security: protect from stack expantion into low vm addresses

Add security checks to make sure we are not attempting to expand the
stack into memory protected by mmap_min_addr
Signed-off-by: NEric Paris <eparis@redhat.com>
Signed-off-by: NJames Morris <jmorris@namei.org>

mm/mmap.c

@@ -1615,6 +1615,12 @@ static inline int expand_downwards(struct vm_area_struct *vma,
 	 */
 	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);
 
 	/*
@@ -1622,8 +1628,6 @@ static inline int expand_downwards(struct vm_area_struct *vma,
 	 * 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) {