[PATCH v2 4/7] malloc: Ensure lower bound on chunk size in __libc_realloc.

[Istvan Kurucsai]

Under some circumstances, a chunk size of SIZE_SZ could lead to an underflow
when calculating the length argument of memcpy.

   * malloc/malloc.c (__libc_realloc): Check chunk size.
---
 malloc/malloc.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/malloc/malloc.c b/malloc/malloc.c
index 51d703c..8e48952 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -3154,8 +3154,9 @@ __libc_realloc (void *oldmem, size_t bytes)
      accident or by "design" from some intruder.  We need to bypass
      this check for dumped fake mmap chunks from the old main arena
      because the new malloc may provide additional alignment.  */
-  if ((__builtin_expect ((uintptr_t) oldp > (uintptr_t) -oldsize, 0)
-       || __builtin_expect (misaligned_chunk (oldp), 0))
+  if ((__glibc_unlikely ((uintptr_t) oldp > (uintptr_t) -oldsize)
+       || __glibc_unlikely (misaligned_chunk (oldp))
+       || __glibc_unlikely (oldsize <= 2 * SIZE_SZ))
       && !DUMPED_MAIN_ARENA_CHUNK (oldp))
       malloc_printerr ("realloc(): invalid pointer");
 
-- 
2.7.4