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

[Christian Brauner <brauner@kernel.org>]

On Wed, Jan 21, 2026 at 10:00:19AM -0800, Andy Lutomirski wrote:
> > On Jan 19, 2026, at 2:21 PM, Jeff Layton <jlayton@kernel.org> wrote:
> >
> > On Mon, 2026-01-19 at 11:05 -0800, Andy Lutomirski wrote:
> >>> On Mon, Jan 19, 2026 at 10:56 AM Askar Safin <safinaskar@gmail.com> wrote:
> >>>
> >>> 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.
> >>
> >> I think this new OPEN_TREE_NAMESPACE is nifty, but I don't think the
> >> path that gives it sensible behavior should be conditional like this.
> >> Either make it *always* mount on top of nullfs (regardless of boot
> >> options) or find some way to have it actually be the root.  I assume
> >> the latter is challenging for some reason.
> >>
> >
> > I think that's the plan. I suggested the same to Christian last week,
> > and he was amenable to removing the option and just always doing a
> > nullfs_rootfs mount.
> >
> > We think that older runtimes should still "just work" with this scheme.
> > Out of an abundance of caution, we _might_ want a command-line option
> > to make it go back to old way, in case we find some userland stuff that
> > doesn't like this for some reason, but hopefully we won't even need
> > that.
> 
> What I mean is: even if for some reason the kernel is running in a
> mode where the *initial* rootfs is a real fs, I think it would be nice
> for OPEN_TREE_NAMESPACE to use nullfs.

The current patchset makes nullfs unconditional. As each mount
namespaces creates a new copy of the namespace root of the namespace it
was created from all mount namespace have nullfs as namespace root.
So every OPEN_TREE_NAMESPACE/FSMOUNT_NAMESPACE will be mounted on top of
nullfs as we always take the namespace root. If we have to make nullfs
conditional then yes, we could still do that - althoug it would be ugly
in various ways.

I would love to keep nullfs unconditional because it means I can wipe a
whole class of MNT_LOCKED nonsense from the face of the earth
afterwards.