From: Kyle Zeng <kylebot@openai.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org, David Hildenbrand <david@kernel.org>,
Lorenzo Stoakes <ljs@kernel.org>, Rik van Riel <riel@surriel.com>,
"Liam R. Howlett" <liam@infradead.org>,
Vlastimil Babka <vbabka@kernel.org>, Harry Yoo <harry@kernel.org>,
Jann Horn <jannh@google.com>, Lance Yang <lance.yang@linux.dev>,
linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH] mm/page_vma_mapped: continue past nonmatching huge PMDs
Date: Thu, 15 Oct 2026 19:56:38 -0700 [thread overview]
Message-ID: <atGSZkBHtsnJ4k1A@com-75606> (raw)
In-Reply-To: <20261015192020.5336a49bc516219e8254dfdf@linux-foundation.org>
On Thu, Oct 15, 2026 at 07:20:20PM -0700, Andrew Morton wrote:
> On Thu, 15 Oct 2026 18:15:49 -0700 Kyle Zeng <kylebot@openai.com> wrote:
>
> > A PTE-mapped large folio can straddle PMD boundaries after an unaligned
> > mremap(). If the first part is discarded and replaced with an unrelated
> > huge PMD, page_vma_mapped_walk() stops instead of looking for the rest
> > of the folio in subsequent PMDs.
> >
> > This also affects migration entries. A walk that stops at an unrelated
> > huge PMD can leave migration PTEs behind when migration or an anonymous
> > folio split restores the mappings. Those entries do not hold mapping
> > references and can outlive the source folio.
> >
> > Skip the current PMD rather than ending the walk on a flag or PFN
> > mismatch. Do the same if the PMD became nonpresent before the locked
> > recheck. Release the PMD lock, clear the per-PMD state and advance with
> > step_forward(), so the existing range limit controls termination.
> > This covers present, migration and device-private huge PMDs while
> > preserving the matching-PMD and split-to-PTE paths.
>
> As always, when fixing a bug please describe the worst-case
> userspace-visible effects of the bug.
>
> I asked $LLM and was told
>
> : The worst credible effects are permanent application hangs, kernel
> : crashes, and potentially system-wide denial of service. Stale
> : migration entries can refer to freed and reused physical pages, causing
> : page faults to access unrelated folio metadata. Memory corruption is
> : possible but unproven; cross-process information disclosure has not
> : been established.
>
> But you have an LLM too! So please never again send a bugfix without
> this info.
>
> Also please explain whether there's a reproducer and if so, at least
> describe it and confirm the before-and-after results.
>
> Thanks.
>
Hi Andrew,
This issue can lead to memory corruption and I have a reproducer for it.
I can update the description and send in a v2 if you think it is proper.
Best,
Kyle
next prev parent reply other threads:[~2026-10-16 2:56 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-10-16 1:15 [PATCH] mm/page_vma_mapped: continue past nonmatching huge PMDs Kyle Zeng
2026-10-16 2:20 ` Andrew Morton
2026-10-16 2:56 ` Kyle Zeng [this message]
2026-10-16 7:46 ` David Hildenbrand (Arm)
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