Handle BN_mod_word failures.

As of 37258dad and the corresponding upstream
change, BN_mod_word may fail, like BN_div_word. Handle this properly. Thanks to
Brian Smith for pointing this out. See BoringSSL's
44bedc348d9491e63c7ed1438db100a4b8a830be.
Signed-off-by: NKurt Roeckx <kurt@roeckx.be>
Reviewed-by: NRich Salz <rsalz@openssl.org>

GH: #1251

crypto/bn/bn_prime.c

@@ -143,7 +143,7 @@ int BN_generate_prime_ex(BIGNUM *ret, int bits, int safe,
                 goto err;
         }
     }
-    /* if (BN_mod_word(ret,(BN_ULONG)3) == 1) goto loop; */
+
     if (!BN_GENCB_call(cb, 0, c1++))
         /* aborted */
         goto err;
@@ -218,9 +218,13 @@ int BN_is_prime_fasttest_ex(const BIGNUM *a, int checks, BN_CTX *ctx_passed,
         /* a is even => a is prime if and only if a == 2 */
         return BN_is_word(a, 2);
     if (do_trial_division) {
-        for (i = 1; i < NUMPRIMES; i++)
-            if (BN_mod_word(a, primes[i]) == 0)
+        for (i = 1; i < NUMPRIMES; i++) {
+            BN_ULONG mod = BN_mod_word(a, primes[i]);
+            if (mod == (BN_ULONG)-1)
+                goto err;
+            if (mod == 0)
                 return 0;
+        }
         if (!BN_GENCB_call(cb, 1, -1))
             goto err;
     }
@@ -313,7 +317,10 @@ int bn_probable_prime_dh_retry(BIGNUM *rnd, int bits, BN_CTX *ctx)
 
     for (i = 1; i < NUMPRIMES; i++) {
         /* check that rnd is a prime */
-        if (BN_mod_word(rnd, (BN_ULONG)primes[i]) <= 1) {
+        BN_ULONG mod = BN_mod_word(rnd, (BN_ULONG)primes[i]);
+        if (mod == (BN_ULONG)-1)
+            goto err;
+        if (mod <= 1) {
             goto loop;
         }
     }
@@ -359,9 +366,11 @@ int bn_probable_prime_dh_coprime(BIGNUM *rnd, int bits, BN_CTX *ctx)
     /* skip coprimes */
     for (i = first_prime_index; i < NUMPRIMES; i++) {
         /* check that rnd is a prime */
-        if (BN_mod_word(rnd, (BN_ULONG)primes[i]) <= 1) {
+        BN_ULONG mod = BN_mod_word(rnd, (BN_ULONG)primes[i]);
+        if (mod == (BN_ULONG)-1)
+            goto err;
+        if (mod <= 1)
             goto loop;
-        }
     }
     ret = 1;
 
@@ -409,8 +418,12 @@ static int probable_prime(BIGNUM *rnd, int bits, prime_t *mods)
     if (!BN_rand(rnd, bits, 1, 1))
         return (0);
     /* we now have a random number 'rnd' to test. */
-    for (i = 1; i < NUMPRIMES; i++)
-        mods[i] = (prime_t) BN_mod_word(rnd, (BN_ULONG)primes[i]);
+    for (i = 1; i < NUMPRIMES; i++) {
+        BN_ULONG mod = BN_mod_word(rnd, (BN_ULONG)primes[i]);
+        if (mod == (BN_ULONG)-1)
+            return 0;
+        mods[i] = (prime_t) mod;
+    }
     /*
      * If bits is so small that it fits into a single word then we
      * additionally don't want to exceed that many bits.
@@ -508,7 +521,10 @@ int bn_probable_prime_dh(BIGNUM *rnd, int bits,
  loop:
     for (i = 1; i < NUMPRIMES; i++) {
         /* check that rnd is a prime */
-        if (BN_mod_word(rnd, (BN_ULONG)primes[i]) <= 1) {
+        BN_ULONG mod = BN_mod_word(rnd, (BN_ULONG)primes[i]);
+        if (mod == (BN_ULONG)-1)
+            goto err;
+        if (mod <= 1) {
             if (!BN_add(rnd, rnd, add))
                 goto err;
             goto loop;
@@ -569,8 +585,11 @@ static int probable_prime_dh_safe(BIGNUM *p, int bits, const BIGNUM *padd,
         /*
          * check that for p and q gcd(p-1,primes) == 1 (except for 2)
          */
-        if ((BN_mod_word(p, (BN_ULONG)primes[i]) == 0) ||
-            (BN_mod_word(q, (BN_ULONG)primes[i]) == 0)) {
+        BN_ULONG pmod = BN_mod_word(p, (BN_ULONG)primes[i]);
+        BN_ULONG qmod = BN_mod_word(q, (BN_ULONG)primes[i]);
+        if (pmod == (BN_ULONG)-1 || qmod == (BN_ULONG)-1)
+            goto err;
+        if (pmod == 0 || qmod == 0) {
             if (!BN_add(p, p, padd))
                 goto err;
             if (!BN_add(q, q, qadd))

crypto/dh/dh_check.c

@@ -68,10 +68,14 @@ int DH_check(const DH *dh, int *ret)
 
     } else if (BN_is_word(dh->g, DH_GENERATOR_2)) {
         l = BN_mod_word(dh->p, 24);
+        if (l == (BN_ULONG)-1)
+            goto err;
         if (l != 11)
             *ret |= DH_NOT_SUITABLE_GENERATOR;
     } else if (BN_is_word(dh->g, DH_GENERATOR_5)) {
         l = BN_mod_word(dh->p, 10);
+        if (l == (BN_ULONG)-1)
+            goto err;
         if ((l != 3) && (l != 7))
             *ret |= DH_NOT_SUITABLE_GENERATOR;
     } else