linux-kernel - [ 24/28] mm/huge_memory.c: fix potential NULL pointer dereference

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Date:	Tue, 24 Sep 2013 17:07:55 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Libin <huawei.libin@...wei.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Michal Hocko <mhocko@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [ 24/28] mm/huge_memory.c: fix potential NULL pointer dereference

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

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

From: Libin <huawei.libin@...wei.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@...wei.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
Reviewed-by: Wanpeng Li <liwanp@...ux.vnet.ibm.com>
Reviewed-by: Michal Hocko <mhocko@...e.cz>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 mm/huge_memory.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1838,6 +1838,8 @@ static void collapse_huge_page(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)

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