From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 626A42798E6 for ; Sun, 21 Dec 2025 08:46:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766306782; cv=none; b=p0LwuIy8wc3V5JZ6kaBBReWMooqx40fHRQ66+g0Xvo/n/e4yKjGFIwnVjxauuuvRCaAPyaHz7grNGMRw9ikGh25sdtSvJvd5id9CFk0Y5Id+JPH7d5Z8sERuKMJlyhpUl7smz9qjsjGhA2yZ/TvgNMyKbR9HBOxw+FIRRG/RHJU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766306782; c=relaxed/simple; bh=C/lFhvRpPUcNkUM6h2eXqKL+1YFxypTZXcHMoqeNcYI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=S+m/6FFNmbForQ/xLEiiit2K/uROX/+SeqGWZNafOz5w6Bxb7XWlH777UDFZGQdWJsYRcBgEPBkJwG0VZGUVIuteCs2Gdyhaj2bOX5VYg1dPV1DNcFfYojdse2O6WFncPpsQ1Mt9Xo9dj2QZCkJ7ij7wnoyMlTAZo+J0RKkruG8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Kl3xf14C; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Kl3xf14C" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4DCCEC4CEFB; Sun, 21 Dec 2025 08:46:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1766306781; bh=C/lFhvRpPUcNkUM6h2eXqKL+1YFxypTZXcHMoqeNcYI=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Kl3xf14CY+PpQ2TO+0BSMolEDYozSQLJ869XFNw/O6Tze1rtIvDiYb6xBx4hzPytB hjR4h1dPWeNntcGydCCMiikxfcdTs0L6qKGIs8mSRkR0xcwtAXNiblJD6IN64EYkd/ c6FM3S6rAsdjX7sLNi1y2LpBrwsoZgeNJLC5W0iQG3hfkcOxe1T65ipSVneqvOimQH rGFu88IRwf/tUpIG/+gqt9qP7qwPkNAbLKTuQmP2qcGH0nCGUXGHnBXg0PlZAJ8hXo mtzWJye9X5EyYPel9ndMfyJ24AZU8G9q3nJlACHwth80o++jYV+amTaSQbK7kMVEs4 CVsc4UWrJ0BUw== Message-ID: Date: Sun, 21 Dec 2025 09:46:16 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/ksm: fix pte_unmap_unlock of wrong address in break_ksm_pmd_entry To: Sasha Levin , akpm@linux-foundation.org Cc: xu.xin16@zte.com.cn, chengming.zhou@linux.dev, pedrodemargomes@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20251220202926.318366-1-sashal@kernel.org> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: <20251220202926.318366-1-sashal@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 12/20/25 21:29, Sasha Levin wrote: > On ARM32 with HIGHMEM/HIGHPTE, break_ksm_pmd_entry() triggers a BUG > during KSM unmerging because pte_unmap_unlock() is passed a pointer > that may be beyond the mapped PTE page. > > The issue occurs when the PTE iteration loop completes without finding > a KSM page. After the loop, 'ptep' has been incremented past the last > PTE entry. On ARM32 LPAE with 512 PTEs per page (512 * 8 = 4096 bytes), > this means ptep points to the next page, outside the kmap'd region. > > When pte_unmap_unlock(ptep, ptl) calls kunmap_local(ptep), it unmaps > the wrong page address, leaving the original kmap slot still mapped. > The next kmap_local then finds this slot unexpectedly occupied: > > WARNING: mm/highmem.c:622 kunmap_local_indexed (address mismatch) > kernel BUG at mm/highmem.c:564 __kmap_local_pfn_prot (slot not empty) > > Fix this by passing start_ptep to pte_unmap_unlock(), which always > points within the originally mapped PTE page. > > Reproducer: Run LTP ksm03 test on ARM32 with HIGHMEM enabled. The test > triggers KSM merging followed by unmerging (writing 0 then 2 to > /sys/kernel/mm/ksm/run), which exercises break_ksm_pmd_entry(). > > Fixes: 5d4939fc2258 ("ksm: perform a range-walk in break_ksm") > Assisted-by: claude-opus-4-5-20251101 > Signed-off-by: Sasha Levin > --- > mm/ksm.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/ksm.c b/mm/ksm.c > index cfc182255c7b..2d89a7c8b4eb 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -650,7 +650,7 @@ static int break_ksm_pmd_entry(pmd_t *pmdp, unsigned long addr, unsigned long en > } > } > out_unlock: > - pte_unmap_unlock(ptep, ptl); > + pte_unmap_unlock(start_ptep, ptl); > return found; > } > Acked-by: David Hildenbrand (Red Hat) -- Cheers David