Timing vulnerability in ECDSA signature generation (CVE-2018-0735)

Preallocate an extra limb for some of the big numbers to avoid a reallocation
that can potentially provide a side channel.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from #7486)

(cherry picked from commit 99540ec)

crypto/ec/ec_mult.c

@@ -206,8 +206,8 @@ int ec_scalar_mul_ladder(const EC_GROUP *group, EC_POINT *r,
      */
     cardinality_bits = BN_num_bits(cardinality);
     group_top = bn_get_top(cardinality);
-    if ((bn_wexpand(k, group_top + 1) == NULL)
-        || (bn_wexpand(lambda, group_top + 1) == NULL)) {
+    if ((bn_wexpand(k, group_top + 2) == NULL)
+        || (bn_wexpand(lambda, group_top + 2) == NULL)) {
         ECerr(EC_F_EC_SCALAR_MUL_LADDER, ERR_R_BN_LIB);
         goto err;
     }
@@ -244,7 +244,7 @@ int ec_scalar_mul_ladder(const EC_GROUP *group, EC_POINT *r,
      * k := scalar + 2*cardinality
      */
     kbit = BN_is_bit_set(lambda, cardinality_bits);
-    BN_consttime_swap(kbit, k, lambda, group_top + 1);
+    BN_consttime_swap(kbit, k, lambda, group_top + 2);
 
     group_top = bn_get_top(group->field);
     if ((bn_wexpand(s->X, group_top) == NULL)