обзор пользовательских пространств имен Linux (overview of Linux user namespaces)
Описание (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)
* 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 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
0 0 4294967295
This mapping tells us that the range starting at user ID 0 in
this namespace maps to a range starting at 0 in the (nonexistent)
parent namespace, and the length of the range is the largest
32-bit unsigned integer. This leaves 4294967295 (the 32-bit
signed -1 value) unmapped. This is deliberate: (uid_t) -1 is
used in several interfaces (e.g., setreuid(2)) as a way to
specify "no user ID". Leaving (uid_t) -1 unmapped and unusable
guarantees that there will be no confusion when using these
interfaces.
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:
* The three fields must be valid numbers, and the last field
must be greater than 0.
* Lines are terminated by newline characters.
* There is a limit on the number of lines in the file. In Linux
4.14 and earlier, this limit was (arbitrarily) set at 5 lines.
Since Linux 4.15, the limit is 340 lines. In addition, the
number of bytes written to the file must be less than the
system page size, and the write must be performed at the start
of the file (i.e., lseek(2) and pwrite(2) can't be used to
write to nonzero offsets in the file).
* The range of user IDs (group IDs) specified in each line
cannot overlap with the ranges in any other lines. In the
initial implementation (Linux 3.8), this requirement was
satisfied by a simplistic implementation that imposed the
further requirement that the values in both field 1 and field
2 of successive lines must be in ascending numerical order,
which prevented some otherwise valid maps from being created.
Linux 3.9 and later fix this limitation, allowing any valid
set of nonoverlapping maps.
* At least one line must be written to the file.
Writes that violate the above rules fail with the error EINVAL.
In order for a process to write to the /proc/[pid]/uid_map
(/proc/[pid]/gid_map) file, all of the following permission
requirements must be met:
1. The writing process must have the CAP_SETUID (CAP_SETGID)
capability in the user namespace of the process pid.
2. The writing process must either be in the user namespace of
the process pid or be in the parent user namespace of the
process pid.
3. The mapped user IDs (group IDs) must in turn have a mapping in
the parent user namespace.
4. 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:
* if writing process is in the parent user namespace, then it
must have the CAP_SETFCAP capability in that user
namespace; or
* 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:
* 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.
* 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).
5. One of the following two cases applies:
* Either the writing process has the CAP_SETUID (CAP_SETGID)
capability in the parent user namespace.
+ No further restrictions apply: the process can make
mappings to arbitrary user IDs (group IDs) in the parent
user namespace.
* Or otherwise all of the following restrictions apply:
+ The data written to uid_map (gid_map) must consist of a
single line that maps the writing process's effective
user ID (group ID) in the parent user namespace to a
user ID (group ID) in the user namespace.
+ The writing process must have the same effective user ID
as the process that created the user namespace.
+ In the case of gid_map, use of the setgroups(2) system
call must first be denied by writing "deny" to the
/proc/[pid]/setgroups file (see below) before writing to
gid_map.
Writes that violate the above rules fail with the error EPERM.
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:
1. 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.
2. 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 "deny" to the /proc/[pid]/setgroups file before writing
to /proc/[pid]/gid_map will permanently disable setgroups(2) in a
user namespace and allow writing to /proc/[pid]/gid_map without
having the CAP_SETGID capability in the parent user namespace.
The /proc/[pid]/setgroups file
The /proc/[pid]/setgroups file displays the string "allow" if
processes in the user namespace that contains the process pid are
permitted to employ the setgroups(2) system call; it displays
"deny" if setgroups(2) is not permitted in that user namespace.
Note that regardless of the value in the /proc/[pid]/setgroups
file (and regardless of the process's capabilities), calls to
setgroups(2) are also not permitted if /proc/[pid]/gid_map has
not yet been set.
A privileged process (one with the CAP_SYS_ADMIN capability in
the namespace) may write either of the strings "allow" or "deny"
to this file before writing a group ID mapping for this user
namespace to the file /proc/[pid]/gid_map. Writing the string
"deny" prevents any process in the user namespace from employing
setgroups(2).
The essence of the restrictions described in the preceding
paragraph is that it is permitted to write to
/proc/[pid]/setgroups only so long as calling setgroups(2) is
disallowed because /proc/[pid]/gid_map has not been set. This
ensures that a process cannot transition from a state where
setgroups(2) is allowed to a state where setgroups(2) is denied;
a process can transition only from setgroups(2) being disallowed
to setgroups(2) being allowed.
The default value of this file in the initial user namespace is
"allow".
Once /proc/[pid]/gid_map has been written to (which has the
effect of enabling setgroups(2) in the user namespace), it is no
longer possible to disallow setgroups(2) by writing "deny" to
/proc/[pid]/setgroups (the write fails with the error EPERM).
A child user namespace inherits the /proc/[pid]/setgroups setting
from its parent.
If the setgroups file has the value "deny", then the setgroups(2)
system call can't subsequently be reenabled (by writing "allow"
to the file) in this user namespace. (Attempts to do so fail
with the error EPERM.) This restriction also propagates down to
all child user namespaces of this user namespace.
The /proc/[pid]/setgroups file was added in Linux 3.19, but was
backported to many earlier stable kernel series, because it
addresses a security issue. The issue concerned files with
permissions such as "rwx---rwx". Such files give fewer
permissions to "group" than they do to "other". This means that
dropping groups using setgroups(2) might allow a process file
access that it did not formerly have. Before the existence of
user namespaces this was not a concern, since only a privileged
process (one with the CAP_SETGID capability) could call
setgroups(2). However, with the introduction of user namespaces,
it became possible for an unprivileged process to create a new
namespace in which the user had all privileges. This then
allowed formerly unprivileged users to drop groups and thus gain
file access that they did not previously have. The
/proc/[pid]/setgroups file was added to address this security
issue, by denying any pathway for an unprivileged process to drop
groups with setgroups(2).
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
of other objects that employ the credentials plus permissions
mask accessibility model, such as System V IPC objects
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: CAP_CHOWN,
CAP_DAC_OVERRIDE, CAP_DAC_READ_SEARCH, CAP_FOWNER, and
CAP_FSETID.
Within a user namespace, these capabilities allow a process to
bypass the rules if the process has the relevant capability over
the file, meaning that:
* the process has the relevant effective capability in its user
namespace; and
* the file's user ID and group ID both have valid mappings in
the user namespace.
The CAP_FOWNER capability is treated somewhat exceptionally: it
allows a process to bypass the corresponding rules so long as at
least the file's user ID has a mapping in the user namespace
(i.e., the file's group ID does not need to have a valid
mapping).
Set-user-ID and set-group-ID programs
When a process inside a user namespace executes a set-user-ID
(set-group-ID) program, the process's effective user (group) ID
inside the namespace is changed to whatever value is mapped for
the user (group) ID of the file. However, if either the user or
the group ID of the file has no mapping inside the namespace, the
set-user-ID (set-group-ID) bit is silently ignored: the new
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).)
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.
