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

[David Hildenbrand]

On 28.11.22 18:57, Jann Horn wrote:
> 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.

Ah, the mail from early October, thanks (drowning in mail).

You're amazingly skilled at writing reproducers.

--
Thanks,

David / dhildenb