linux-kernel - [PATCH] mm: mempolicy: fix potential pte_unmap

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Date:   Mon, 19 Oct 2020 03:48:53 -0400
From:   Shijie Luo <luoshijie1@...wei.com>
To:     <akpm@...ux-foundation.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <osalvador@...e.de>, <mhocko@...e.com>, <linmiaohe@...wei.com>,
        <linfeilong@...wei.com>, <luoshijie1@...wei.com>
Subject: [PATCH] mm: mempolicy: fix potential pte_unmap_unlock pte error

When flags in queue_pages_pte_range don't have MPOL_MF_MOVE or MPOL_MF_MOVE_ALL
 bits, code breaks and passing origin pte - 1 to pte_unmap_unlock seems like
not a good idea.

queue_pages_pte_range can run in MPOL_MF_MOVE_ALL mode which doesn't migrate
misplaced pages but returns with EIO when encountering such a page. Since
commit a7f40cfe3b7a ("mm: mempolicy: make mbind() return -EIO when MPOL_MF_STRICT
 is specified") and early break on the first pte in the range results in
pte_unmap_unlock on an underflow pte. This can lead to lockups later on when
 somebody  tries to lock the pte resp. page_table_lock again..

Fixes: a7f40cfe3b7a ("mm: mempolicy: make mbind() return -EIO when
MPOL_MF_STRICT is specified")

Signed-off-by: Shijie Luo <luoshijie1@...wei.com>
Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
---
 mm/mempolicy.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 3fde772ef5ef..3ca4898f3f24 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -525,7 +525,7 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
 	unsigned long flags = qp->flags;
 	int ret;
 	bool has_unmovable = false;
-	pte_t *pte;
+	pte_t *pte, *mapped_pte;
 	spinlock_t *ptl;
 
 	ptl = pmd_trans_huge_lock(pmd, vma);
@@ -539,7 +539,7 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
 	if (pmd_trans_unstable(pmd))
 		return 0;
 
-	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
+	mapped_pte = pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
 	for (; addr != end; pte++, addr += PAGE_SIZE) {
 		if (!pte_present(*pte))
 			continue;
@@ -571,7 +571,7 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
 		} else
 			break;
 	}
-	pte_unmap_unlock(pte - 1, ptl);
+	pte_unmap_unlock(mapped_pte, ptl);
 	cond_resched();
 
 	if (has_unmovable)
-- 
2.19.1