linux-kernel - [PATCH] mm/memory.c: fix potential pte_unmap

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Date:   Fri, 5 Feb 2021 03:19:25 -0500
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     <akpm@...ux-foundation.org>
CC:     <jeremy@...p.org>, <mingo@...e.hu>, <ian.pratt@...source.com>,
        <chrisw@...s-sol.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <linmiaohe@...wei.com>
Subject: [PATCH] mm/memory.c: fix potential pte_unmap_unlock pte error

If all pte entry is none in 'non-create' case, we would break the loop with
pte unchanged.  Then the wrong pte - 1 would be passed to pte_unmap_unlock.
This is a theoretical issue which may not be a real bug. So it's not worth
cc stable.

Fixes: aee16b3cee27 ("Add apply_to_page_range() which applies a function to a pte range")
Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
---
 mm/memory.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 9d68a2340589..b2d6a0a6088b 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2394,18 +2394,18 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
 				     pte_fn_t fn, void *data, bool create,
 				     pgtbl_mod_mask *mask)
 {
-	pte_t *pte;
+	pte_t *pte, *mapped_pte;
 	int err = 0;
 	spinlock_t *ptl;
 
 	if (create) {
-		pte = (mm == &init_mm) ?
+		mapped_pte = pte = (mm == &init_mm) ?
 			pte_alloc_kernel_track(pmd, addr, mask) :
 			pte_alloc_map_lock(mm, pmd, addr, &ptl);
 		if (!pte)
 			return -ENOMEM;
 	} else {
-		pte = (mm == &init_mm) ?
+		mapped_pte = pte = (mm == &init_mm) ?
 			pte_offset_kernel(pmd, addr) :
 			pte_offset_map_lock(mm, pmd, addr, &ptl);
 	}
@@ -2428,7 +2428,7 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
 	arch_leave_lazy_mmu_mode();
 
 	if (mm != &init_mm)
-		pte_unmap_unlock(pte-1, ptl);
+		pte_unmap_unlock(mapped_pte, ptl);
 	return err;
 }
 
-- 
2.19.1