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precompiles: Fix expmod gas calculation
Fix and refactor gas cost calculation for the expmod precompile. The issue was that some computations could overflow and the result was incorrect. This was found by a fuzzer so some test vectors are included.
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-41
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2 files changed

+86
-41
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Diff for: ‎test/state/precompiles.cpp

+55-41
Original file line numberDiff line numberDiff line change
@@ -100,63 +100,77 @@ PrecompileAnalysis expmod_analyze(bytes_view input, evmc_revision rev) noexcept
100100
{
101101
using namespace intx;
102102

103-
static constexpr size_t INPUT_HEADER_REQUIRED_SIZE = 3 * sizeof(uint256);
104-
const int64_t min_gas = (rev >= EVMC_BERLIN) ? 200 : 0;
105-
106-
uint8_t input_header[INPUT_HEADER_REQUIRED_SIZE]{};
107-
// NOLINTNEXTLINE(bugprone-suspicious-stringview-data-usage)
108-
std::copy_n(input.data(), std::min(input.size(), INPUT_HEADER_REQUIRED_SIZE), input_header);
109-
110-
const auto base_len = be::unsafe::load<uint256>(&input_header[0]);
111-
const auto exp_len = be::unsafe::load<uint256>(&input_header[32]);
112-
const auto mod_len = be::unsafe::load<uint256>(&input_header[64]);
103+
const auto calc_adjusted_exp_len = [input](size_t offset, uint32_t len) noexcept {
104+
const auto head_len = std::min(size_t{len}, size_t{32});
105+
const auto head_explicit_bytes =
106+
offset < input.size() ?
107+
input.substr(offset, std::min(head_len, input.size() - offset)) :
108+
bytes_view{};
113109

114-
if (base_len == 0 && mod_len == 0)
115-
return {min_gas, 0};
116-
117-
static constexpr auto LEN_LIMIT = std::numeric_limits<size_t>::max();
118-
if (base_len > LEN_LIMIT || exp_len > LEN_LIMIT || mod_len > LEN_LIMIT)
119-
return {GasCostMax, 0};
120-
121-
const auto adjusted_len = [input](size_t offset, size_t len) {
122-
const auto head_len = std::min(len, size_t{32});
123-
const auto head_explicit_len =
124-
std::max(std::min(offset + head_len, input.size()), offset) - offset;
125-
const bytes_view head_explicit_bytes(&input[offset], head_explicit_len);
126110
const auto top_byte_index = head_explicit_bytes.find_first_not_of(uint8_t{0});
127-
const size_t exp_bit_width =
111+
const auto exp_bit_width =
128112
(top_byte_index != bytes_view::npos) ?
129-
(head_len - top_byte_index - 1) * 8 +
130-
static_cast<size_t>(std::bit_width(head_explicit_bytes[top_byte_index])) :
113+
8 * (head_len - top_byte_index - 1) +
114+
static_cast<unsigned>(std::bit_width(head_explicit_bytes[top_byte_index])) :
131115
0;
132116

133-
return std::max(
134-
8 * (std::max(len, size_t{32}) - 32) + (std::max(exp_bit_width, size_t{1}) - 1),
135-
size_t{1});
117+
const auto tail_len = len - head_len;
118+
const auto head_bits = std::max(exp_bit_width, size_t{1}) - 1;
119+
return std::max(8 * uint64_t{tail_len} + uint64_t{head_bits}, uint64_t{1});
136120
};
137121

138-
static constexpr auto mult_complexity_eip2565 = [](const uint256& x) noexcept {
139-
const auto w = (x + 7) >> 3;
140-
return w * w;
122+
static constexpr auto calc_mult_complexity_eip2565 = [](uint32_t y) noexcept {
123+
const auto x = uint64_t{y};
124+
const auto w = (x + 7) / 8;
125+
return w * w; // max value: 0x04000000'00000000
141126
};
142-
static constexpr auto mult_complexity_eip198 = [](const uint256& x) noexcept {
127+
static constexpr auto calc_mult_complexity_eip198 = [](uint32_t y) noexcept {
128+
const auto x = uint64_t{y};
143129
const auto xx = x * x;
144130
if (x <= 64)
145131
return xx;
146132
else if (x <= 1024)
147-
return (xx >> 2) + 96 * x - 3072;
133+
return xx / 4 + 96 * x - 3072;
148134
else
149-
return (xx >> 4) + 480 * x - 199680;
135+
return xx / 16 + 480 * x - 199680; // max value: 0x100001df'dffcf220
136+
};
137+
138+
struct Params
139+
{
140+
int64_t min_gas;
141+
unsigned final_divisor;
142+
uint64_t (*calc_mult_complexity)(uint32_t y) noexcept;
150143
};
144+
static constexpr Params byzantium_params{0, 20, calc_mult_complexity_eip198};
145+
static constexpr Params berlin_params{200, 3, calc_mult_complexity_eip2565};
146+
const auto& [min_gas, final_divisor, calc_mult_complexity] =
147+
(rev >= EVMC_BERLIN) ? berlin_params : byzantium_params;
148+
149+
static constexpr size_t INPUT_HEADER_REQUIRED_SIZE = 3 * sizeof(uint256);
150+
uint8_t input_header[INPUT_HEADER_REQUIRED_SIZE]{};
151+
// NOLINTNEXTLINE(bugprone-suspicious-stringview-data-usage)
152+
std::copy_n(input.data(), std::min(input.size(), INPUT_HEADER_REQUIRED_SIZE), input_header);
153+
154+
const auto base_len256 = be::unsafe::load<uint256>(&input_header[0]);
155+
const auto exp_len256 = be::unsafe::load<uint256>(&input_header[32]);
156+
const auto mod_len256 = be::unsafe::load<uint256>(&input_header[64]);
157+
158+
if (base_len256 == 0 && mod_len256 == 0)
159+
return {min_gas, 0};
160+
161+
static constexpr auto LEN_LIMIT = std::numeric_limits<uint32_t>::max();
162+
if (base_len256 > LEN_LIMIT || exp_len256 > LEN_LIMIT || mod_len256 > LEN_LIMIT)
163+
return {GasCostMax, 0};
164+
165+
const auto base_len = static_cast<uint32_t>(base_len256);
166+
const auto exp_len = static_cast<uint32_t>(exp_len256);
167+
const auto mod_len = static_cast<uint32_t>(mod_len256);
151168

169+
const auto adjusted_exp_len = calc_adjusted_exp_len(sizeof(input_header) + base_len, exp_len);
152170
const auto max_len = std::max(mod_len, base_len);
153-
const auto adjusted_exp_len = adjusted_len(
154-
sizeof(input_header) + static_cast<size_t>(base_len), static_cast<size_t>(exp_len));
155-
const auto gas = (rev >= EVMC_BERLIN) ?
156-
mult_complexity_eip2565(max_len) * adjusted_exp_len / 3 :
157-
mult_complexity_eip198(max_len) * adjusted_exp_len / 20;
158-
return {std::max(min_gas, static_cast<int64_t>(std::min(gas, uint256{GasCostMax}))),
159-
static_cast<size_t>(mod_len)};
171+
const auto gas = umul(calc_mult_complexity(max_len), adjusted_exp_len) / final_divisor;
172+
const auto gas_clamped = std::clamp<uint128>(gas, min_gas, GasCostMax);
173+
return {static_cast<int64_t>(gas_clamped), mod_len};
160174
}
161175

162176
PrecompileAnalysis point_evaluation_analyze(bytes_view, evmc_revision) noexcept

Diff for: ‎test/unittests/precompiles_expmod_test.cpp

+31
Original file line numberDiff line numberDiff line change
@@ -56,3 +56,34 @@ TEST(expmod, test_vectors)
5656
EXPECT_EQ(hex(result), expected_result);
5757
}
5858
}
59+
60+
TEST(expmod, analysis_oog)
61+
{
62+
// Tests the gas cost calculation of the expmod precompile.
63+
// The result cost is expected to prohibit execution.
64+
static constexpr auto GAS_LIMIT = 1'000'000'000;
65+
static constexpr std::array inputs{
66+
// clang-format off
67+
"0000000000000000000000000000000000000000000000000000000000000001 000000000000000000000000000000000000000000000000 80000000 00000020 0000000000000000000000000000000000000000000000000000000000000001 80",
68+
"0000000000000000000000000000000000000000000000000000000000000001 000000000000000000000000000000000000000000000000 40000000 00000020 0000000000000000000000000000000000000000000000000000000000000001 80",
69+
"0000000000000000000000000000000000000000000000000000000000000001 000000000000000000000000000000000000000000000000 20000000 00000020 0000000000000000000000000000000000000000000000000000000000000001 80",
70+
"0000000000000000000000000000000000000000000000000000000000000001 000000000000000000000000000000000000000000000000 10000000 00000020 0000000000000000000000000000000000000000000000000000000000000001 80",
71+
"0000000000000000000000000000000000000000000000000000000000000001 000000000000000000000000000000000000000000000000 00000000 80000020 0000000000000000000000000000000000000000000000000000000000000001 80",
72+
"0000000000000000000000000000000000000000000000000000000000000001 000000000000000000000000000000000000000000000000 00000000 40000020 0000000000000000000000000000000000000000000000000000000000000001 80",
73+
"0000000000000000000000000000000000000000000000000000000000000001 000000000000000000000000000000000000000000000000 00000000 20000020 0000000000000000000000000000000000000000000000000000000000000001 80",
74+
"fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff9 0000000000000000000000000000000000000000000000000000000000000001 0000000000000000000000000000000000000000000000000000000000000001",
75+
"0000000000000000000000000000000000000000000000000000000000000001 0000000000000000000000000000000000000000000000000000000000000040 00000000000000000000000000000000000000000000000000000000ffffffff 80"
76+
"0000000000000000000000000000000000000000000000000000000000000001 0000000000000000000000000000000000000000000000000000000000000040 00000000000000000000000000000000000000000000000000000000ffffffff 80",
77+
"00000000000000000000000000000000000000000000000000000000ffffffff 0000000000000000000000000000000000000000000000000000000000000001 0000000000000000000000000000000000000000000000000000000000000001 80",
78+
"0000000000000000000000000000000000000000000000000000000000000001 00000000000000000000000000000000000000000000000000000000ffffffff 0000000000000000000000000000000000000000000000000000000000000001 80",
79+
"0000000000000000000000000000000000000000000000000000000080000000 0000000000000000000000000000000000000000000000000000000080000000 0000000000000000000000000000000000000000000000000000000000000001 80",
80+
// clang-format on
81+
};
82+
83+
for (const auto& input_hex : inputs)
84+
{
85+
const auto input = evmc::from_spaced_hex(input_hex).value();
86+
const auto [gas_cost, max_output_size] = evmone::state::expmod_analyze(input, EVMC_PRAGUE);
87+
EXPECT_GT(gas_cost, GAS_LIMIT);
88+
}
89+
}

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