Tell ASan about the OPENSSL_malloc prefix.

OpenSSL's BN_mul function had a single-word buffer underflow (see
576129cd72ae054d246221f111aabf42b9c6d76d). We already independently
fixed this but, if we hadn't, ASan wouldn't have noticed because of
OPENSSL_malloc.

ASan has runtime hooks we can call to make it more accurate.

Change-Id: Ifc9c3837ece2bc456c5bdc960be707d7b1759904
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/35165
Reviewed-by: Adam Langley <agl@google.com>

crypto/mem.c

diff --git a/crypto/mem.c b/crypto/mem.c
index 6af532e..14e0bdf 100644
--- a/crypto/mem.c
+++ b/crypto/mem.c
@@ -71,6 +71,14 @@
 
 #define OPENSSL_MALLOC_PREFIX 8
 
+#if defined(OPENSSL_ASAN)
+void __asan_poison_memory_region(const volatile void *addr, size_t size);
+void __asan_unpoison_memory_region(const volatile void *addr, size_t size);
+#else
+static void __asan_poison_memory_region(const void *addr, size_t size) {}
+static void __asan_unpoison_memory_region(const void *addr, size_t size) {}
+#endif
+
 #if defined(__GNUC__) || defined(__clang__)
 // sdallocx is a sized |free| function. By passing the size (which we happen to
 // always know in BoringSSL), the malloc implementation can save work. We cannot
@@ -99,6 +107,7 @@
 
   *(size_t *)ptr = size;
 
+  __asan_poison_memory_region(ptr, OPENSSL_MALLOC_PREFIX);
   return ((uint8_t *)ptr) + OPENSSL_MALLOC_PREFIX;
 }
 
@@ -108,6 +117,7 @@
   }
 
   void *ptr = ((uint8_t *)orig_ptr) - OPENSSL_MALLOC_PREFIX;
+  __asan_unpoison_memory_region(ptr, OPENSSL_MALLOC_PREFIX);
 
   size_t size = *(size_t *)ptr;
   OPENSSL_cleanse(ptr, size + OPENSSL_MALLOC_PREFIX);
@@ -120,7 +130,9 @@
   }
 
   void *ptr = ((uint8_t *)orig_ptr) - OPENSSL_MALLOC_PREFIX;
+  __asan_unpoison_memory_region(ptr, OPENSSL_MALLOC_PREFIX);
   size_t old_size = *(size_t *)ptr;
+  __asan_poison_memory_region(ptr, OPENSSL_MALLOC_PREFIX);
 
   void *ret = OPENSSL_malloc(new_size);
   if (ret == NULL) {