malloc: Verify size of top chunk.

The House of Force is a well-known technique to exploit heap
overflow. In essence, this exploit takes three steps:
1. Overwrite the size of top chunk with very large value (e.g. -1).
2. Request x bytes from top chunk. As the size of top chunk
   is corrupted, x can be arbitrarily large and top chunk will
   still be offset by x.
3. The next allocation from top chunk will thus be controllable.

If we verify the size of top chunk at step 2, we can stop such attack.

ChangeLog

@@ -1,3 +1,7 @@
+2018-08-16  Pochang Chen  <johnchen902@gmail.com>
+
+	* malloc/malloc.c (_int_malloc.c): Verify size of top chunk.
+
 2018-08-16  Siddhesh Poyarekar  <siddhesh@sourceware.org>
 
 	* benchtests/bench-strlen.c (do_test): Allocate buffers before

malloc/malloc.c

@@ -4076,6 +4076,9 @@ _int_malloc (mstate av, size_t bytes)
       victim = av->top;
       size = chunksize (victim);
 
+      if (__glibc_unlikely (size > av->system_mem))
+        malloc_printerr ("malloc(): corrupted top size");
+
       if ((unsigned long) (size) >= (unsigned long) (nb + MINSIZE))
         {
           remainder_size = size - nb;