[PATCH] mm/ksm: fix pte_unmap_unlock of wrong address in break_ksm_pmd_entry

[Sasha Levin <sashal@kernel.org>]

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 <sashal@kernel.org>
---
 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;
 }
 
-- 
2.51.0