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

[Jann Horn]

On Mon, Nov 28, 2022 at 6:37 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> 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?

You're on the CC list of my bug report to security@xxxxxxxxxx
with title "khugepaged races with rmap-based zap, races with GUP-fast,
and fails to call MMU notifiers". That has an attached reproducer
thp_ro_no_notify_kvm.c that is able to read PAGE_POISON out of freed
file THP pages through KVM.