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Date:   Mon, 28 Nov 2022 18:37:47 +0100
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Subject: Re: [PATCH v3 3/3] mm/khugepaged: Invoke MMU notifiers in shmem/file
 collapse paths
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To:     Jann Horn <jannh@google.com>, security@kernel.org,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Yang Shi <shy828301@gmail.com>, Peter Xu <peterx@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
References: <20221125213714.4115729-1-jannh@google.com>
 <20221125213714.4115729-3-jannh@google.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20221125213714.4115729-3-jannh@google.com>
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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@kernel.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