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

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Date:   Mon, 28 Nov 2022 18:57:58 +0100
From:   Jann Horn <jannh@...gle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     security@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
        Yang Shi <shy828301@...il.com>, Peter Xu <peterx@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 3/3] mm/khugepaged: Invoke MMU notifiers in shmem/file
 collapse paths

On Mon, Nov 28, 2022 at 6:37 PM David Hildenbrand <david@...hat.com> 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@...nel.org
> > 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@...nel.org
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.