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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=MMPJjA4n; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B9AC960A3828 for ; Mon, 2 Nov 2026 12:49:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=MMPJjA4n Received: by smtp.kernel.org (Postfix) with ESMTPSA id D3C4F1F00A16; Mon, 2 Nov 2026 12:49:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793623747; bh=yJLZmPBZv+CycFtwzr5pSIbvk5UEpxdi0CgzoRsWvzw=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=MMPJjA4na183XXSB/egQ9V5ON4+8CaoBH9DPVdEWUHbIAMBb04H9NjYoh9ROfyUbn 3pxOf7dLjNDEHak6Ud5YERJgeEe5E9Qf6D6/drb9lpM+YqWN0nPKhpZ81h1/DSCxrM kyCVvJO4dp130ffKnTQDY6VKVTmASgwZw535cMSHQltvaTEMqXeclFU1VczrKrOf8r pOrZd/Ui6t37y6G/6TmpRitEfUg9mNGuxA4kTijCkQgBYqQDYhBSFfksoo7ufUVBfQ 65L5YXuwcPAK9CLNF2gIJx2KZmh4+OAevQcAA5Mnd1bLaKSdC158CDdWNrITIDb2Rj ojzlCG5UIafdw== Message-ID: <54e20549-ca49-4d86-90fd-9f5e483d362d@kernel.org> Date: Mon, 2 Nov 2026 13:49:02 +0100 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/ksm: avoid unsafe anon rmap dereference in KSM merge To: "Lorenzo Stoakes (ARM)" , Xu Xin , Chengming Zhou , Andrew Morton , Rik van Riel Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Zhao Gongyi References: <20261102-ksm-anon-rmap-fix-v1-1-68962acfb59b@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/2/26 13:32, Lorenzo Stoakes (ARM) wrote: > When KSM examines candidate VMAs in scan_get_next_rmap_item(), it > accumulates (mm, address) pairs in ksm_rmap_item objects, assigning them > via get_next_rmap_item(), pinning folios via a page walk in > ksm_next_page_pmd_entry(). > > Throughout this operation, scan_get_next_rmap_item() holds mmap read lock, > which it drops afterwards. > > After the mmap lock is dropped the VMAs might be freed and remapped where > the VMA belonging to the folio was previously mapped. > > mremap()'s ability to change where anon folios are mapped is dealt with by > KSM by essentially avoiding the problem - prep_move_vma() unshares KSM > folios, and the scanning logic is tolerant of concurrent remaps, discarding > stale ksm_rmap_item objects and allocating new ones at the new address. > > This doesn't account for a new VMA being mapped over an old one where a > folio was previously mapped, so KSM accounts for this by checking things at > the page table level. > > However, once an anon_vma's reference count reaches zero, it is freed, but > nothing clears folio->mapping, which is where the KSM pin interacts badly > with the current KSM implementation. > > page_address_in_vma() is consistently used to look up the folio's address, > but page_address_in_vma() explicitly states: > > "It is the caller's responsibility to check the page is actually within the > VMA." > > And KSM violates this, as it cannot know this to be true. This results in > this function performing a use-after-free: > > struct anon_vma *anon_vma = folio_anon_vma(folio); > > ... > > if (!vma_has_anon_rmap(vma) || !anon_vma || > vma->anon_vma->root != anon_vma->root) > return -EFAULT; > > As anon_vma->root is dereferenced when anon_vma can already have been > freed. > > This is currently benign in practice, as anon_vma's are slab-allocated > objects so this will always be within the direct mapping and arbitrary > memory will be treated as if it were a pointer to be compared. > > This either results in an -EFAULT or a spurious match, which will be > rejected at the page table level. > > However we mustn't UAF even if it's harmless, and KASAN rightly flags this. > > It turns out we don't even need to use page_address_in_vma() as we already > know the folio's address, it simply needs to be passed through. > > This change therefore updates KSM to do this, sidestepping the issue > altogether. > > Explicitly check for KSM folios in try_to_merge_one_page(), which are > already merged, and by definition never have an anon_vma. > > This was previously handled implicitly by page_address_in_vma() as > folio_anon_vma() returns NULL for those. > > In many cases cmp_and_merge_page() will reject these, but it's possible for > KSM folios to fall into the unstable tree there, so eliminate that > possibility by checking for that early. > > This maintains existing behaviour. > > The bug was introduced by commit 21d0d443cdc1 ("rmap: resurrect > page_address_in_vma anon_vma check") which first dereferenced > folio->mapping unsafely, but commit b8c8ba73c68b ("slub: Introduce > CONFIG_SLUB_RCU_DEBUG") made it visible with CONFIG_SLUB_RCU_DEBUG enabled. > > Fixes: 21d0d443cdc1 ("rmap: resurrect page_address_in_vma anon_vma check") > Reported-by: Zhao Gongyi > Closes: https://lore.kernel.org/all/20261031182256.40205-1-zhaogongyi@bytedance.com/ > Signed-off-by: Lorenzo Stoakes (ARM) > --- > David - this seems benign in practice, so I'm not sure whether a backport > is required here. Ok, thanks. Is there a similar problem as with the other anon_vma->root issue where a false positive match could be problematic? I suspect not. In that case I would pick this for the next merge window after a couple of days. [...] > { > struct folio *folio = page_folio(page); > pte_t orig_pte = __pte(0); > @@ -1515,7 +1507,7 @@ static int try_to_merge_one_page(struct vm_area_struct *vma, > if (page == kpage) /* ksm page forked */ > return 0; > > - if (!folio_test_anon(folio)) > + if (!folio_test_anon(folio) || folio_test_ksm(folio)) > goto out; Okay, that makes sense. We don't have to check under the folio lock as KSM doesn't merge concurrently. Do we want to assert in write_protect_page() that we are not getting called with a KSM folio, similar to what we do with large folios in there? -- Cheers, David