From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7CAFA23ABBF; Tue, 13 Oct 2026 09:30:44 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791883845; cv=none; b=Ro0WHpw+J/DMOw4A3ErlrkKGXZERu1oU93l/1qnDW060X1E2GHil/w9FBIrWmnrGbXjkj+3AqYqTQG5CRMAsMpnC3OiQTUK+c9PwTL3NNY2s2owTj345GGbX1QrUcJgqn9Gop41ZF9p8npnvB6LRHomwOu5QIrdXbv2nEt3AFfg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791883845; c=relaxed/simple; bh=iQy3zVnbluN2KJGjJ0s/IVJ7Cbz/6lovy1SbuMmgvbg=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=cf0mkctRKdbIRZaGzm5uP4PYpZ5C6bWgVhhZqa6u51GQvXeJI5RaYWPKnrYgptELGocaQ1IagE/lobHfJFmTwoesN7B0TKVaduYK97ni/h3O+ke/sMWPf4TfR6scMr2UZpfD+0UdeRIzvO9d/fUTcjcet0zNdVE29lHxIlAtZnI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=mihHl0rN; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="mihHl0rN" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 31AA61F000FF; Tue, 13 Oct 2026 09:30:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791883844; bh=GDCBsEYoZUsLsXk6wyffkPfdwkrEP+ByhgCH/FzXMM0=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=mihHl0rNLCQg4W0s0+z1en8+7AJqOHOYrKKJKz5IhLlWBh/WFVyTbeSQjkYoIVSzt DGPTrHwtRRyDPBfIaEcFHjYmdI9jiru9Fxn6+uf2Fg3fjT5HSDPt1gvNul1q3TwYl4 I7Oeat+WqwQguzZL/k3f5B0O2N10bHH4poIAs2B5oxBRQTxPbNdopI0BoH6kenivHs 6n0br9igd382Ry5UeEOmzq6VovM50w6O4ULlzX2G2Ubxn/ahH/pFLWqkEBX+PnDLhY nZ11PYfc9VBqj2iBPNUVlwOq1CvyCOPPJ5AomYRjP4Nk7omM6k+IILv674Q4Xl2REy LS/CIKh05DgiA== Date: Tue, 13 Oct 2026 10:30:37 +0100 From: "Lorenzo Stoakes (ARM)" To: Zi Yan Cc: Andrew Morton , David Hildenbrand , Rik van Riel , "Liam R. Howlett" , Vlastimil Babka , Harry Yoo , Jann Horn , Lance Yang , Kyle Zeng , stable@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm/rmap: don't discard lazyfree THPs at PMD level Message-ID: References: <20261012-lazyfree-stable-fix-v2-1-6387d6e30063@nvidia.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261012-lazyfree-stable-fix-v2-1-6387d6e30063@nvidia.com> On Mon, Oct 12, 2026 at 10:20:16PM -0400, Zi Yan wrote: > __discard_anon_folio_pmd_locked() clears a lazyfree THP's PMD before it > determines whether the folio can be discarded, and restores it if the folio > is dirty or has extra references. It is done under only the PMD lock and > the anon_vma read lock inside try_to_unmap_one() and no mmap lock is held. > The temporarily none PMD causes UAF. > > A parallel munmap() or mremap() is supposed to serialize with it on the PMD > lock, but zap_pmd_range() (called by munmap()) and move_page_tables() > (called by mremap()) do not take the lock when they see the temporarily > none PMD and skip the PMD. There is no more page table walk after the none > PMD is restored: > > - munmap(): free_pgtables() waits in unlink_anon_vmas() for the anon_vma > lock, then free_pmd_range() drops the restored PMD as bad without > removing the rmap, so the folio keeps a stale mapcount. > > - whole-VMA MREMAP_DONTUNMAP: dontunmap_complete() unlinks the source VMA > from its anon_vma while the restored PMD still maps the folio. > > - plain mremap(): the THP is not moved but unmapped, so the THP's data is > lost. > > In the first two cases, the folio stays mapped, but no VMA that maps it is > linked to its anon_vma. When the anon_vma is freed, it is still reachable > when one rmap walks the folio. Racing lazyfree reclaim against > MREMAP_DONTUNMAP as an unprivileged user reproduces a KASAN use-after-free > in folio_lock_anon_vma_read() as reported in the Closes link. > > One solution is to use an invalidated PMD instead of a cleared one until > the discard is committed, but it needs arch fixes first. > 1. GUP-fast still follows an invalidated PMD on riscv, LoongArch and s390. > 2. sparc64 miscounts thp_pte_count when an invalidated PMD is set again or > cleared. > > To keep the fix simple for backport, stop discarding lazyfree THPs at PMD > level instead. try_to_unmap_one() splits the PMD and discards the folio at > PTE level. > > unmap_huge_pmd_locked() and __discard_anon_folio_pmd_locked() are left in > place, unused, to minimize this fix for backport and the invalidated PMD > approach is planned as a follow-up series to use them again. > Amazing, thanks for that! All LGTM. > Fixes: 735ecdfaf4e8 ("mm/vmscan: avoid split lazyfree THP during shrink_folio_list()") > Reported-by: Kyle Zeng > Closes: https://lore.kernel.org/r/20261009165214.40212-2-kylebot@openai.com > Cc: stable@vger.kernel.org > Assisted-by: LLM > Reviewed-by: Kyle Zeng > Reviewed-by: Lance Yang > Acked-by: David Hildenbrand (Arm) > Reviewed-by: Lorenzo Stoakes (ARM) > Signed-off-by: Zi Yan > --- > The proposed fix[1] requires additional arch code fixes[2]. Disable it > first for stable fix. The function will be re-enabled along with arch code > fixes. > > Link: https://lore.kernel.org/all/20261009165214.40212-2-kylebot@openai.com/ [1] > Link: https://lore.kernel.org/all/DM0II2QHWWS1.2RXEJXC8S0101@nvidia.com/ [2] > --- > Changes in v2: > 1. added more details on how UAF can happen. > 2. provides more details on why the invalidated PMD approach is not used. > 3. explained why unmap_huge_pmd_locked() and > __discard_anon_folio_pmd_locked() are left unused. > - Link to v1: https://patch.msgid.link/20261009-lazyfree-stable-fix-v1-1-70997958ef8d@nvidia.com > --- > mm/rmap.c | 11 ----------- > 1 file changed, 11 deletions(-) > > diff --git a/mm/rmap.c b/mm/rmap.c > index 805db93fe0428..1131b76bbbc28 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -2275,17 +2275,6 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, > } > > if (!pvmw.pte) { > - if (folio_test_lazyfree(folio)) { > - if (unmap_huge_pmd_locked(vma, pvmw.address, pvmw.pmd, folio)) > - goto walk_done; > - /* > - * unmap_huge_pmd_locked has either already marked > - * the folio as swap-backed or decided to retain it > - * due to GUP or speculative references. > - */ > - goto walk_abort; > - } > - > if (flags & TTU_SPLIT_HUGE_PMD) { > /* > * We temporarily have to drop the PTL and > > --- > base-commit: 8b38ed9ab5b09c8ba168cbcc49524e9b380ee5c4 > change-id: 20261009-lazyfree-stable-fix-67712b2049dd > > Best regards, > -- > Yan, Zi > -- Cheers, Lorenzo