.\" Copyright (c) 2013, 2014 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" and Copyright (c) 2012, 2014 by Eric W. Biederman <ebiederm@xmission.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\"
.TH user_namespaces 7 2024-06-15 "Linux man-pages (unreleased)"
.SH NAME
user_namespaces \- overview of Linux user namespaces
.SH DESCRIPTION
For an overview of namespaces, see
.BR namespaces (7).
.P
User namespaces isolate security-related identifiers and attributes,
in particular,
user IDs and group IDs (see
.BR credentials (7)),
the root directory,
keys (see
.BR keyrings (7)),
.\" FIXME: This page says very little about the interaction
.\" of user namespaces and keys. Add something on this topic.
and capabilities (see
.BR capabilities (7)).
A process's user and group IDs can be different
inside and outside a user namespace.
In particular,
a process can have a normal unprivileged user ID outside a user namespace
while at the same time having a user ID of 0 inside the namespace;
in other words,
the process has full privileges for operations inside the user namespace,
but is unprivileged for operations outside the namespace.
.\"
.\" ============================================================
.\"
.SS 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
.BR unshare (2)
or
.BR clone (2)
with the
.B CLONE_NEWUSER
flag.
.P
The kernel imposes (since Linux 3.11) a limit of 32 nested levels of
.\" commit 8742f229b635bf1c1c84a3dfe5e47c814c20b5c8
user namespaces.
.\" FIXME Explain the rationale for this limit. (What is the rationale?)
Calls to
.BR unshare (2)
or
.BR clone (2)
that would cause this limit to be exceeded fail with the error
.BR EUSERS .
.P
Each process is a member of exactly one user namespace.
A process created via
.BR fork (2)
or
.BR clone (2)
without the
.B CLONE_NEWUSER
flag is a member of the same user namespace as its parent.
A single-threaded process can join another user namespace with
.BR setns (2)
if it has the
.B CAP_SYS_ADMIN
in that namespace;
upon doing so, it gains a full set of capabilities in that namespace.
.P
A call to
.BR clone (2)
or
.BR unshare (2)
with the
.B CLONE_NEWUSER
flag makes the new child process (for
.BR clone (2))
or the caller (for
.BR unshare (2))
a member of the new user namespace created by the call.
.P
The
.B NS_GET_PARENT
.BR ioctl (2)
operation can be used to discover the parental relationship
between user namespaces; see
.BR ioctl_nsfs (2).
.P
A task that changes one of its effective IDs
will have its dumpability reset to the value in
.IR /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
.B PR_SET_DUMPABLE
in a call to
.BR prctl (2),
before creating a child process in a new user namespace
may rectify this problem.
See
.BR prctl (2)
and
.BR proc (5)
for details on how ownership is affected.
.\"
.\" ============================================================
.\"
.SS Capabilities
The child process created by
.BR clone (2)
with the
.B CLONE_NEWUSER
flag starts out with a complete set
of capabilities in the new user namespace.
Likewise, a process that creates a new user namespace using
.BR unshare (2)
or joins an existing user namespace using
.BR setns (2)
gains a full set of capabilities in that namespace.
On the other hand,
that process has no capabilities in the parent (in the case of
.BR clone (2))
or previous (in the case of
.BR unshare (2)
and
.BR setns (2))
user namespace,
even if the new namespace is created or joined by the root user
(i.e., a process with user ID 0 in the root namespace).
.P
Note that a call to
.BR execve (2)
will cause a process's capabilities to be recalculated in the usual way (see
.BR 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.
.P
A call to
.BR clone (2)
or
.BR unshare (2)
using the
.B CLONE_NEWUSER
flag
or a call to
.BR setns (2)
that moves the caller into another user namespace
sets the "securebits" flags
(see
.BR capabilities (7))
to their default values (all flags disabled) in the child (for
.BR clone (2))
or caller (for
.BR unshare (2)
or
.BR setns (2)).
Note that because the caller no longer has capabilities
in its original user namespace after a call to
.BR setns (2),
it is not possible for a process to reset its "securebits" flags while
retaining its user namespace membership by using a pair of
.BR setns (2)
calls to move to another user namespace and then return to
its original user namespace.
.P
The rules for determining whether or not a process has a capability
in a particular user namespace are as follows:
.IP \[bu] 3
A process has a capability inside a user namespace
if it is a member of that namespace and
it has the capability in its effective capability set.
A process can gain capabilities in its effective capability
set in various ways.
For example, it may execute a set-user-ID program or an
executable with associated file capabilities.
In addition,
a process may gain capabilities via the effect of
.BR clone (2),
.BR unshare (2),
or
.BR setns (2),
as already described.
.\" In the 3.8 sources, see security/commoncap.c::cap_capable():
.IP \[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.
.IP \[bu]
.\" * The owner of the user namespace in the parent of the
.\" * user namespace has all caps.
When a user namespace is created, the kernel records the effective
user ID of the creating process as being the "owner" of the namespace.
.\" (and likewise associates the effective group ID of the creating process
.\" with the namespace).
A process that resides
in the parent of the user namespace
.\" See kernel commit 520d9eabce18edfef76a60b7b839d54facafe1f9 for a fix
.\" on this point
and whose effective user ID matches the owner of the namespace
has all capabilities in the namespace.
.\"     This includes the case where the process executes a set-user-ID
.\"     program that confers the effective UID of the creator of the namespace.
By virtue of the previous rule,
this means that the process has all capabilities in all
further removed descendant user namespaces as well.
The
.B NS_GET_OWNER_UID
.BR ioctl (2)
operation can be used to discover the user ID of the owner of the namespace;
see
.BR ioctl_nsfs (2).
.\"
.\" ============================================================
.\"
.SS 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).
.P
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
.BR CAP_SYS_TIME ),
loading a kernel module (governed by
.BR CAP_SYS_MODULE ),
and creating a device (governed by
.BR CAP_MKNOD ).
Only a process with privileges in the
.I initial
user namespace can perform such operations.
.P
Holding
.B 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:
.\" fs_flags = FS_USERNS_MOUNT in kernel sources
.P
.RS 4
.PD 0
.IP \[bu] 3
.I /proc
(since Linux 3.8)
.IP \[bu]
.I /sys
(since Linux 3.8)
.IP \[bu]
.I devpts
(since Linux 3.9)
.IP \[bu]
.BR tmpfs (5)
(since Linux 3.9)
.IP \[bu]
.I ramfs
(since Linux 3.9)
.IP \[bu]
.I mqueue
(since Linux 3.9)
.IP \[bu]
.I bpf
.\" commit b2197755b2633e164a439682fb05a9b5ea48f706
(since Linux 4.4)
.IP \[bu]
.I overlayfs
.\" commit 92dbc9dedccb9759c7f9f2f0ae6242396376988f
.\" commit 4cb2c00c43b3fe88b32f29df4f76da1b92c33224
(since Linux 5.11)
.PD
.RE
.P
Holding
.B 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
.I \[dq]none,name=\[dq]
option).
.P
Holding
.B CAP_SYS_ADMIN
within the user namespace that owns a process's PID namespace
allows (since Linux 3.8)
that process to mount
.I /proc
filesystems.
.P
Note, however, that mounting block-based filesystems can be done
only by a process that holds
.B CAP_SYS_ADMIN
in the initial user namespace.
.\"
.\" ============================================================
.\"
.SS 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
.B CAP_SYS_ADMIN
capability in the caller's user namespace.
.P
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.
.P
If
.B CLONE_NEWUSER
is specified along with other
.B CLONE_NEW*
flags in a single
.BR clone (2)
or
.BR unshare (2)
call, the user namespace is guaranteed to be created first,
giving the child
.RB ( clone (2))
or caller
.RB ( unshare (2))
privileges over the remaining namespaces created by the call.
Thus, it is possible for an unprivileged caller to specify this combination
of flags.
.P
When a new namespace (other than a user namespace) is created via
.BR clone (2)
or
.BR 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
.RB ( 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
.RB ( CAP_SYS_ADMIN )
in that user namespace.
.P
The
.B NS_GET_USERNS
.BR ioctl (2)
operation can be used to discover the user namespace
that owns a nonuser namespace; see
.BR ioctl_nsfs (2).
.\"
.\" ============================================================
.\"
.SS User and group ID mappings: uid_map and gid_map
When a user namespace is created,
it starts out without a mapping of user IDs (group IDs)
to the parent user namespace.
The
.IR /proc/ pid /uid_map
and
.IR /proc/ pid /gid_map
files (available since Linux 3.5)
.\" commit 22d917d80e842829d0ca0a561967d728eb1d6303
expose the mappings for user and group IDs
inside the user namespace for the process
.IR pid .
These files can be read to view the mappings in a user namespace and
written to (once) to define the mappings.
.P
The description in the following paragraphs explains the details for
.IR uid_map ;
.I gid_map
is exactly the same,
but each instance of "user ID" is replaced by "group ID".
.P
The
.I uid_map
file exposes the mapping of user IDs from the user namespace
of the process
.I pid
to the user namespace of the process that opened
.I uid_map
(but see a qualification to this point below).
In other words, processes that are in different user namespaces
will potentially see different values when reading from a particular
.I uid_map
file, depending on the user ID mappings for the user namespaces
of the reading processes.
.P
Each line in the
.I uid_map
file specifies a 1-to-1 mapping of a range of contiguous
user IDs between two user namespaces.
(When a user namespace is first created, this file is empty.)
The specification in each line takes the form of
three numbers delimited by white space.
The first two numbers specify the starting user ID in
each of the two user namespaces.
The third number specifies the length of the mapped range.
In detail, the fields are interpreted as follows:
.IP (1) 5
The start of the range of user IDs in
the user namespace of the process
.IR pid .
.IP (2)
The start of the range of user
IDs to which the user IDs specified by field one map.
How field two is interpreted depends on whether the process that opened
.I uid_map
and the process
.I pid
are in the same user namespace, as follows:
.RS
.IP (a) 5
If the two processes are in different user namespaces:
field two is the start of a range of
user IDs in the user namespace of the process that opened
.IR uid_map .
.IP (b)
If the two processes are in the same user namespace:
field two is the start of the range of
user IDs in the parent user namespace of the process
.IR pid .
This case enables the opener of
.I uid_map
(the common case here is opening
.IR /proc/self/uid_map )
to see the mapping of user IDs into the user namespace of the process
that created this user namespace.
.RE
.IP (3)
The length of the range of user IDs that is mapped between the two
user namespaces.
.P
System calls that return user IDs (group IDs)\[em]for example,
.BR getuid (2),
.BR getgid (2),
and the credential fields in the structure returned by
.BR stat (2)\[em]return
the user ID (group ID) mapped into the caller's user namespace.
.P
When a process accesses a file, its user and group IDs
are mapped into the initial user namespace for the purpose of permission
checking and assigning IDs when creating a file.
When a process retrieves file user and group IDs via
.BR stat (2),
the IDs are mapped in the opposite direction,
to produce values relative to the process user and group ID mappings.
.P
The initial user namespace has no parent namespace,
but, for consistency, the kernel provides dummy user and group
ID mapping files for this namespace.
Looking at the
.I uid_map
file
.RI ( gid_map
is the same) from a shell in the initial namespace shows:
.P
.in +4n
.EX
$ \fBcat /proc/$$/uid_map\fP
         0          0 4294967295
.EE
.in
.P
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:
.I (uid_t)\~\-1
is used in several interfaces (e.g.,
.BR setreuid (2))
as a way to specify "no user ID".
Leaving
.I (uid_t)\~\-1
unmapped and unusable guarantees that there will be no
confusion when using these interfaces.
.\"
.\" ============================================================
.\"
.SS Defining user and group ID mappings: writing to uid_map and gid_map
After the creation of a new user namespace, the
.I uid_map
file of
.I one
of the processes in the namespace may be written to
.I once
to define the mapping of user IDs in the new user namespace.
An attempt to write more than once to a
.I uid_map
file in a user namespace fails with the error
.BR EPERM .
Similar rules apply for
.I gid_map
files.
.P
The lines written to
.I uid_map
.RI ( gid_map )
must conform to the following validity rules:
.IP \[bu] 3
The three fields must be valid numbers,
and the last field must be greater than 0.
.IP \[bu]
Lines are terminated by newline characters.
.IP \[bu]
There is a limit on the number of lines in the file.
In Linux 4.14 and earlier, this limit was (arbitrarily)
.\" 5*12-byte records could fit in a 64B cache line
set at 5 lines.
Since Linux 4.15,
.\" commit 6397fac4915ab3002dc15aae751455da1a852f25
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.,
.BR lseek (2)
and
.BR pwrite (2)
can't be used to write to nonzero offsets in the file).
.IP \[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.
Linux 3.9 and later
.\" commit 0bd14b4fd72afd5df41e9fd59f356740f22fceba
fix this limitation, allowing any valid set of nonoverlapping maps.
.IP \[bu]
At least one line must be written to the file.
.P
Writes that violate the above rules fail with the error
.BR EINVAL .
.P
In order for a process to write to the
.IR /proc/ pid /uid_map
.RI ( /proc/ pid /gid_map )
file, all of the following permission requirements must be met:
.IP \[bu] 3
The writing process must have the
.B CAP_SETUID
.RB ( CAP_SETGID )
capability in the user namespace of the process
.IR pid .
.IP \[bu]
The writing process must either be in the user namespace of the process
.I pid
or be in the parent user namespace of the process
.IR pid .
.IP \[bu]
The mapped user IDs (group IDs) must in turn have a mapping
in the parent user namespace.
.IP \[bu]
If updating
.IR /proc/ pid /uid_map
to create a mapping that maps UID 0 in the parent namespace,
then one of the following must be true:
.RS
.IP (a) 5
if writing process is in the parent user namespace,
then it must have the
.B CAP_SETFCAP
capability in that user namespace; or
.IP (b)
if the writing process is in the child user namespace,
then the process that created the user namespace must have had the
.B CAP_SETFCAP
capability when the namespace was created.
.RE
.IP
This rule has been in place since
.\" commit db2e718a47984b9d71ed890eb2ea36ecf150de18
Linux 5.12.
It eliminates an earlier security bug whereby
a UID 0 process that lacks the
.B CAP_SETFCAP
capability,
which is needed to create a binary with namespaced file capabilities
(as described in
.BR capabilities (7)),
could nevertheless create such a binary,
by the following steps:
.RS
.IP (1) 5
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.
.IP (2)
Since the child process has the
.B 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).
.RE
.IP \[bu]
One of the following two cases applies:
.RS
.IP (a) 5
.I Either
the writing process has the
.B CAP_SETUID
.RB ( CAP_SETGID )
capability in the
.I parent
user namespace.
.RS
.IP \[bu] 3
No further restrictions apply:
the process can make mappings to arbitrary user IDs (group IDs)
in the parent user namespace.
.RE
.IP (b)
.I Or
otherwise all of the following restrictions apply:
.RS
.IP \[bu] 3
The data written to
.I uid_map
.RI ( 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.
.IP \[bu]
The writing process must have the same effective user ID as the process
that created the user namespace.
.IP \[bu]
In the case of
.IR gid_map ,
use of the
.BR setgroups (2)
system call must first be denied by writing
.RI \[dq] deny \[dq]
to the
.IR /proc/ pid /setgroups
file (see below) before writing to
.IR gid_map .
.RE
.RE
.P
Writes that violate the above rules fail with the error
.BR EPERM .
.\"
.\" ============================================================
.\"
.SS 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
.BR setquota (8)
and
.BR quotactl (2).)
.P
Project ID mappings are defined by writing to the
.IR /proc/ pid /projid_map
file (present since
.\" commit f76d207a66c3a53defea67e7d36c3eb1b7d6d61d
Linux 3.7).
.P
The validity rules for writing to the
.IR /proc/ pid /projid_map
file are as for writing to the
.I uid_map
file; violation of these rules causes
.BR write (2)
to fail with the error
.BR EINVAL .
.P
The permission rules for writing to the
.IR /proc/ pid /projid_map
file are as follows:
.IP \[bu] 3
The writing process must either be in the user namespace of the process
.I pid
or be in the parent user namespace of the process
.IR pid .
.IP \[bu]
The mapped project IDs must in turn have a mapping
in the parent user namespace.
.P
Violation of these rules causes
.BR write (2)
to fail with the error
.BR EPERM .
.\"
.\" ============================================================
.\"
.SS Interaction with system calls that change process UIDs or GIDs
In a user namespace where the
.I uid_map
file has not been written, the system calls that change user IDs will fail.
Similarly, if the
.I gid_map
file has not been written, the system calls that change group IDs will fail.
After the
.I uid_map
and
.I gid_map
files have been written, only the mapped values may be used in
system calls that change user and group IDs.
.P
For user IDs, the relevant system calls include
.BR setuid (2),
.BR setfsuid (2),
.BR setreuid (2),
and
.BR setresuid (2).
For group IDs, the relevant system calls include
.BR setgid (2),
.BR setfsgid (2),
.BR setregid (2),
.BR setresgid (2),
and
.BR setgroups (2).
.P
Writing
.RI \[dq] deny \[dq]
to the
.IR /proc/ pid /setgroups
file before writing to
.IR /proc/ pid /gid_map
.\" Things changed in Linux 3.19
.\" commit 9cc46516ddf497ea16e8d7cb986ae03a0f6b92f8
.\" commit 66d2f338ee4c449396b6f99f5e75cd18eb6df272
.\" http://lwn.net/Articles/626665/
will permanently disable
.BR setgroups (2)
in a user namespace and allow writing to
.IR /proc/ pid /gid_map
without having the
.B CAP_SETGID
capability in the parent user namespace.
.\"
.\" ============================================================
.\"
.SS The \fI/proc/\fPpid\fI/setgroups\fP file
.\"
.\" commit 9cc46516ddf497ea16e8d7cb986ae03a0f6b92f8
.\" commit 66d2f338ee4c449396b6f99f5e75cd18eb6df272
.\" http://lwn.net/Articles/626665/
.\" http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2014-8989
.\"
The
.IR /proc/ pid /setgroups
file displays the string
.RI \[dq] allow \[dq]
if processes in the user namespace that contains the process
.I pid
are permitted to employ the
.BR setgroups (2)
system call; it displays
.RI \[dq] deny \[dq]
if
.BR setgroups (2)
is not permitted in that user namespace.
Note that regardless of the value in the
.IR /proc/ pid /setgroups
file (and regardless of the process's capabilities), calls to
.BR setgroups (2)
are also not permitted if
.IR /proc/ pid /gid_map
has not yet been set.
.P
A privileged process (one with the
.B CAP_SYS_ADMIN
capability in the namespace) may write either of the strings
.RI \[dq] allow \[dq]
or
.RI \[dq] deny \[dq]
to this file
.I before
writing a group ID mapping
for this user namespace to the file
.IR /proc/ pid /gid_map .
Writing the string
.RI \[dq] deny \[dq]
prevents any process in the user namespace from employing
.BR setgroups (2).
.P
The essence of the restrictions described in the preceding
paragraph is that it is permitted to write to
.IR /proc/ pid /setgroups
only so long as calling
.BR setgroups (2)
is disallowed because
.IR /proc/ pid /gid_map
has not been set.
This ensures that a process cannot transition from a state where
.BR setgroups (2)
is allowed to a state where
.BR setgroups (2)
is denied;
a process can transition only from
.BR setgroups (2)
being disallowed to
.BR setgroups (2)
being allowed.
.P
The default value of this file in the initial user namespace is
.RI \[dq] allow \[dq].
.P
Once
.IR /proc/ pid /gid_map
has been written to
(which has the effect of enabling
.BR setgroups (2)
in the user namespace),
it is no longer possible to disallow
.BR setgroups (2)
by writing
.RI \[dq] deny \[dq]
to
.IR /proc/ pid /setgroups
(the write fails with the error
.BR EPERM ).
.P
A child user namespace inherits the
.IR /proc/ pid /setgroups
setting from its parent.
.P
If the
.I setgroups
file has the value
.RI \[dq] deny \[dq],
then the
.BR setgroups (2)
system call can't subsequently be reenabled (by writing
.RI \[dq] allow \[dq]
to the file) in this user namespace.
(Attempts to do so fail with the error
.BR EPERM .)
This restriction also propagates down to all child user namespaces of
this user namespace.
.P
The
.IR /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
.BR 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
.B CAP_SETGID
capability) could call
.BR 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
.IR /proc/ pid /setgroups
file was added to address this security issue,
by denying any pathway for an unprivileged process to drop groups with
.BR setgroups (2).
.\"
.\" /proc/PID/setgroups
.\"	[allow == setgroups() is allowed, "deny" == setgroups() is disallowed]
.\"	* Can write if have CAP_SYS_ADMIN in NS
.\"	* Must write BEFORE writing to /proc/PID/gid_map
.\"
.\" setgroups()
.\"	* Must already have written to gid_map
.\"	* /proc/PID/setgroups must be "allow"
.\"
.\" /proc/PID/gid_map -- writing
.\"	* Must already have written "deny" to /proc/PID/setgroups
.\"
.\" ============================================================
.\"
.SS 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
.BR getuid (2)
before a user ID mapping has been defined for the namespace.
In most such cases, an unmapped user ID is converted
.\" from_kuid_munged(), from_kgid_munged()
to the overflow user ID (group ID);
the default value for the overflow user ID (group ID) is 65534.
See the descriptions of
.I /proc/sys/kernel/overflowuid
and
.I /proc/sys/kernel/overflowgid
in
.BR proc (5).
.P
The cases where unmapped IDs are mapped in this fashion include
system calls that return user IDs
.RB ( getuid (2),
.BR getgid (2),
and similar),
credentials passed over a UNIX domain socket,
.\" also SO_PEERCRED
credentials returned by
.BR stat (2),
.BR waitid (2),
and the System V IPC "ctl"
.B IPC_STAT
operations,
credentials exposed by
.IR /proc/ pid /status
and the files in
.IR /proc/sysvipc/* ,
credentials returned via the
.I si_uid
field in the
.I siginfo_t
received with a signal (see
.BR sigaction (2)),
credentials written to the process accounting file (see
.BR acct (5)),
and credentials returned with POSIX message queue notifications (see
.BR mq_notify (3)).
.P
There is one notable case where unmapped user and group IDs are
.I not
.\" from_kuid(), from_kgid()
.\" Also F_GETOWNER_UIDS is an exception
converted to the corresponding overflow ID value.
When viewing a
.I uid_map
or
.I gid_map
file in which there is no mapping for the second field,
that field is displayed as 4294967295 (\-1 as an unsigned integer).
.\"
.\" ============================================================
.\"
.SS 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.
.\"
.\" ============================================================
.\"
.SS Operation of file-related capabilities
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:
.BR CAP_CHOWN ,
.BR CAP_DAC_OVERRIDE ,
.BR CAP_DAC_READ_SEARCH ,
.BR CAP_FOWNER ,
and
.BR CAP_FSETID .
.P
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:
.IP \[bu] 3
the process has the relevant effective capability in its user namespace; and
.IP \[bu]
the file's user ID and group ID both have valid mappings
in the user namespace.
.P
The
.B CAP_FOWNER
capability is treated somewhat exceptionally:
.\" These are the checks performed by the kernel function
.\" inode_owner_or_capable(). There is one exception to the exception:
.\" overriding the directory sticky permission bit requires that
.\" the file has a valid mapping for both its UID and GID.
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).
.\"
.\" ============================================================
.\"
.SS Set-user-ID and set-group-ID programs
When a process inside a user namespace executes
a set-user-ID (set-group-ID) program,
the process's effective user (group) ID inside the namespace is changed
to whatever value is mapped for the user (group) ID of the file.
However, if either the user
.I or
the group ID of the file has no mapping inside the namespace,
the set-user-ID (set-group-ID) bit is silently ignored:
the new program is executed,
but the process's effective user (group) ID is left unchanged.
(This mirrors the semantics of executing a set-user-ID or set-group-ID
program that resides on a filesystem that was mounted with the
.B MS_NOSUID
flag, as described in
.BR mount (2).)
.\"
.\" ============================================================
.\"
.SS 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
.B SCM_CREDENTIALS
in
.BR unix (7)),
they are translated into the corresponding values as per the
receiving process's user and group ID mappings.
.\"
.SH STANDARDS
Linux.
.\"
.SH NOTES
Over the years, there have been a lot of features that have been added
to the Linux kernel that have been made available only to privileged users
because of their potential to confuse set-user-ID-root applications.
In general, it becomes safe to allow the root user in a user namespace to
use those features because it is impossible, while in a user namespace,
to gain more privilege than the root user of a user namespace has.
.\"
.\" ============================================================
.\"
.SS Global root
The term "global root" is sometimes used as a shorthand for
user ID 0 in the initial user namespace.
.\"
.\" ============================================================
.\"
.SS Availability
Use of user namespaces requires a kernel that is configured with the
.B 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.
.P
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,
.\" commit d6970d4b726cea6d7a9bc4120814f95c09571fc3
XFS.
.\"
.SH 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
.IR usage ()
function inside the program provide a full explanation of the program.
The following shell session demonstrates its use.
.P
First, we look at the run-time environment:
.P
.in +4n
.EX
$ \fBuname \-rs\fP     # Need Linux 3.8 or later
Linux 3.8.0
$ \fBid \-u\fP         # Running as unprivileged user
1000
$ \fBid \-g\fP
1000
.EE
.in
.P
Now start a new shell in new user
.RI ( \-U ),
mount
.RI ( \-m ),
and PID
.RI ( \-p )
namespaces, with user ID
.RI ( \-M )
and group ID
.RI ( \-G )
1000 mapped to 0 inside the user namespace:
.P
.in +4n
.EX
$ \fB./userns_child_exec \-p \-m \-U \-M \[aq]0 1000 1\[aq] \-G \[aq]0 1000 1\[aq] bash\fP
.EE
.in
.P
The shell has PID 1, because it is the first process in the new
PID namespace:
.P
.in +4n
.EX
bash$ \fBecho $$\fP
1
.EE
.in
.P
Mounting a new
.I /proc
filesystem and listing all of the processes visible
in the new PID namespace shows that the shell can't see
any processes outside the PID namespace:
.P
.in +4n
.EX
bash$ \fBmount \-t proc proc /proc\fP
bash$ \fBps ax\fP
  PID TTY      STAT   TIME COMMAND
    1 pts/3    S      0:00 bash
   22 pts/3    R+     0:00 ps ax
.EE
.in
.P
Inside the user namespace, the shell has user and group ID 0,
and a full set of permitted and effective capabilities:
.P
.in +4n
.EX
bash$ \fBcat /proc/$$/status | egrep \[aq]\[ha][UG]id\[aq]\fP
Uid:	0	0	0	0
Gid:	0	0	0	0
bash$ \fBcat /proc/$$/status | egrep \[aq]\[ha]Cap(Prm|Inh|Eff)\[aq]\fP
CapInh:	0000000000000000
CapPrm:	0000001fffffffff
CapEff:	0000001fffffffff
.EE
.in
.SS Program source
\&
.EX
/* 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   len\[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   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;
    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);
}
.EE
.SH SEE ALSO
.BR newgidmap (1),      \" From the shadow package
.BR newuidmap (1),      \" From the shadow package
.BR clone (2),
.BR ptrace (2),
.BR setns (2),
.BR unshare (2),
.BR proc (5),
.BR subgid (5),         \" From the shadow package
.BR subuid (5),         \" From the shadow package
.BR capabilities (7),
.BR cgroup_namespaces (7),
.BR credentials (7),
.BR namespaces (7),
.BR pid_namespaces (7)
.P
The kernel source file
.IR Documentation/admin\-guide/namespaces/resource\-control.rst .