Re: [PATCH v3 3/3] mm/khugepaged: Invoke MMU notifiers in shmem/file collapse paths

[David Hildenbrand]

On 25.11.22 22:37, Jann Horn wrote:
> Any codepath that zaps page table entries must invoke MMU notifiers to
> ensure that secondary MMUs (like KVM) don't keep accessing pages which
> aren't mapped anymore. Secondary MMUs don't hold their own references to
> pages that are mirrored over, so failing to notify them can lead to page
> use-after-free.
>
> I'm marking this as addressing an issue introduced in commit f3f0e1d2150b
> ("khugepaged: add support of collapse for tmpfs/shmem pages"), but most of
> the security impact of this only came in commit 27e1f8273113 ("khugepaged:
> enable collapse pmd for pte-mapped THP"), which actually omitted flushes
> for the removal of present PTEs, not just for the removal of empty page
> tables.
>
> Cc: stable@xxxxxxxxxx
> Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages")

I'm curious, do you have a working reproducer for this?

Change looks sane on quick glimpse.

--
Thanks,

David / dhildenb