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

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Message-ID: <66cfc9ba-868c-9620-fbfc-38931c76ff50@redhat.com>
Date:   Mon, 28 Nov 2022 18:37:47 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Jann Horn <jannh@...gle.com>, security@...nel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     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 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?

Change looks sane on quick glimpse.

-- 
Thanks,

David / dhildenb