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: 80110bbfbba6 ("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>

diff --git a/mm/page_table_check.c b/mm/page_table_check.c
index 2458281..bc55be2 100644 (file)
--- a/mm/page_table_check.c
+++ b/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);
        }
 }