[PATCH] mm: Fix potential NULL pointer dereference

[Libin <huawei.libin@huawei.com>]

v1->v2: Add description about the bug potential trigger condition.
	Thanks for the review/suggestion of Michal Hocko &
	Wanpeng Li.

In collapse_huge_page, there is a race window between release
the mmap_sem read lock and hold the mmap_sem write lock, so
find_vma() may return NULL, thus check the return value to
avoid NULL pointer dereference.

collapse_huge_page
	khugepaged_alloc_page
		up_read(&mm->mmap_sem)
	down_write(&mm->mmap_sem)
	vma = find_vma(mm, address)

Signed-off-by: Libin <huawei.libin@huawei.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: <stable@vger.kernel.org> # v3.0+
---
 mm/huge_memory.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 243e710..d4423f4 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2294,6 +2294,8 @@ static void collapse_huge_page(struct mm_struct *mm,
 		goto out;
 
 	vma = find_vma(mm, address);
+	if (!vma)
+		goto out;
 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
 	hend = vma->vm_end & HPAGE_PMD_MASK;
 	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
-- 
1.8.2.1


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