Re: [PATCH 0/2] mount: add OPEN_TREE_NAMESPACE

[Askar Safin <safinaskar@gmail.com>]

Christian Brauner <brauner@kernel.org>:
> Extend open_tree() with a new OPEN_TREE_NAMESPACE flag. Similar to
> OPEN_TREE_CLONE only the indicated mount tree is copied. Instead of
> returning a file descriptor referring to that mount tree
> OPEN_TREE_NAMESPACE will cause open_tree() to return a file descriptor
> to a new mount namespace. In that new mount namespace the copied mount
> tree has been mounted on top of a copy of the real rootfs.

I want to point at security benefits of this.

[[ TL;DR: [1] and [2] are very big changes to how mount namespaces work.
I like them, and I think they should get wider exposure. ]]

If this patchset ([1]) and [2] both land (they are both in "next" now and
likely will be submitted to mainline soon) and "nullfs_rootfs" is passed on
command line, then mount namespace created by open_tree(OPEN_TREE_NAMESPACE) will
usually contain exactly 2 mounts: nullfs and whatever was passed to
open_tree(OPEN_TREE_NAMESPACE).

This means that even if attacker somehow is able to unmount its root and
get access to underlying mounts, then the only underlying thing they will
get is nullfs.

Also this means that other mounts are not only hidden in new namespace, they
are fully absent. This prevents attacks discussed here: [3], [4].

Also this means that (assuming we have both [1] and [2] and "nullfs_rootfs"
is passed), there is no anymore hidden writable mount shared by all containers,
potentially available to attackers. This is concern raised in [5]:

> You want rootfs to be a NULLFS instead of ramfs. You don't seem to want it to
> actually _be_ a filesystem. Even with your "fix", containers could communicate
> with each _other_ through it if it becomes accessible. If a container can get
> access to an empty initramfs and write into it, it can ask/answer the question
> "Are there any other containers on this machine running stux24" and then coordinate.

Note: as well as I understand all actual security bugs are already fixed in kernel,
runc and similar tools. But still [1] and [2] reduce chances of similar bugs
in the future, and this is very good thing.

Also: [1] and [2] are pretty big changes to how mount namespaces work, so
I added more people and lists to CC.

This mail is answer to [1].

[1] https://lore.kernel.org/all/20251229-work-empty-namespace-v1-0-bfb24c7b061f@kernel.org/
[2] https://lore.kernel.org/all/20260112-work-immutable-rootfs-v2-0-88dd1c34a204@kernel.org/

[3] https://lore.kernel.org/all/rxh6knvencwjajhgvdgzmrkwmyxwotu3itqyreun3h2pmaujhr@snhuqoq44kkf/
[4] https://github.com/opencontainers/runc/pull/1962
[5] https://lore.kernel.org/all/cec90924-e7ec-377c-fb02-e0f25ab9db73@landley.net/

-- 
Askar Safin