[3.1 -> main] Add subjective limit to mod_exp host function

libraries/chain/webassembly/crypto.cpp

@@ -158,6 +158,14 @@ namespace eosio { namespace chain { namespace webassembly {
                               span<const char> exp,
                               span<const char> modulus,
                               span<char> out) const {
+      if (context.control.is_producing_block()) {
+         static constexpr int byte_size_limit = 256; // Allow up to 2048-bit values for base, exp, and modulus.
+
+         if (std::max(std::max(base.size(), exp.size()), modulus.size()) > byte_size_limit) {
+            EOS_THROW(subjective_block_production_exception, "Bit size too large for values passed into mod_exp");
+         }
+      }
+
       bytes bbase(base.data(), base.data() + base.size());
       bytes bexp(exp.data(), exp.data() + exp.size());
       bytes bmod(modulus.data(), modulus.data() + modulus.size());

unittests/crypto_primitives_tests.cpp

@@ -386,6 +386,7 @@ BOOST_AUTO_TEST_CASE( modexp_test ) { try {
          return_code::failure,
          "",
       },
+
    };
 
    for(const auto& test : tests) {
@@ -409,6 +410,59 @@ BOOST_AUTO_TEST_CASE( modexp_test ) { try {
 } FC_LOG_AND_RETHROW() }
 
 
+BOOST_AUTO_TEST_CASE( modexp_subjective_limit_test ) { try {
+
+   // Given the need to respect the deadline timer and the current limitation that the deadline timer is not plumbed into the
+   // inner loops of the implementation of mod_exp (which currently exists in the gmp shared library), only a small enough duration for
+   // mod_exp can be tolerated to avoid going over the deadline timer by too much. A good threshold for small may be less than 5 ms.
+   // Then based on benchmarks within the test_modular_arithmetic test within fc, it seems safe to limit the bit size to 2048 bits.
+
+   // This test case verifies that the subjective bit size limit for mod_exp is properly enforced within libchain.
+
+   // To allow mod_exp to be more useful, the limits on bit size need to be removed and the deadline timer plumbing into the implementation
+   // needs to occur. When that happens, this test case can be removed.
+
+   tester c( setup_policy::preactivate_feature_and_new_bios );
+
+   const auto& tester1_account = account_name("tester1");
+   c.create_accounts( {tester1_account} );
+   c.produce_block();
+
+   const auto& pfm = c.control->get_protocol_feature_manager();
+   const auto& d = pfm.get_builtin_digest( builtin_protocol_feature_t::crypto_primitives );
+   BOOST_REQUIRE( d );
+
+   c.preactivate_protocol_features( {*d} );
+   c.produce_block();
+
+   c.set_code( tester1_account, contracts::crypto_primitives_test_wasm() );
+   c.set_abi( tester1_account, contracts::crypto_primitives_test_abi().data() );
+   c.produce_block();
+
+   bytes exponent(256); // 2048 bits of all zeros is fine
+
+   c.push_action( tester1_account, "testmodexp"_n, tester1_account, mutable_variant_object()
+      ("base", h2bin("01"))
+      ("exp", exponent)
+      ("modulo", h2bin("0F"))
+      ("expected_error", static_cast<int32_t>(return_code::success))
+      ("expected_result", h2bin("01"))
+   );
+
+   exponent.push_back(0); // But 2056 bits of all zeros crosses the subjective limit (even if the value is still technically only zero).
+
+   BOOST_CHECK_EXCEPTION(c.push_action( tester1_account, "testmodexp"_n, tester1_account, mutable_variant_object()
+                                        ("base", h2bin("01"))
+                                        ("exp", exponent)
+                                        ("modulo", h2bin("0F"))
+                                        ("expected_error", static_cast<int32_t>(return_code::success))
+                                        ("expected_result", h2bin("01"))),
+                         eosio::chain::subjective_block_production_exception,
+                         fc_exception_message_is("Bit size too large for values passed into mod_exp")
+   );
+
+} FC_LOG_AND_RETHROW() }
+
 BOOST_AUTO_TEST_CASE( blake2f_test ) { try {
    tester c( setup_policy::preactivate_feature_and_new_bios );