Add a deterministic PRNG for fuzzing.

If running the stack through a fuzzer, we would like execution to be
completely deterministic. This is gated on a
BORINGSSL_UNSAFE_FUZZER_MODE #ifdef.

For now, this just uses the zero ChaCha20 key and a global counter. As
needed, we can extend this to a thread-local counter and a separate
ChaCha20 stream and counter per input length.

Change-Id: Ic6c9d8a25e70d68e5dc6804e2c234faf48e51395
Reviewed-on: https://boringssl-review.googlesource.com/7286
Reviewed-by: Adam Langley <agl@google.com>

CMakeLists.txt

@@ -96,6 +96,8 @@ if(FUZZ)
     message("You need to build with Clang for fuzzing to work")
   endif()
 
+  add_definitions(-DBORINGSSL_UNSAFE_FUZZER_MODE)
+
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fsanitize=address -fsanitize-coverage=edge,indirect-calls")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=address -fsanitize-coverage=edge,indirect-calls")
   set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_C_FLAGS} -fsanitize=address")

crypto/rand/CMakeLists.txt

@@ -13,6 +13,7 @@ add_library(
 
   OBJECT
 
+  deterministic.c
   rand.c
   urandom.c
   windows.c

crypto/rand/deterministic.c

@@ -0,0 +1,49 @@
+/* Copyright (c) 2016, Google Inc.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
+
+#include <openssl/rand.h>
+
+#if defined(BORINGSSL_UNSAFE_FUZZER_MODE)
+
+#include <string.h>
+
+#include <openssl/chacha.h>
+
+#include "internal.h"
+
+
+/* g_num_calls is the number of calls to |CRYPTO_sysrand| that have occured.
+ *
+ * TODO(davidben): This is intentionally not thread-safe. If the fuzzer mode is
+ * ever used in a multi-threaded program, replace this with a thread-local. (A
+ * mutex would not be deterministic.) */
+static uint64_t g_num_calls = 0;
+
+void RAND_cleanup(void) {}
+
+void RAND_reset_for_fuzzing(void) { g_num_calls = 0; }
+
+void CRYPTO_sysrand(uint8_t *out, size_t requested) {
+  static const uint8_t kZeroKey[32];
+
+  uint8_t nonce[12];
+  memset(nonce, 0, sizeof(nonce));
+  memcpy(nonce, &g_num_calls, sizeof(g_num_calls));
+
+  memset(out, 0, requested);
+  CRYPTO_chacha_20(out, out, requested, kZeroKey, nonce, 0);
+  g_num_calls++;
+}
+
+#endif  /* BORINGSSL_UNSAFE_FUZZER_MODE */

crypto/rand/rand.c

@@ -72,7 +72,8 @@ static void rand_thread_state_free(void *state) {
   OPENSSL_free(state);
 }
 
-#if defined(OPENSSL_X86_64) && !defined(OPENSSL_NO_ASM)
+#if defined(OPENSSL_X86_64) && !defined(OPENSSL_NO_ASM) && \
+    !defined(BORINGSSL_UNSAFE_FUZZER_MODE)
 
 /* These functions are defined in asm/rdrand-x86_64.pl */
 extern int CRYPTO_rdrand(uint8_t out[8]);

crypto/rand/urandom.c

@@ -14,7 +14,7 @@
 
 #include <openssl/rand.h>
 
-#if !defined(OPENSSL_WINDOWS)
+#if !defined(OPENSSL_WINDOWS) && !defined(BORINGSSL_UNSAFE_FUZZER_MODE)
 
 #include <assert.h>
 #include <errno.h>
@@ -220,4 +220,4 @@ void CRYPTO_sysrand(uint8_t *out, size_t requested) {
   }
 }
 
-#endif  /* !OPENSSL_WINDOWS */
+#endif  /* !OPENSSL_WINDOWS && !BORINGSSL_UNSAFE_FUZZER_MODE */

crypto/rand/windows.c

@@ -14,7 +14,7 @@
 
 #include <openssl/rand.h>
 
-#if defined(OPENSSL_WINDOWS)
+#if defined(OPENSSL_WINDOWS) && !defined(BORINGSSL_UNSAFE_FUZZER_MODE)
 
 #include <limits.h>
 #include <stdlib.h>
@@ -53,4 +53,4 @@ void CRYPTO_sysrand(uint8_t *out, size_t requested) {
   return;
 }
 
-#endif  /* OPENSSL_WINDOWS */
+#endif  /* OPENSSL_WINDOWS && !BORINGSSL_UNSAFE_FUZZER_MODE */

fuzz/client.cc

@@ -1,5 +1,6 @@
 #include <assert.h>
 
+#include <openssl/rand.h>
 #include <openssl/ssl.h>
 
 struct GlobalState {
@@ -15,6 +16,8 @@ struct GlobalState {
 static GlobalState g_state;
 
 extern "C" int LLVMFuzzerTestOneInput(uint8_t *buf, size_t len) {
+  RAND_reset_for_fuzzing();
+
   // This only fuzzes the initial flow from the server so far.
   SSL *client = SSL_new(g_state.ctx);
   BIO *in = BIO_new(BIO_s_mem());

fuzz/server.cc

@@ -1,5 +1,6 @@
 #include <assert.h>
 
+#include <openssl/rand.h>
 #include <openssl/ssl.h>
 
 static const uint8_t kCertificateDER[] = {
@@ -204,6 +205,8 @@ struct GlobalState {
 static GlobalState g_state;
 
 extern "C" int LLVMFuzzerTestOneInput(uint8_t *buf, size_t len) {
+  RAND_reset_for_fuzzing();
+
   // This only fuzzes the initial flow from the client so far.
   SSL *server = SSL_new(g_state.ctx);
   BIO *in = BIO_new(BIO_s_mem());

include/openssl/rand.h

@@ -62,6 +62,12 @@ OPENSSL_EXPORT void RAND_set_urandom_fd(int fd);
 OPENSSL_EXPORT void RAND_enable_fork_unsafe_buffering(int fd);
 #endif
 
+#if defined(BORINGSSL_UNSAFE_FUZZER_MODE)
+/* RAND_reset_for_fuzzing resets the fuzzer-only deterministic RNG. This
+ * function is only defined in the fuzzer-only build configuration. */
+OPENSSL_EXPORT void RAND_reset_for_fuzzing(void);
+#endif
+
 
 /* Deprecated functions */