[PATCH 4.9 04/78] mm: mempolicy: fix potential pte_unmap_unlock pte error

[Greg Kroah-Hartman]

From: Shijie Luo <luoshijie1@xxxxxxxxxx>

[ Upstream commit 3f08842098e842c51e3b97d0dcdebf810b32558e ]

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@xxxxxxxxxx>
Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Cc: Feilong Lin <linfeilong@xxxxxxxxxx>
Cc: Shijie Luo <luoshijie1@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20201019074853.50856-1-luoshijie1@xxxxxxxxxx
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 mm/mempolicy.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index a2be65bf5d8cc..2f443767fd1b4 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -487,7 +487,7 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
 	struct queue_pages *qp = walk->private;
 	unsigned long flags = qp->flags;
 	int nid, ret;
-	pte_t *pte;
+	pte_t *pte, *mapped_pte;
 	spinlock_t *ptl;
 
 	if (pmd_trans_huge(*pmd)) {
@@ -515,7 +515,7 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
 	if (pmd_trans_unstable(pmd))
 		return 0;
 retry:
-	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;
@@ -554,7 +554,7 @@ retry:
 		} else
 			break;
 	}
-	pte_unmap_unlock(pte - 1, ptl);
+	pte_unmap_unlock(mapped_pte, ptl);
 	cond_resched();
 	return addr != end ? -EIO : 0;
 }
-- 
2.27.0