[PATCH] sys_swapon bad arg causing slab corruption

There's an error in sys_swapon() that can cause slab corruption if you
pass in a bad specialfile pointer. getname() then returns
ERR_PTR(-EFAULT), but sys_swapon() doesn't clear name before calling
putname() on it (thus freeing 0xfffffff2, corrupting slab). An ltp test
case repeatedly crashed in later tests due to thus, irk.

mm/swapfile.c

@@ -1269,8 +1269,10 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
 	swap_list_unlock();
 	name = getname(specialfile);
 	error = PTR_ERR(name);
-	if (IS_ERR(name))
+	if (IS_ERR(name)) {
+		name = NULL;
 		goto bad_swap_2;
+	}
 	swap_file = filp_open(name, O_RDWR, 0);
 	error = PTR_ERR(swap_file);
 	if (IS_ERR(swap_file)) {