mmap: protect from stack expantion into low vm addresses

When adding the new security hooks into mmap to enhance protection against NULL pointers in the kernel I overlooked that a user may be able to expand the stack all the way down to low addresses. This flaw was pointed out by a PaX/grsecurity developer and this patch should provide protection against this missed code path.

---

** Be very careful applying/rediffing this patch. Standard 3 lines of context from git diff will misapply the first hunk to expand_upwards instead of properly in expand_downwards. This patch was generated using -U 4 to make sure it applies in the correct place. **

 mm/mmap.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

@@ -1614,17 +1614,21 @@ static inline int expand_downwards(struc
 	 * so that the anon_vma locking is not a noop.
 	 */
 	if (unlikely(anon_vma_prepare(vma)))
 		return -ENOMEM;
+
+	address &= PAGE_MASK;
+	error = security_file_mmap(0, 0, 0, 0, address, 1);
+	if (error)
+		return error;
+
 	anon_vma_lock(vma);
 
 	/*
 	 * vma->vm_start/vm_end cannot change under us because the caller
 	 * is required to hold the mmap_sem in read mode.  We need the
 	 * anon_vma lock to serialize against concurrent expand_stacks.
 	 */
-	address &= PAGE_MASK;
-	error = 0;
 
 	/* Somebody else might have raced and expanded it already */
 	if (address < vma->vm_start) {
 		unsigned long size, grow;

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