fs: don't silently unmount busy mounts

Summary

In the Linux kernel, the following vulnerability has been resolved:

fs: don't silently unmount busy mounts

The propagate_mount_busy() helper exists to decide whether a synchronous can suceed. It does that by looking at the copies of the victims in the mounts its parent propagates to. One of those functions from propagation hell.

It skips a copy that has child mounts except for the case of a single child mount covering the copy's root. That algorithm used to be exact. It isn't anymore.

Originally only childless copies were unmounted during propagation. And 1064f874abc0 ("mnt: Tuck mounts under others instead of creating shadow/side mounts.") ended up adding covered mounts to both sides in one go.

Along came 99b19d16471e ("mnt: In propgate_umount handle visiting mounts in any order") and that started to unmount more than before:

[1]: A copy of a mount is unmounted when each of its children is either its overmount or a copy of the victim itself. In the former case the overmount gets reparented. In the latter case the copy of the victim will end up being unmounted.

So propagate_umount() ended up being rewritten without adjusting propagate_mount_busy(). And currently propagate_mount_busy() doesn't look at copies of the victim that have two or more children nor a single child that is not the overmount.

So any such copy in [1] is not checked for references. The result is that a synchronous unmount succeeds, pulling out a mount that is still in use.

A container can run into this with its own mounts. The host shares a tree with the container and the host has a mount in that tree:

mount -t tmpfs none /mnt mount --make-shared /mnt mount -t tmpfs none /mnt/a

The container's /mnt is a slave copy of the host mount. The container now takes a detached copy of the tree and mounts that copy beneath its propagated copy of /mnt/a and keeps the file descriptor to the tree open:

unshare -m --propagation unchanged mount --make-rslave / fd = open_tree(AT_FDCWD, "/mnt", OPEN_TREE_CLONE | AT_RECURSIVE) move_mount(fd, "", AT_FDCWD, "/mnt/a", MOVE_MOUNT_BENEATH)

grep /mnt /proc/self/mountinfo 775 355 ... /mnt ... master:646 777 775 ... /mnt/a ... master:646 776 777 ... /mnt/a ... master:647 778 777 ... /mnt/a/a ... master:647

The now attached tree (777) is now mounted beneath the container's copy of /mnt/a. That's also where the host mount sits (as seen from the host mount namespace). So the propagated copy (776) of the host's mount is on top of its root and the copy of that (778) is inside it. Now the host runs:

umount /mnt/a

and it succeeds. The moved tree tree and the copy inside of it are gone from the container. Now only the propagated copy is left and it got reparented to where it was before:

grep /mnt /proc/self/mountinfo 775 355 ... /mnt ... master:646 776 775 ... /mnt/a ...

But the file descriptor still refers to the moved tree. So a synchronous umount detached a mount that is still in use.

Teach propagate_mount_busy() the same checks that propagate_umount() does. It now checks for a single victim mount without children for which it can assume that MNT_LOCKED is cleared on every copy like propagate_umount() does. The copies form chains. IOW, when a copy itself receives propagation from the victim's parent then the mount at the same mountpoint inside that copy is a copy as well. Whether a mount receives propagation from the victim's can simply be checked by walking its masters. Walk each chain once from its top so we are sure that nested copies stay linear. Check every reference once of every copy that gets unmounted.