mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-26 10:49:33 +00:00
![Randy Dunlap](/assets/img/avatar_default.png)
Change occurrences of "it's" that are possessive to "its" so that they don't read as "it is". For f2fs.rst, reword one description for better clarity. Signed-off-by: Randy Dunlap <rdunlap@infradead.org> Cc: Jonathan Corbet <corbet@lwn.net> Cc: linux-fsdevel@vger.kernel.org Cc: linux-f2fs-devel@lists.sourceforge.net Cc: linux-xfs@vger.kernel.org Cc: Christian Brauner <brauner@kernel.org> Cc: Seth Forshee <sforshee@kernel.org> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Theodore Ts'o <tytso@mit.edu> Cc: Jaegeuk Kim <jaegeuk@kernel.org> Reviewed-by: Darrick J. Wong <djwong@kernel.org> Reviewed-by: "Christian Brauner (Microsoft)" <brauner@kernel.org> Reviewed-by: Chao Yu <chao@kernel.org> Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org> Link: https://lore.kernel.org/r/20220901002828.25102-1-rdunlap@infradead.org Signed-off-by: Jonathan Corbet <corbet@lwn.net>
959 lines
37 KiB
ReStructuredText
959 lines
37 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
Idmappings
|
|
==========
|
|
|
|
Most filesystem developers will have encountered idmappings. They are used when
|
|
reading from or writing ownership to disk, reporting ownership to userspace, or
|
|
for permission checking. This document is aimed at filesystem developers that
|
|
want to know how idmappings work.
|
|
|
|
Formal notes
|
|
------------
|
|
|
|
An idmapping is essentially a translation of a range of ids into another or the
|
|
same range of ids. The notational convention for idmappings that is widely used
|
|
in userspace is::
|
|
|
|
u:k:r
|
|
|
|
``u`` indicates the first element in the upper idmapset ``U`` and ``k``
|
|
indicates the first element in the lower idmapset ``K``. The ``r`` parameter
|
|
indicates the range of the idmapping, i.e. how many ids are mapped. From now
|
|
on, we will always prefix ids with ``u`` or ``k`` to make it clear whether
|
|
we're talking about an id in the upper or lower idmapset.
|
|
|
|
To see what this looks like in practice, let's take the following idmapping::
|
|
|
|
u22:k10000:r3
|
|
|
|
and write down the mappings it will generate::
|
|
|
|
u22 -> k10000
|
|
u23 -> k10001
|
|
u24 -> k10002
|
|
|
|
From a mathematical viewpoint ``U`` and ``K`` are well-ordered sets and an
|
|
idmapping is an order isomorphism from ``U`` into ``K``. So ``U`` and ``K`` are
|
|
order isomorphic. In fact, ``U`` and ``K`` are always well-ordered subsets of
|
|
the set of all possible ids useable on a given system.
|
|
|
|
Looking at this mathematically briefly will help us highlight some properties
|
|
that make it easier to understand how we can translate between idmappings. For
|
|
example, we know that the inverse idmapping is an order isomorphism as well::
|
|
|
|
k10000 -> u22
|
|
k10001 -> u23
|
|
k10002 -> u24
|
|
|
|
Given that we are dealing with order isomorphisms plus the fact that we're
|
|
dealing with subsets we can embedd idmappings into each other, i.e. we can
|
|
sensibly translate between different idmappings. For example, assume we've been
|
|
given the three idmappings::
|
|
|
|
1. u0:k10000:r10000
|
|
2. u0:k20000:r10000
|
|
3. u0:k30000:r10000
|
|
|
|
and id ``k11000`` which has been generated by the first idmapping by mapping
|
|
``u1000`` from the upper idmapset down to ``k11000`` in the lower idmapset.
|
|
|
|
Because we're dealing with order isomorphic subsets it is meaningful to ask
|
|
what id ``k11000`` corresponds to in the second or third idmapping. The
|
|
straightfoward algorithm to use is to apply the inverse of the first idmapping,
|
|
mapping ``k11000`` up to ``u1000``. Afterwards, we can map ``u1000`` down using
|
|
either the second idmapping mapping or third idmapping mapping. The second
|
|
idmapping would map ``u1000`` down to ``21000``. The third idmapping would map
|
|
``u1000`` down to ``u31000``.
|
|
|
|
If we were given the same task for the following three idmappings::
|
|
|
|
1. u0:k10000:r10000
|
|
2. u0:k20000:r200
|
|
3. u0:k30000:r300
|
|
|
|
we would fail to translate as the sets aren't order isomorphic over the full
|
|
range of the first idmapping anymore (However they are order isomorphic over
|
|
the full range of the second idmapping.). Neither the second or third idmapping
|
|
contain ``u1000`` in the upper idmapset ``U``. This is equivalent to not having
|
|
an id mapped. We can simply say that ``u1000`` is unmapped in the second and
|
|
third idmapping. The kernel will report unmapped ids as the overflowuid
|
|
``(uid_t)-1`` or overflowgid ``(gid_t)-1`` to userspace.
|
|
|
|
The algorithm to calculate what a given id maps to is pretty simple. First, we
|
|
need to verify that the range can contain our target id. We will skip this step
|
|
for simplicity. After that if we want to know what ``id`` maps to we can do
|
|
simple calculations:
|
|
|
|
- If we want to map from left to right::
|
|
|
|
u:k:r
|
|
id - u + k = n
|
|
|
|
- If we want to map from right to left::
|
|
|
|
u:k:r
|
|
id - k + u = n
|
|
|
|
Instead of "left to right" we can also say "down" and instead of "right to
|
|
left" we can also say "up". Obviously mapping down and up invert each other.
|
|
|
|
To see whether the simple formulas above work, consider the following two
|
|
idmappings::
|
|
|
|
1. u0:k20000:r10000
|
|
2. u500:k30000:r10000
|
|
|
|
Assume we are given ``k21000`` in the lower idmapset of the first idmapping. We
|
|
want to know what id this was mapped from in the upper idmapset of the first
|
|
idmapping. So we're mapping up in the first idmapping::
|
|
|
|
id - k + u = n
|
|
k21000 - k20000 + u0 = u1000
|
|
|
|
Now assume we are given the id ``u1100`` in the upper idmapset of the second
|
|
idmapping and we want to know what this id maps down to in the lower idmapset
|
|
of the second idmapping. This means we're mapping down in the second
|
|
idmapping::
|
|
|
|
id - u + k = n
|
|
u1100 - u500 + k30000 = k30600
|
|
|
|
General notes
|
|
-------------
|
|
|
|
In the context of the kernel an idmapping can be interpreted as mapping a range
|
|
of userspace ids into a range of kernel ids::
|
|
|
|
userspace-id:kernel-id:range
|
|
|
|
A userspace id is always an element in the upper idmapset of an idmapping of
|
|
type ``uid_t`` or ``gid_t`` and a kernel id is always an element in the lower
|
|
idmapset of an idmapping of type ``kuid_t`` or ``kgid_t``. From now on
|
|
"userspace id" will be used to refer to the well known ``uid_t`` and ``gid_t``
|
|
types and "kernel id" will be used to refer to ``kuid_t`` and ``kgid_t``.
|
|
|
|
The kernel is mostly concerned with kernel ids. They are used when performing
|
|
permission checks and are stored in an inode's ``i_uid`` and ``i_gid`` field.
|
|
A userspace id on the other hand is an id that is reported to userspace by the
|
|
kernel, or is passed by userspace to the kernel, or a raw device id that is
|
|
written or read from disk.
|
|
|
|
Note that we are only concerned with idmappings as the kernel stores them not
|
|
how userspace would specify them.
|
|
|
|
For the rest of this document we will prefix all userspace ids with ``u`` and
|
|
all kernel ids with ``k``. Ranges of idmappings will be prefixed with ``r``. So
|
|
an idmapping will be written as ``u0:k10000:r10000``.
|
|
|
|
For example, the id ``u1000`` is an id in the upper idmapset or "userspace
|
|
idmapset" starting with ``u1000``. And it is mapped to ``k11000`` which is a
|
|
kernel id in the lower idmapset or "kernel idmapset" starting with ``k10000``.
|
|
|
|
A kernel id is always created by an idmapping. Such idmappings are associated
|
|
with user namespaces. Since we mainly care about how idmappings work we're not
|
|
going to be concerned with how idmappings are created nor how they are used
|
|
outside of the filesystem context. This is best left to an explanation of user
|
|
namespaces.
|
|
|
|
The initial user namespace is special. It always has an idmapping of the
|
|
following form::
|
|
|
|
u0:k0:r4294967295
|
|
|
|
which is an identity idmapping over the full range of ids available on this
|
|
system.
|
|
|
|
Other user namespaces usually have non-identity idmappings such as::
|
|
|
|
u0:k10000:r10000
|
|
|
|
When a process creates or wants to change ownership of a file, or when the
|
|
ownership of a file is read from disk by a filesystem, the userspace id is
|
|
immediately translated into a kernel id according to the idmapping associated
|
|
with the relevant user namespace.
|
|
|
|
For instance, consider a file that is stored on disk by a filesystem as being
|
|
owned by ``u1000``:
|
|
|
|
- If a filesystem were to be mounted in the initial user namespaces (as most
|
|
filesystems are) then the initial idmapping will be used. As we saw this is
|
|
simply the identity idmapping. This would mean id ``u1000`` read from disk
|
|
would be mapped to id ``k1000``. So an inode's ``i_uid`` and ``i_gid`` field
|
|
would contain ``k1000``.
|
|
|
|
- If a filesystem were to be mounted with an idmapping of ``u0:k10000:r10000``
|
|
then ``u1000`` read from disk would be mapped to ``k11000``. So an inode's
|
|
``i_uid`` and ``i_gid`` would contain ``k11000``.
|
|
|
|
Translation algorithms
|
|
----------------------
|
|
|
|
We've already seen briefly that it is possible to translate between different
|
|
idmappings. We'll now take a closer look how that works.
|
|
|
|
Crossmapping
|
|
~~~~~~~~~~~~
|
|
|
|
This translation algorithm is used by the kernel in quite a few places. For
|
|
example, it is used when reporting back the ownership of a file to userspace
|
|
via the ``stat()`` system call family.
|
|
|
|
If we've been given ``k11000`` from one idmapping we can map that id up in
|
|
another idmapping. In order for this to work both idmappings need to contain
|
|
the same kernel id in their kernel idmapsets. For example, consider the
|
|
following idmappings::
|
|
|
|
1. u0:k10000:r10000
|
|
2. u20000:k10000:r10000
|
|
|
|
and we are mapping ``u1000`` down to ``k11000`` in the first idmapping . We can
|
|
then translate ``k11000`` into a userspace id in the second idmapping using the
|
|
kernel idmapset of the second idmapping::
|
|
|
|
/* Map the kernel id up into a userspace id in the second idmapping. */
|
|
from_kuid(u20000:k10000:r10000, k11000) = u21000
|
|
|
|
Note, how we can get back to the kernel id in the first idmapping by inverting
|
|
the algorithm::
|
|
|
|
/* Map the userspace id down into a kernel id in the second idmapping. */
|
|
make_kuid(u20000:k10000:r10000, u21000) = k11000
|
|
|
|
/* Map the kernel id up into a userspace id in the first idmapping. */
|
|
from_kuid(u0:k10000:r10000, k11000) = u1000
|
|
|
|
This algorithm allows us to answer the question what userspace id a given
|
|
kernel id corresponds to in a given idmapping. In order to be able to answer
|
|
this question both idmappings need to contain the same kernel id in their
|
|
respective kernel idmapsets.
|
|
|
|
For example, when the kernel reads a raw userspace id from disk it maps it down
|
|
into a kernel id according to the idmapping associated with the filesystem.
|
|
Let's assume the filesystem was mounted with an idmapping of
|
|
``u0:k20000:r10000`` and it reads a file owned by ``u1000`` from disk. This
|
|
means ``u1000`` will be mapped to ``k21000`` which is what will be stored in
|
|
the inode's ``i_uid`` and ``i_gid`` field.
|
|
|
|
When someone in userspace calls ``stat()`` or a related function to get
|
|
ownership information about the file the kernel can't simply map the id back up
|
|
according to the filesystem's idmapping as this would give the wrong owner if
|
|
the caller is using an idmapping.
|
|
|
|
So the kernel will map the id back up in the idmapping of the caller. Let's
|
|
assume the caller has the slighly unconventional idmapping
|
|
``u3000:k20000:r10000`` then ``k21000`` would map back up to ``u4000``.
|
|
Consequently the user would see that this file is owned by ``u4000``.
|
|
|
|
Remapping
|
|
~~~~~~~~~
|
|
|
|
It is possible to translate a kernel id from one idmapping to another one via
|
|
the userspace idmapset of the two idmappings. This is equivalent to remapping
|
|
a kernel id.
|
|
|
|
Let's look at an example. We are given the following two idmappings::
|
|
|
|
1. u0:k10000:r10000
|
|
2. u0:k20000:r10000
|
|
|
|
and we are given ``k11000`` in the first idmapping. In order to translate this
|
|
kernel id in the first idmapping into a kernel id in the second idmapping we
|
|
need to perform two steps:
|
|
|
|
1. Map the kernel id up into a userspace id in the first idmapping::
|
|
|
|
/* Map the kernel id up into a userspace id in the first idmapping. */
|
|
from_kuid(u0:k10000:r10000, k11000) = u1000
|
|
|
|
2. Map the userspace id down into a kernel id in the second idmapping::
|
|
|
|
/* Map the userspace id down into a kernel id in the second idmapping. */
|
|
make_kuid(u0:k20000:r10000, u1000) = k21000
|
|
|
|
As you can see we used the userspace idmapset in both idmappings to translate
|
|
the kernel id in one idmapping to a kernel id in another idmapping.
|
|
|
|
This allows us to answer the question what kernel id we would need to use to
|
|
get the same userspace id in another idmapping. In order to be able to answer
|
|
this question both idmappings need to contain the same userspace id in their
|
|
respective userspace idmapsets.
|
|
|
|
Note, how we can easily get back to the kernel id in the first idmapping by
|
|
inverting the algorithm:
|
|
|
|
1. Map the kernel id up into a userspace id in the second idmapping::
|
|
|
|
/* Map the kernel id up into a userspace id in the second idmapping. */
|
|
from_kuid(u0:k20000:r10000, k21000) = u1000
|
|
|
|
2. Map the userspace id down into a kernel id in the first idmapping::
|
|
|
|
/* Map the userspace id down into a kernel id in the first idmapping. */
|
|
make_kuid(u0:k10000:r10000, u1000) = k11000
|
|
|
|
Another way to look at this translation is to treat it as inverting one
|
|
idmapping and applying another idmapping if both idmappings have the relevant
|
|
userspace id mapped. This will come in handy when working with idmapped mounts.
|
|
|
|
Invalid translations
|
|
~~~~~~~~~~~~~~~~~~~~
|
|
|
|
It is never valid to use an id in the kernel idmapset of one idmapping as the
|
|
id in the userspace idmapset of another or the same idmapping. While the kernel
|
|
idmapset always indicates an idmapset in the kernel id space the userspace
|
|
idmapset indicates a userspace id. So the following translations are forbidden::
|
|
|
|
/* Map the userspace id down into a kernel id in the first idmapping. */
|
|
make_kuid(u0:k10000:r10000, u1000) = k11000
|
|
|
|
/* INVALID: Map the kernel id down into a kernel id in the second idmapping. */
|
|
make_kuid(u10000:k20000:r10000, k110000) = k21000
|
|
~~~~~~~
|
|
|
|
and equally wrong::
|
|
|
|
/* Map the kernel id up into a userspace id in the first idmapping. */
|
|
from_kuid(u0:k10000:r10000, k11000) = u1000
|
|
|
|
/* INVALID: Map the userspace id up into a userspace id in the second idmapping. */
|
|
from_kuid(u20000:k0:r10000, u1000) = k21000
|
|
~~~~~
|
|
|
|
Idmappings when creating filesystem objects
|
|
-------------------------------------------
|
|
|
|
The concepts of mapping an id down or mapping an id up are expressed in the two
|
|
kernel functions filesystem developers are rather familiar with and which we've
|
|
already used in this document::
|
|
|
|
/* Map the userspace id down into a kernel id. */
|
|
make_kuid(idmapping, uid)
|
|
|
|
/* Map the kernel id up into a userspace id. */
|
|
from_kuid(idmapping, kuid)
|
|
|
|
We will take an abbreviated look into how idmappings figure into creating
|
|
filesystem objects. For simplicity we will only look at what happens when the
|
|
VFS has already completed path lookup right before it calls into the filesystem
|
|
itself. So we're concerned with what happens when e.g. ``vfs_mkdir()`` is
|
|
called. We will also assume that the directory we're creating filesystem
|
|
objects in is readable and writable for everyone.
|
|
|
|
When creating a filesystem object the caller will look at the caller's
|
|
filesystem ids. These are just regular ``uid_t`` and ``gid_t`` userspace ids
|
|
but they are exclusively used when determining file ownership which is why they
|
|
are called "filesystem ids". They are usually identical to the uid and gid of
|
|
the caller but can differ. We will just assume they are always identical to not
|
|
get lost in too many details.
|
|
|
|
When the caller enters the kernel two things happen:
|
|
|
|
1. Map the caller's userspace ids down into kernel ids in the caller's
|
|
idmapping.
|
|
(To be precise, the kernel will simply look at the kernel ids stashed in the
|
|
credentials of the current task but for our education we'll pretend this
|
|
translation happens just in time.)
|
|
2. Verify that the caller's kernel ids can be mapped up to userspace ids in the
|
|
filesystem's idmapping.
|
|
|
|
The second step is important as regular filesystem will ultimately need to map
|
|
the kernel id back up into a userspace id when writing to disk.
|
|
So with the second step the kernel guarantees that a valid userspace id can be
|
|
written to disk. If it can't the kernel will refuse the creation request to not
|
|
even remotely risk filesystem corruption.
|
|
|
|
The astute reader will have realized that this is simply a varation of the
|
|
crossmapping algorithm we mentioned above in a previous section. First, the
|
|
kernel maps the caller's userspace id down into a kernel id according to the
|
|
caller's idmapping and then maps that kernel id up according to the
|
|
filesystem's idmapping.
|
|
|
|
Let's see some examples with caller/filesystem idmapping but without mount
|
|
idmappings. This will exhibit some problems we can hit. After that we will
|
|
revisit/reconsider these examples, this time using mount idmappings, to see how
|
|
they can solve the problems we observed before.
|
|
|
|
Example 1
|
|
~~~~~~~~~
|
|
|
|
::
|
|
|
|
caller id: u1000
|
|
caller idmapping: u0:k0:r4294967295
|
|
filesystem idmapping: u0:k0:r4294967295
|
|
|
|
Both the caller and the filesystem use the identity idmapping:
|
|
|
|
1. Map the caller's userspace ids into kernel ids in the caller's idmapping::
|
|
|
|
make_kuid(u0:k0:r4294967295, u1000) = k1000
|
|
|
|
2. Verify that the caller's kernel ids can be mapped to userspace ids in the
|
|
filesystem's idmapping.
|
|
|
|
For this second step the kernel will call the function
|
|
``fsuidgid_has_mapping()`` which ultimately boils down to calling
|
|
``from_kuid()``::
|
|
|
|
from_kuid(u0:k0:r4294967295, k1000) = u1000
|
|
|
|
In this example both idmappings are the same so there's nothing exciting going
|
|
on. Ultimately the userspace id that lands on disk will be ``u1000``.
|
|
|
|
Example 2
|
|
~~~~~~~~~
|
|
|
|
::
|
|
|
|
caller id: u1000
|
|
caller idmapping: u0:k10000:r10000
|
|
filesystem idmapping: u0:k20000:r10000
|
|
|
|
1. Map the caller's userspace ids down into kernel ids in the caller's
|
|
idmapping::
|
|
|
|
make_kuid(u0:k10000:r10000, u1000) = k11000
|
|
|
|
2. Verify that the caller's kernel ids can be mapped up to userspace ids in the
|
|
filesystem's idmapping::
|
|
|
|
from_kuid(u0:k20000:r10000, k11000) = u-1
|
|
|
|
It's immediately clear that while the caller's userspace id could be
|
|
successfully mapped down into kernel ids in the caller's idmapping the kernel
|
|
ids could not be mapped up according to the filesystem's idmapping. So the
|
|
kernel will deny this creation request.
|
|
|
|
Note that while this example is less common, because most filesystem can't be
|
|
mounted with non-initial idmappings this is a general problem as we can see in
|
|
the next examples.
|
|
|
|
Example 3
|
|
~~~~~~~~~
|
|
|
|
::
|
|
|
|
caller id: u1000
|
|
caller idmapping: u0:k10000:r10000
|
|
filesystem idmapping: u0:k0:r4294967295
|
|
|
|
1. Map the caller's userspace ids down into kernel ids in the caller's
|
|
idmapping::
|
|
|
|
make_kuid(u0:k10000:r10000, u1000) = k11000
|
|
|
|
2. Verify that the caller's kernel ids can be mapped up to userspace ids in the
|
|
filesystem's idmapping::
|
|
|
|
from_kuid(u0:k0:r4294967295, k11000) = u11000
|
|
|
|
We can see that the translation always succeeds. The userspace id that the
|
|
filesystem will ultimately put to disk will always be identical to the value of
|
|
the kernel id that was created in the caller's idmapping. This has mainly two
|
|
consequences.
|
|
|
|
First, that we can't allow a caller to ultimately write to disk with another
|
|
userspace id. We could only do this if we were to mount the whole fileystem
|
|
with the caller's or another idmapping. But that solution is limited to a few
|
|
filesystems and not very flexible. But this is a use-case that is pretty
|
|
important in containerized workloads.
|
|
|
|
Second, the caller will usually not be able to create any files or access
|
|
directories that have stricter permissions because none of the filesystem's
|
|
kernel ids map up into valid userspace ids in the caller's idmapping
|
|
|
|
1. Map raw userspace ids down to kernel ids in the filesystem's idmapping::
|
|
|
|
make_kuid(u0:k0:r4294967295, u1000) = k1000
|
|
|
|
2. Map kernel ids up to userspace ids in the caller's idmapping::
|
|
|
|
from_kuid(u0:k10000:r10000, k1000) = u-1
|
|
|
|
Example 4
|
|
~~~~~~~~~
|
|
|
|
::
|
|
|
|
file id: u1000
|
|
caller idmapping: u0:k10000:r10000
|
|
filesystem idmapping: u0:k0:r4294967295
|
|
|
|
In order to report ownership to userspace the kernel uses the crossmapping
|
|
algorithm introduced in a previous section:
|
|
|
|
1. Map the userspace id on disk down into a kernel id in the filesystem's
|
|
idmapping::
|
|
|
|
make_kuid(u0:k0:r4294967295, u1000) = k1000
|
|
|
|
2. Map the kernel id up into a userspace id in the caller's idmapping::
|
|
|
|
from_kuid(u0:k10000:r10000, k1000) = u-1
|
|
|
|
The crossmapping algorithm fails in this case because the kernel id in the
|
|
filesystem idmapping cannot be mapped up to a userspace id in the caller's
|
|
idmapping. Thus, the kernel will report the ownership of this file as the
|
|
overflowid.
|
|
|
|
Example 5
|
|
~~~~~~~~~
|
|
|
|
::
|
|
|
|
file id: u1000
|
|
caller idmapping: u0:k10000:r10000
|
|
filesystem idmapping: u0:k20000:r10000
|
|
|
|
In order to report ownership to userspace the kernel uses the crossmapping
|
|
algorithm introduced in a previous section:
|
|
|
|
1. Map the userspace id on disk down into a kernel id in the filesystem's
|
|
idmapping::
|
|
|
|
make_kuid(u0:k20000:r10000, u1000) = k21000
|
|
|
|
2. Map the kernel id up into a userspace id in the caller's idmapping::
|
|
|
|
from_kuid(u0:k10000:r10000, k21000) = u-1
|
|
|
|
Again, the crossmapping algorithm fails in this case because the kernel id in
|
|
the filesystem idmapping cannot be mapped to a userspace id in the caller's
|
|
idmapping. Thus, the kernel will report the ownership of this file as the
|
|
overflowid.
|
|
|
|
Note how in the last two examples things would be simple if the caller would be
|
|
using the initial idmapping. For a filesystem mounted with the initial
|
|
idmapping it would be trivial. So we only consider a filesystem with an
|
|
idmapping of ``u0:k20000:r10000``:
|
|
|
|
1. Map the userspace id on disk down into a kernel id in the filesystem's
|
|
idmapping::
|
|
|
|
make_kuid(u0:k20000:r10000, u1000) = k21000
|
|
|
|
2. Map the kernel id up into a userspace id in the caller's idmapping::
|
|
|
|
from_kuid(u0:k0:r4294967295, k21000) = u21000
|
|
|
|
Idmappings on idmapped mounts
|
|
-----------------------------
|
|
|
|
The examples we've seen in the previous section where the caller's idmapping
|
|
and the filesystem's idmapping are incompatible causes various issues for
|
|
workloads. For a more complex but common example, consider two containers
|
|
started on the host. To completely prevent the two containers from affecting
|
|
each other, an administrator may often use different non-overlapping idmappings
|
|
for the two containers::
|
|
|
|
container1 idmapping: u0:k10000:r10000
|
|
container2 idmapping: u0:k20000:r10000
|
|
filesystem idmapping: u0:k30000:r10000
|
|
|
|
An administrator wanting to provide easy read-write access to the following set
|
|
of files::
|
|
|
|
dir id: u0
|
|
dir/file1 id: u1000
|
|
dir/file2 id: u2000
|
|
|
|
to both containers currently can't.
|
|
|
|
Of course the administrator has the option to recursively change ownership via
|
|
``chown()``. For example, they could change ownership so that ``dir`` and all
|
|
files below it can be crossmapped from the filesystem's into the container's
|
|
idmapping. Let's assume they change ownership so it is compatible with the
|
|
first container's idmapping::
|
|
|
|
dir id: u10000
|
|
dir/file1 id: u11000
|
|
dir/file2 id: u12000
|
|
|
|
This would still leave ``dir`` rather useless to the second container. In fact,
|
|
``dir`` and all files below it would continue to appear owned by the overflowid
|
|
for the second container.
|
|
|
|
Or consider another increasingly popular example. Some service managers such as
|
|
systemd implement a concept called "portable home directories". A user may want
|
|
to use their home directories on different machines where they are assigned
|
|
different login userspace ids. Most users will have ``u1000`` as the login id
|
|
on their machine at home and all files in their home directory will usually be
|
|
owned by ``u1000``. At uni or at work they may have another login id such as
|
|
``u1125``. This makes it rather difficult to interact with their home directory
|
|
on their work machine.
|
|
|
|
In both cases changing ownership recursively has grave implications. The most
|
|
obvious one is that ownership is changed globally and permanently. In the home
|
|
directory case this change in ownership would even need to happen everytime the
|
|
user switches from their home to their work machine. For really large sets of
|
|
files this becomes increasingly costly.
|
|
|
|
If the user is lucky, they are dealing with a filesystem that is mountable
|
|
inside user namespaces. But this would also change ownership globally and the
|
|
change in ownership is tied to the lifetime of the filesystem mount, i.e. the
|
|
superblock. The only way to change ownership is to completely unmount the
|
|
filesystem and mount it again in another user namespace. This is usually
|
|
impossible because it would mean that all users currently accessing the
|
|
filesystem can't anymore. And it means that ``dir`` still can't be shared
|
|
between two containers with different idmappings.
|
|
But usually the user doesn't even have this option since most filesystems
|
|
aren't mountable inside containers. And not having them mountable might be
|
|
desirable as it doesn't require the filesystem to deal with malicious
|
|
filesystem images.
|
|
|
|
But the usecases mentioned above and more can be handled by idmapped mounts.
|
|
They allow to expose the same set of dentries with different ownership at
|
|
different mounts. This is achieved by marking the mounts with a user namespace
|
|
through the ``mount_setattr()`` system call. The idmapping associated with it
|
|
is then used to translate from the caller's idmapping to the filesystem's
|
|
idmapping and vica versa using the remapping algorithm we introduced above.
|
|
|
|
Idmapped mounts make it possible to change ownership in a temporary and
|
|
localized way. The ownership changes are restricted to a specific mount and the
|
|
ownership changes are tied to the lifetime of the mount. All other users and
|
|
locations where the filesystem is exposed are unaffected.
|
|
|
|
Filesystems that support idmapped mounts don't have any real reason to support
|
|
being mountable inside user namespaces. A filesystem could be exposed
|
|
completely under an idmapped mount to get the same effect. This has the
|
|
advantage that filesystems can leave the creation of the superblock to
|
|
privileged users in the initial user namespace.
|
|
|
|
However, it is perfectly possible to combine idmapped mounts with filesystems
|
|
mountable inside user namespaces. We will touch on this further below.
|
|
|
|
Remapping helpers
|
|
~~~~~~~~~~~~~~~~~
|
|
|
|
Idmapping functions were added that translate between idmappings. They make use
|
|
of the remapping algorithm we've introduced earlier. We're going to look at
|
|
two:
|
|
|
|
- ``i_uid_into_mnt()`` and ``i_gid_into_mnt()``
|
|
|
|
The ``i_*id_into_mnt()`` functions translate filesystem's kernel ids into
|
|
kernel ids in the mount's idmapping::
|
|
|
|
/* Map the filesystem's kernel id up into a userspace id in the filesystem's idmapping. */
|
|
from_kuid(filesystem, kid) = uid
|
|
|
|
/* Map the filesystem's userspace id down ito a kernel id in the mount's idmapping. */
|
|
make_kuid(mount, uid) = kuid
|
|
|
|
- ``mapped_fsuid()`` and ``mapped_fsgid()``
|
|
|
|
The ``mapped_fs*id()`` functions translate the caller's kernel ids into
|
|
kernel ids in the filesystem's idmapping. This translation is achieved by
|
|
remapping the caller's kernel ids using the mount's idmapping::
|
|
|
|
/* Map the caller's kernel id up into a userspace id in the mount's idmapping. */
|
|
from_kuid(mount, kid) = uid
|
|
|
|
/* Map the mount's userspace id down into a kernel id in the filesystem's idmapping. */
|
|
make_kuid(filesystem, uid) = kuid
|
|
|
|
Note that these two functions invert each other. Consider the following
|
|
idmappings::
|
|
|
|
caller idmapping: u0:k10000:r10000
|
|
filesystem idmapping: u0:k20000:r10000
|
|
mount idmapping: u0:k10000:r10000
|
|
|
|
Assume a file owned by ``u1000`` is read from disk. The filesystem maps this id
|
|
to ``k21000`` according to its idmapping. This is what is stored in the
|
|
inode's ``i_uid`` and ``i_gid`` fields.
|
|
|
|
When the caller queries the ownership of this file via ``stat()`` the kernel
|
|
would usually simply use the crossmapping algorithm and map the filesystem's
|
|
kernel id up to a userspace id in the caller's idmapping.
|
|
|
|
But when the caller is accessing the file on an idmapped mount the kernel will
|
|
first call ``i_uid_into_mnt()`` thereby translating the filesystem's kernel id
|
|
into a kernel id in the mount's idmapping::
|
|
|
|
i_uid_into_mnt(k21000):
|
|
/* Map the filesystem's kernel id up into a userspace id. */
|
|
from_kuid(u0:k20000:r10000, k21000) = u1000
|
|
|
|
/* Map the filesystem's userspace id down ito a kernel id in the mount's idmapping. */
|
|
make_kuid(u0:k10000:r10000, u1000) = k11000
|
|
|
|
Finally, when the kernel reports the owner to the caller it will turn the
|
|
kernel id in the mount's idmapping into a userspace id in the caller's
|
|
idmapping::
|
|
|
|
from_kuid(u0:k10000:r10000, k11000) = u1000
|
|
|
|
We can test whether this algorithm really works by verifying what happens when
|
|
we create a new file. Let's say the user is creating a file with ``u1000``.
|
|
|
|
The kernel maps this to ``k11000`` in the caller's idmapping. Usually the
|
|
kernel would now apply the crossmapping, verifying that ``k11000`` can be
|
|
mapped to a userspace id in the filesystem's idmapping. Since ``k11000`` can't
|
|
be mapped up in the filesystem's idmapping directly this creation request
|
|
fails.
|
|
|
|
But when the caller is accessing the file on an idmapped mount the kernel will
|
|
first call ``mapped_fs*id()`` thereby translating the caller's kernel id into
|
|
a kernel id according to the mount's idmapping::
|
|
|
|
mapped_fsuid(k11000):
|
|
/* Map the caller's kernel id up into a userspace id in the mount's idmapping. */
|
|
from_kuid(u0:k10000:r10000, k11000) = u1000
|
|
|
|
/* Map the mount's userspace id down into a kernel id in the filesystem's idmapping. */
|
|
make_kuid(u0:k20000:r10000, u1000) = k21000
|
|
|
|
When finally writing to disk the kernel will then map ``k21000`` up into a
|
|
userspace id in the filesystem's idmapping::
|
|
|
|
from_kuid(u0:k20000:r10000, k21000) = u1000
|
|
|
|
As we can see, we end up with an invertible and therefore information
|
|
preserving algorithm. A file created from ``u1000`` on an idmapped mount will
|
|
also be reported as being owned by ``u1000`` and vica versa.
|
|
|
|
Let's now briefly reconsider the failing examples from earlier in the context
|
|
of idmapped mounts.
|
|
|
|
Example 2 reconsidered
|
|
~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
::
|
|
|
|
caller id: u1000
|
|
caller idmapping: u0:k10000:r10000
|
|
filesystem idmapping: u0:k20000:r10000
|
|
mount idmapping: u0:k10000:r10000
|
|
|
|
When the caller is using a non-initial idmapping the common case is to attach
|
|
the same idmapping to the mount. We now perform three steps:
|
|
|
|
1. Map the caller's userspace ids into kernel ids in the caller's idmapping::
|
|
|
|
make_kuid(u0:k10000:r10000, u1000) = k11000
|
|
|
|
2. Translate the caller's kernel id into a kernel id in the filesystem's
|
|
idmapping::
|
|
|
|
mapped_fsuid(k11000):
|
|
/* Map the kernel id up into a userspace id in the mount's idmapping. */
|
|
from_kuid(u0:k10000:r10000, k11000) = u1000
|
|
|
|
/* Map the userspace id down into a kernel id in the filesystem's idmapping. */
|
|
make_kuid(u0:k20000:r10000, u1000) = k21000
|
|
|
|
2. Verify that the caller's kernel ids can be mapped to userspace ids in the
|
|
filesystem's idmapping::
|
|
|
|
from_kuid(u0:k20000:r10000, k21000) = u1000
|
|
|
|
So the ownership that lands on disk will be ``u1000``.
|
|
|
|
Example 3 reconsidered
|
|
~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
::
|
|
|
|
caller id: u1000
|
|
caller idmapping: u0:k10000:r10000
|
|
filesystem idmapping: u0:k0:r4294967295
|
|
mount idmapping: u0:k10000:r10000
|
|
|
|
The same translation algorithm works with the third example.
|
|
|
|
1. Map the caller's userspace ids into kernel ids in the caller's idmapping::
|
|
|
|
make_kuid(u0:k10000:r10000, u1000) = k11000
|
|
|
|
2. Translate the caller's kernel id into a kernel id in the filesystem's
|
|
idmapping::
|
|
|
|
mapped_fsuid(k11000):
|
|
/* Map the kernel id up into a userspace id in the mount's idmapping. */
|
|
from_kuid(u0:k10000:r10000, k11000) = u1000
|
|
|
|
/* Map the userspace id down into a kernel id in the filesystem's idmapping. */
|
|
make_kuid(u0:k0:r4294967295, u1000) = k1000
|
|
|
|
2. Verify that the caller's kernel ids can be mapped to userspace ids in the
|
|
filesystem's idmapping::
|
|
|
|
from_kuid(u0:k0:r4294967295, k21000) = u1000
|
|
|
|
So the ownership that lands on disk will be ``u1000``.
|
|
|
|
Example 4 reconsidered
|
|
~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
::
|
|
|
|
file id: u1000
|
|
caller idmapping: u0:k10000:r10000
|
|
filesystem idmapping: u0:k0:r4294967295
|
|
mount idmapping: u0:k10000:r10000
|
|
|
|
In order to report ownership to userspace the kernel now does three steps using
|
|
the translation algorithm we introduced earlier:
|
|
|
|
1. Map the userspace id on disk down into a kernel id in the filesystem's
|
|
idmapping::
|
|
|
|
make_kuid(u0:k0:r4294967295, u1000) = k1000
|
|
|
|
2. Translate the kernel id into a kernel id in the mount's idmapping::
|
|
|
|
i_uid_into_mnt(k1000):
|
|
/* Map the kernel id up into a userspace id in the filesystem's idmapping. */
|
|
from_kuid(u0:k0:r4294967295, k1000) = u1000
|
|
|
|
/* Map the userspace id down into a kernel id in the mounts's idmapping. */
|
|
make_kuid(u0:k10000:r10000, u1000) = k11000
|
|
|
|
3. Map the kernel id up into a userspace id in the caller's idmapping::
|
|
|
|
from_kuid(u0:k10000:r10000, k11000) = u1000
|
|
|
|
Earlier, the caller's kernel id couldn't be crossmapped in the filesystems's
|
|
idmapping. With the idmapped mount in place it now can be crossmapped into the
|
|
filesystem's idmapping via the mount's idmapping. The file will now be created
|
|
with ``u1000`` according to the mount's idmapping.
|
|
|
|
Example 5 reconsidered
|
|
~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
::
|
|
|
|
file id: u1000
|
|
caller idmapping: u0:k10000:r10000
|
|
filesystem idmapping: u0:k20000:r10000
|
|
mount idmapping: u0:k10000:r10000
|
|
|
|
Again, in order to report ownership to userspace the kernel now does three
|
|
steps using the translation algorithm we introduced earlier:
|
|
|
|
1. Map the userspace id on disk down into a kernel id in the filesystem's
|
|
idmapping::
|
|
|
|
make_kuid(u0:k20000:r10000, u1000) = k21000
|
|
|
|
2. Translate the kernel id into a kernel id in the mount's idmapping::
|
|
|
|
i_uid_into_mnt(k21000):
|
|
/* Map the kernel id up into a userspace id in the filesystem's idmapping. */
|
|
from_kuid(u0:k20000:r10000, k21000) = u1000
|
|
|
|
/* Map the userspace id down into a kernel id in the mounts's idmapping. */
|
|
make_kuid(u0:k10000:r10000, u1000) = k11000
|
|
|
|
3. Map the kernel id up into a userspace id in the caller's idmapping::
|
|
|
|
from_kuid(u0:k10000:r10000, k11000) = u1000
|
|
|
|
Earlier, the file's kernel id couldn't be crossmapped in the filesystems's
|
|
idmapping. With the idmapped mount in place it now can be crossmapped into the
|
|
filesystem's idmapping via the mount's idmapping. The file is now owned by
|
|
``u1000`` according to the mount's idmapping.
|
|
|
|
Changing ownership on a home directory
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
We've seen above how idmapped mounts can be used to translate between
|
|
idmappings when either the caller, the filesystem or both uses a non-initial
|
|
idmapping. A wide range of usecases exist when the caller is using
|
|
a non-initial idmapping. This mostly happens in the context of containerized
|
|
workloads. The consequence is as we have seen that for both, filesystem's
|
|
mounted with the initial idmapping and filesystems mounted with non-initial
|
|
idmappings, access to the filesystem isn't working because the kernel ids can't
|
|
be crossmapped between the caller's and the filesystem's idmapping.
|
|
|
|
As we've seen above idmapped mounts provide a solution to this by remapping the
|
|
caller's or filesystem's idmapping according to the mount's idmapping.
|
|
|
|
Aside from containerized workloads, idmapped mounts have the advantage that
|
|
they also work when both the caller and the filesystem use the initial
|
|
idmapping which means users on the host can change the ownership of directories
|
|
and files on a per-mount basis.
|
|
|
|
Consider our previous example where a user has their home directory on portable
|
|
storage. At home they have id ``u1000`` and all files in their home directory
|
|
are owned by ``u1000`` whereas at uni or work they have login id ``u1125``.
|
|
|
|
Taking their home directory with them becomes problematic. They can't easily
|
|
access their files, they might not be able to write to disk without applying
|
|
lax permissions or ACLs and even if they can, they will end up with an annoying
|
|
mix of files and directories owned by ``u1000`` and ``u1125``.
|
|
|
|
Idmapped mounts allow to solve this problem. A user can create an idmapped
|
|
mount for their home directory on their work computer or their computer at home
|
|
depending on what ownership they would prefer to end up on the portable storage
|
|
itself.
|
|
|
|
Let's assume they want all files on disk to belong to ``u1000``. When the user
|
|
plugs in their portable storage at their work station they can setup a job that
|
|
creates an idmapped mount with the minimal idmapping ``u1000:k1125:r1``. So now
|
|
when they create a file the kernel performs the following steps we already know
|
|
from above:::
|
|
|
|
caller id: u1125
|
|
caller idmapping: u0:k0:r4294967295
|
|
filesystem idmapping: u0:k0:r4294967295
|
|
mount idmapping: u1000:k1125:r1
|
|
|
|
1. Map the caller's userspace ids into kernel ids in the caller's idmapping::
|
|
|
|
make_kuid(u0:k0:r4294967295, u1125) = k1125
|
|
|
|
2. Translate the caller's kernel id into a kernel id in the filesystem's
|
|
idmapping::
|
|
|
|
mapped_fsuid(k1125):
|
|
/* Map the kernel id up into a userspace id in the mount's idmapping. */
|
|
from_kuid(u1000:k1125:r1, k1125) = u1000
|
|
|
|
/* Map the userspace id down into a kernel id in the filesystem's idmapping. */
|
|
make_kuid(u0:k0:r4294967295, u1000) = k1000
|
|
|
|
2. Verify that the caller's kernel ids can be mapped to userspace ids in the
|
|
filesystem's idmapping::
|
|
|
|
from_kuid(u0:k0:r4294967295, k1000) = u1000
|
|
|
|
So ultimately the file will be created with ``u1000`` on disk.
|
|
|
|
Now let's briefly look at what ownership the caller with id ``u1125`` will see
|
|
on their work computer:
|
|
|
|
::
|
|
|
|
file id: u1000
|
|
caller idmapping: u0:k0:r4294967295
|
|
filesystem idmapping: u0:k0:r4294967295
|
|
mount idmapping: u1000:k1125:r1
|
|
|
|
1. Map the userspace id on disk down into a kernel id in the filesystem's
|
|
idmapping::
|
|
|
|
make_kuid(u0:k0:r4294967295, u1000) = k1000
|
|
|
|
2. Translate the kernel id into a kernel id in the mount's idmapping::
|
|
|
|
i_uid_into_mnt(k1000):
|
|
/* Map the kernel id up into a userspace id in the filesystem's idmapping. */
|
|
from_kuid(u0:k0:r4294967295, k1000) = u1000
|
|
|
|
/* Map the userspace id down into a kernel id in the mounts's idmapping. */
|
|
make_kuid(u1000:k1125:r1, u1000) = k1125
|
|
|
|
3. Map the kernel id up into a userspace id in the caller's idmapping::
|
|
|
|
from_kuid(u0:k0:r4294967295, k1125) = u1125
|
|
|
|
So ultimately the caller will be reported that the file belongs to ``u1125``
|
|
which is the caller's userspace id on their workstation in our example.
|
|
|
|
The raw userspace id that is put on disk is ``u1000`` so when the user takes
|
|
their home directory back to their home computer where they are assigned
|
|
``u1000`` using the initial idmapping and mount the filesystem with the initial
|
|
idmapping they will see all those files owned by ``u1000``.
|