cryptonote_basic: sanity check on key offsets

Nice early check on allowed n key offsets per tx based on the
guaranteed ceiling for the entire chain.

src/cryptonote_basic/cryptonote_format_utils.cpp

@@ -99,6 +99,27 @@ namespace cryptonote
     const uint64_t bp_clawback = (bp_base * n_padded_outputs - bp_size) * 4 / 5;
     return bp_clawback;
   }
+
+  std::size_t max_total_key_offsets()
+  {
+    // This is a 100% guaranteed ceiling for the entire chain
+    return get_max_tx_size() / sizeof(crypto::public_key);
+  }
+
+  bool n_key_offsets_exceeds_max(const transaction_prefix& tx)
+  {
+    const std::size_t max_allowed = max_total_key_offsets();
+    std::size_t total_key_offsets = 0;
+    for (const auto &vin : tx.vin)
+    {
+      if (vin.type() != typeid(cryptonote::txin_to_key))
+        continue;
+      const std::size_t n_key_offsets = boost::get<cryptonote::txin_to_key>(vin).key_offsets.size();
+      CHECK_AND_ASSERT_MES((n_key_offsets + total_key_offsets) >= total_key_offsets, true, "key offsets overflow");
+      total_key_offsets += n_key_offsets;
+    }
+    return total_key_offsets >= max_allowed;
+  }
   //---------------------------------------------------------------
 }
 
@@ -203,6 +224,7 @@ namespace cryptonote
     binary_archive<false> ba{epee::strspan<std::uint8_t>(tx_blob)};
     bool r = ::serialization::serialize(ba, tx);
     CHECK_AND_ASSERT_MES(r, false, "Failed to parse transaction from blob");
+    CHECK_AND_ASSERT_MES(!n_key_offsets_exceeds_max(tx), false, "Transaction contains too many ring members");
     CHECK_AND_ASSERT_MES(expand_transaction_1(tx, false), false, "Failed to expand transaction data");
     tx.invalidate_hashes();
     tx.set_blob_size(tx_blob.size());
@@ -214,6 +236,7 @@ namespace cryptonote
     binary_archive<false> ba{epee::strspan<std::uint8_t>(tx_blob)};
     bool r = tx.serialize_base(ba);
     CHECK_AND_ASSERT_MES(r, false, "Failed to parse transaction from blob");
+    CHECK_AND_ASSERT_MES(!n_key_offsets_exceeds_max(tx), false, "Transaction contains too many ring members");
     CHECK_AND_ASSERT_MES(expand_transaction_1(tx, true), false, "Failed to expand transaction data");
     tx.invalidate_hashes();
     return true;
@@ -224,6 +247,7 @@ namespace cryptonote
     binary_archive<false> ba{epee::strspan<std::uint8_t>(tx_blob)};
     bool r = ::serialization::serialize_noeof(ba, tx);
     CHECK_AND_ASSERT_MES(r, false, "Failed to parse transaction prefix from blob");
+    CHECK_AND_ASSERT_MES(!n_key_offsets_exceeds_max(tx), false, "Transaction contains too many ring members");
     return true;
   }
   //---------------------------------------------------------------
@@ -232,6 +256,7 @@ namespace cryptonote
     binary_archive<false> ba{epee::strspan<std::uint8_t>(tx_blob)};
     bool r = ::serialization::serialize(ba, tx);
     CHECK_AND_ASSERT_MES(r, false, "Failed to parse transaction from blob");
+    CHECK_AND_ASSERT_MES(!n_key_offsets_exceeds_max(tx), false, "Transaction contains too many ring members");
     CHECK_AND_ASSERT_MES(expand_transaction_1(tx, false), false, "Failed to expand transaction data");
     tx.invalidate_hashes();
     tx.set_blob_size(tx_blob.size());