Merge bitcoin#839: Prevent arithmetic on NULL pointer if the scratch …

…space is too small

29a299e Run the undefined behaviour sanitizer on Travis (Fabien)
7506e06 Prevent arithmetic on NULL pointer if the scratch space is too small (Fabien)

Pull request description:

ACKs for top commit:
  sipa:
    ACK 29a299e. Reviewed the code changes and verified that building with these sanitizer flags catches the existing error, as well as a signed integer overflow if introduced.
  real-or-random:
    ACK 29a299e code inspection
  jonasnick:
    utACK 29a299e

Tree-SHA512: 4d788f12f3d7b48018e884910adb9b530a05d88f504de83dadeab8a22d75da83c05a1518f7317de5f536c4dd243ea7b347b1eaddb2ca1d804c663e41b85db69d

.travis.yml

@@ -31,6 +31,7 @@ env:
     - BUILD=distcheck WITH_VALGRIND=no CTIMETEST=no BENCH=no
     - CPPFLAGS=-DDETERMINISTIC
     - CFLAGS=-O0 CTIMETEST=no
+    - CFLAGS="-fsanitize=undefined -fno-omit-frame-pointer" LDFLAGS="-fsanitize=undefined -fno-omit-frame-pointer" UBSAN_OPTIONS="print_stacktrace=1:halt_on_error=1" BIGNUM=no ASM=x86_64 ECDH=yes RECOVERY=yes EXPERIMENTAL=yes SCHNORRSIG=yes CTIMETEST=no
     - ECMULTGENPRECISION=2
     - ECMULTGENPRECISION=8
     - RUN_VALGRIND=yes BIGNUM=no ASM=x86_64 ECDH=yes  RECOVERY=yes EXPERIMENTAL=yes SCHNORRSIG=yes EXTRAFLAGS="--disable-openssl-tests" BUILD=

src/ecmult_impl.h

@@ -595,11 +595,11 @@ static int secp256k1_ecmult_strauss_batch(const secp256k1_callback* error_callba
     scalars = (secp256k1_scalar*)secp256k1_scratch_alloc(error_callback, scratch, n_points * sizeof(secp256k1_scalar));
     state.prej = (secp256k1_gej*)secp256k1_scratch_alloc(error_callback, scratch, n_points * ECMULT_TABLE_SIZE(WINDOW_A) * sizeof(secp256k1_gej));
     state.zr = (secp256k1_fe*)secp256k1_scratch_alloc(error_callback, scratch, n_points * ECMULT_TABLE_SIZE(WINDOW_A) * sizeof(secp256k1_fe));
-    state.pre_a = (secp256k1_ge*)secp256k1_scratch_alloc(error_callback, scratch, n_points * 2 * ECMULT_TABLE_SIZE(WINDOW_A) * sizeof(secp256k1_ge));
-    state.pre_a_lam = state.pre_a + n_points * ECMULT_TABLE_SIZE(WINDOW_A);
+    state.pre_a = (secp256k1_ge*)secp256k1_scratch_alloc(error_callback, scratch, n_points * ECMULT_TABLE_SIZE(WINDOW_A) * sizeof(secp256k1_ge));
+    state.pre_a_lam = (secp256k1_ge*)secp256k1_scratch_alloc(error_callback, scratch, n_points * ECMULT_TABLE_SIZE(WINDOW_A) * sizeof(secp256k1_ge));
     state.ps = (struct secp256k1_strauss_point_state*)secp256k1_scratch_alloc(error_callback, scratch, n_points * sizeof(struct secp256k1_strauss_point_state));
 
-    if (points == NULL || scalars == NULL || state.prej == NULL || state.zr == NULL || state.pre_a == NULL) {
+    if (points == NULL || scalars == NULL || state.prej == NULL || state.zr == NULL || state.pre_a == NULL || state.pre_a_lam == NULL || state.ps == NULL) {
         secp256k1_scratch_apply_checkpoint(error_callback, scratch, scratch_checkpoint);
         return 0;
     }