unsupported subsystem is configured into the kernel,
it is not possible to configure user namespaces support.
As at Linux 3.8,
most relevant subsystems supported user namespaces,
but a number of filesystems did not have the infrastructure
needed to map user and group IDs between user namespaces.
Linux 3.9 added the required infrastructure support
for many of the remaining unsupported filesystems
(Plan 9 (9P), Andrew File System (AFS), Ceph, CIFS, CODA, NFS, and OCFS2).
Linux 3.12 added support for the last of the unsupported major filesystems,
XFS.
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 comman-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 ru-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 [aq]0 1000 1[aq] -G [aq]0 1000 1[aq] 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/$$/status | egrep [aq][ha][UG]id[aq];
Uid:
0
0
0
0
Gid:
0
0
0
0
bash$
cat /proc/$$/status | egrep [aq][ha]Cap(Prm|Inh|Eff)[aq];
CapInh:
0000000000000000
CapPrm:
0000001fffffffff
CapEff:
0000001fffffffff
Program source
/* userns_child_exec.c
Licensed under GNU General Public License v2 or later
Create a child process that executes a shell command in new
namespace(s); allow UID and GID mappings to be specified when
creating a user namespace.
*/
#define _GNU_SOURCE
#include <
err.h>
#include <
sched.h>
#include <
unistd.h>
#include <
stdint.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>
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...][rs]n[rs]n", pname);
fprintf(stderr, "Create a child process that executes a shell "
"command in a new user namespace,[rs]n"
"and possibly also other new namespace(s).[rs]n[rs]n");
fprintf(stderr, "Options can be:[rs]n[rs]n");
#define fpe(str) fprintf(stderr, " %s", str);
fpe("-i New IPC namespace[rs]n");
fpe("-m New mount namespace[rs]n");
fpe("-n New network namespace[rs]n");
fpe("-p New PID namespace[rs]n");
fpe("-u New UTS namespace[rs]n");
fpe("-U New user namespace[rs]n");
fpe("-M uid_map Specify UID map for user namespace[rs]n");
fpe("-G gid_map Specify GID map for user namespace[rs]n");
fpe("-z Map user[aq]s UID and GID to 0 in user namespace[rs]n");
fpe(" (equivalent to: -M [aq]0 <uid> 1[aq] -G [aq]0 <gid> 1[aq])[rs]n");
fpe("-v Display verbose messages[rs]n");
fpe("[rs]n");
fpe("If -z, -M, or -G is specified, -U is required.[rs]n");
fpe("It is not permitted to specify both -z and either -M or -G.[rs]n");
fpe("[rs]n");
fpe("Map strings for -M and -G consist of records of the form:[rs]n");
fpe("[rs]n");
fpe(" ID-inside-ns ID-outside-ns size[rs]n");
fpe("[rs]n");
fpe("A map string can contain multiple records, separated"
" by commas;[rs]n");
fpe("the commas are replaced by newlines before writing"
" to map files.[rs]n");
exit(EXIT_FAILURE);
}
/* Update the mapping file [aq]map_file[aq], with the value provided in
[aq]mapping[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 size
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;
size_t map_len; /* Length of [aq]mapping[aq] */
/* Replace commas in mapping string with newlines. */
map_len = strlen(mapping);
for (size_t j = 0; j < map_len; j++)
if (mapping[j] == [aq],[aq])
mapping[j] = [aq][rs]n[aq];
fd = open(map_file, O_RDWR);
if (fd == -1) {
fprintf(stderr, "ERROR: open %s: %s[rs]n", map_file,
strerror(errno));
exit(EXIT_FAILURE);
}
if (write(fd, mapping, map_len) != map_len) {
fprintf(stderr, "ERROR: write %s: %s[rs]n", map_file,
strerror(errno));
exit(EXIT_FAILURE);
}
close(fd);
}
/* Linux 3.19 made a change in the handling of
setgroups(2) and
the [aq]gid_map[aq] file to address a security issue. The issue
allowed *unprivileged* users to employ user namespaces in
order to drop groups. The upshot of the 3.19 changes is that
in order to update the [aq]gid_maps[aq] 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/%jd/setgroups",
(intmax_t) child_pid);
fd = open(setgroups_path, O_RDWR);
if (fd == -1) {
/* We may be on a system that doesn[aq]t support
/proc/PID/setgroups. In that case, the file won[aq]t exist,
and the system won[aq]t impose the restrictions that Linux 3.19
added. That[aq]s fine: we don[aq]t need to do anything in order
to permit [aq]gid_map[aq] 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[rs]n", setgroups_path,
strerror(errno));
return;
}
if (write(fd, str, strlen(str)) == -1)
fprintf(stderr, "ERROR: write %s: %s[rs]n", setgroups_path,
strerror(errno));
close(fd);
}
static int /* Start function for cloned child */
childFunc(void *arg)
{
struct child_args *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[rs]n");
exit(EXIT_FAILURE);
}
close(args->pipe_fd[0]);
/* Execute a shell command. */
printf("About to exec %s[rs]n", args->argv[0]);
execvp(args->argv[0], args->argv);
err(EXIT_FAILURE, "execvp");
}
#define STACK_SIZE (1024 * 1024)
static char child_stack[STACK_SIZE]; /* Space for child[aq]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 [aq]+[aq] character in
the final getopt() argument prevents GNU-style permutation
of command-line options. That[aq]s useful, since sometimes
the [aq]command[aq] to be executed by this program itself
has command-line options. We don[aq]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 [aq]i[aq]: flags |= CLONE_NEWIPC; break;
case [aq]m[aq]: flags |= CLONE_NEWNS; break;
case [aq]n[aq]: flags |= CLONE_NEWNET; break;
case [aq]p[aq]: flags |= CLONE_NEWPID; break;
case [aq]u[aq]: flags |= CLONE_NEWUTS; break;
case [aq]v[aq]: verbose = 1; break;
case [aq]z[aq]: map_zero = 1; break;
case [aq]M[aq]: uid_map = optarg; break;
case [aq]G[aq]: gid_map = optarg; break;
case [aq]U[aq]: 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[aq]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[aq]s capabilities during execve()). */
if (pipe(args.pipe_fd) == -1)
err(EXIT_FAILURE, "pipe");
/* Create the child in new namespace(s). */
child_pid = clone(childFunc, child_stack + STACK_SIZE,
flags | SIGCHLD, &args);
if (child_pid == -1)
err(EXIT_FAILURE, "clone");
/* Parent falls through to here. */
if (verbose)
printf("%s: PID of child created by clone() is %jd[rs]n",
argv[0], (intmax_t) child_pid);
/* Update the UID and GID maps in the child. */
if (uid_map != NULL || map_zero) {
snprintf(map_path, PATH_MAX, "/proc/%jd/uid_map",
(intmax_t) child_pid);
if (map_zero) {
snprintf(map_buf, MAP_BUF_SIZE, "0 %jd 1",
(intmax_t) 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/%jd/gid_map",
(intmax_t) child_pid);
if (map_zero) {
snprintf(map_buf, MAP_BUF_SIZE, "0 %ld 1",
(intmax_t) 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 */
err(EXIT_FAILURE, "waitpid");
if (verbose)
printf("%s: terminating[rs]n", argv[0]);
exit(EXIT_SUCCESS);
}
SEE ALSO
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/admin-guide/namespaces/resource-control.rst.
Index
- NAME
-
- DESCRIPTION
-
- Nested namespaces, namespace membership
-
- Capabilities
-
- Effect of capabilities within a user namespace
-
- Interaction of user namespaces and other types of namespaces
-
- User and group ID mappings: uid_map and gid_map
-
- Defining user and group ID mappings: writing to uid_map and gid_map
-
- Project ID mappings: projid_map
-
- Interaction with system calls that change process UIDs or GIDs
-
- The I]/proc/]pidI]/setgroups] file
-
- Unmapped user and group IDs
-
- Accessing files
-
- Operation of file-related capabilities
-
- Set-user-ID and set-group-ID programs
-
- Miscellaneous
-
- STANDARDS
-
- NOTES
-
- Global root
-
- Availability
-
- EXAMPLES
-
- Program source
-
- SEE ALSO
-
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