mm/ksm: fix pte_unmap_unlock of wrong address in break_ksm_pmd_entry

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().

Link: https://lkml.kernel.org/r/20251220202926.318366-1-sashal@kernel.org
Fixes: 5d4939fc2258 ("ksm: perform a range-walk in break_ksm")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Assisted-by: claude-opus-4-5-20251101
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Pedro Demarchi Gomes <pedrodemargomes@gmail.com>
Cc: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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;
 }