[PATCH 091/104] mm/huge_memory.c: fix potential NULL pointer dereference

[Luis Henriques]

3.5.7.22 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Libin <huawei.libin at huawei.com>

commit a8f531ebc33052642b4bd7b812eedf397108ce64 upstream.

In collapse_huge_page() there is a race window between releasing the
mmap_sem read lock and taking the mmap_sem write lock, so find_vma() may
return NULL.  So 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 at huawei.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
Reviewed-by: Wanpeng Li <liwanp at linux.vnet.ibm.com>
Reviewed-by: Michal Hocko <mhocko at suse.cz>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 mm/huge_memory.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 7d0eb9e..613d436 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1900,6 +1900,8 @@ 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)
-- 
1.8.3.2