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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 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 X-Stat-Signature: f87dw4qbk6yro1x4twkgn9uyzm98w4xs X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 07A35C0008 X-Rspam-User: X-HE-Tag: 1766306781-6049 X-HE-Meta: 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 Y6cgC/xm jSKRCxf3wKQ9RoIM74E4Yyc2reSL+y57lYBMS2aTehWSqn89o7TpnZNCGSzYG8v1efh3deL0LgnAKC/jOJ/e63bpXMMm3vRQpJqWetjONnjyxt6wq28EV3uh/FMDPKiRkSKvmqW0EjgKhHDQxTfPnknC3GfkSkEp767FZ6on9UDZ/B2ZJmKy2KPhCHhkMIBMXPJFbNRr/3pC15DcOZ/sbgXG8VLkHMJJecmBK7+pLWYtpFKx91emEByPmo6PEsSg+GVKYbAzqNNTuGZypz/Jb/Z0f5Q/dYNkkug+BfddGljbdE1CLnaZnRVy+U9ntonUgSUh5Qx95qAg1jPY= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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