mm/page_table_check: fix accessing unmapped ptep

ptep is unmapped too early, so ptep could theoretically be accessed while
it's unmapped.  This might become a problem if/when CONFIG_HIGHPTE becomes
available on riscv.

Fix it by deferring pte_unmap() until page table checking is done.

[akpm@linux-foundation.org: account for ptep alteration, per Matthew]
Link: https://lkml.kernel.org/r/20220526113350.30806-1-linmiaohe@huawei.com
Fixes: 80110bbf ("mm/page_table_check: check entries at pmd levels")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/page_table_check.c

@@ -251,11 +251,11 @@ void __page_table_check_pte_clear_range(struct mm_struct *mm,
 		pte_t *ptep = pte_offset_map(&pmd, addr);
 		unsigned long i;
 
-		pte_unmap(ptep);
 		for (i = 0; i < PTRS_PER_PTE; i++) {
 			__page_table_check_pte_clear(mm, addr, *ptep);
 			addr += PAGE_SIZE;
 			ptep++;
 		}
+		pte_unmap(ptep - PTRS_PER_PTE);
 	}
 }