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.

diff --git a/ChangeLog b/ChangeLog
index cd60ef8399b97d005c999671140bccbd8a02aa70..dc72e54f0038907f19039efb6ceadfab7ccbe441 100644 (file)
--- a/ChangeLog
+++ b/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

diff --git a/malloc/malloc.c b/malloc/malloc.c
index e247c77b7d4de26e0f2fbec16e352889bac3781b..9431108626cdc0b5c1972ee00126228c8dd7166f 100644 (file)
--- a/malloc/malloc.c
+++ b/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;