2022 | 中国科学技术大学第九届信息安全大赛 | General
链上记忆大师
题目¶
听说你在区块链上部署的智能合约有过目不忘的能力。
main.py
from web3 import Web3
from web3.middleware import geth_poa_middleware
import os
import json
import time
import shutil
challenge_id = int(input('The challenge you want to play (1 or 2 or 3): '))
assert challenge_id == 1 or challenge_id == 2 or challenge_id == 3
player_bytecode = bytes.fromhex(input('Player bytecode: '))
print('Launching geth...')
shutil.copytree('/data', '/dev/shm/geth')
os.system('geth --datadir /dev/shm/geth --nodiscover --mine --unlock 0x2022af4DCbb9dA7F41cBD3dD8CdB4134D4e6DDe6 --password password.txt --verbosity 0 --datadir.minfreedisk 0 &')
time.sleep(2)
w3 = Web3(Web3.IPCProvider('/dev/shm/geth/geth.ipc'))
w3.middleware_onion.inject(geth_poa_middleware, layer=0)
w3.eth.default_account = w3.eth.accounts[0]
w3.geth.personal.unlock_account(w3.eth.default_account, open('password.txt').read().strip())
print('Deploying challenge contract...')
bytecode, abi = json.load(open(f'contract{challenge_id}.json'))
Challenge = w3.eth.contract(abi=abi, bytecode=bytecode)
tx_hash = Challenge.constructor().transact()
tx_receipt = w3.eth.wait_for_transaction_receipt(tx_hash)
print('Challenge contract address:', tx_receipt.contractAddress)
challenge = w3.eth.contract(address=tx_receipt.contractAddress, abi=abi)
print('Deploying player contract...')
tx_hash = w3.eth.send_transaction({'to': None, 'data': player_bytecode})
tx_receipt = w3.eth.wait_for_transaction_receipt(tx_hash)
print('Player contract address:', tx_receipt.contractAddress)
for i in range(10):
print(f'Testing {i + 1}/10...')
if challenge_id == 2:
n = int.from_bytes(os.urandom(2), 'big')
else:
n = int.from_bytes(os.urandom(32), 'big')
print(f'n = {n}')
if challenge.functions.test(tx_receipt.contractAddress, n).call():
print('Test passed!')
else:
print('Test failed!')
exit(-1)
print(open(f'flag{challenge_id}').read())
compile.py
from solcx import compile_source
import json
for i in 1, 2, 3:
compiled_sol = compile_source(open(f'challenge{i}.sol').read(), output_values=['abi', 'bin'])
contract_interface = compiled_sol['<stdin>:Challenge']
bytecode = contract_interface['bin']
abi = contract_interface['abi']
json.dump((bytecode, abi), open(f'contract{i}.json', 'w'))
genesis.json
{
"config": {
"chainId": 2022,
"homesteadBlock": 0,
"eip150Block": 0,
"eip155Block": 0,
"eip158Block": 0,
"byzantiumBlock": 0,
"constantinopleBlock": 0,
"petersburgBlock": 0,
"istanbulBlock": 0,
"muirGlacierBlock": 0,
"berlinBlock": 0,
"londonBlock": 0,
"arrowGlacierBlock": 0,
"grayGlacierBlock": 0,
"clique": {
"period": 0,
"epoch": 30000
}
},
"alloc": {
"0x2022af4DCbb9dA7F41cBD3dD8CdB4134D4e6DDe6": {"balance": "0x56bc75e2d63100000"}
},
"coinbase": "0x0000000000000000000000000000000000000000",
"difficulty": "0x1",
"gasLimit": "0x1c9c380",
"extraData": "0x00000000000000000000000000000000000000000000000000000000000000002022af4dcbb9da7f41cbd3dd8cdb4134d4e6dde60000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
"nonce": "0x0000000000000042",
"mixhash": "0x0000000000000000000000000000000000000000000000000000000000000000",
"parentHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
"timestamp": "0x00"
}
Dockerfile
FROM ubuntu:22.04
RUN apt update && apt install -y software-properties-common && add-apt-repository -y ppa:ethereum/ethereum && apt update && apt install -y ethereum python3-pip
RUN python3 -m pip install web3 py-solc-x
RUN python3 -c "from solcx import install_solc; install_solc(version='latest')"
COPY genesis.json privatekey.txt password.txt main.py challenge1.sol challenge2.sol challenge3.sol compile.py /
RUN geth init --datadir data genesis.json
RUN geth --datadir data account import --password password.txt privatekey.txt
RUN python3 compile.py
CMD ["/usr/bin/python3", "-u", "/main.py"]
记忆练习¶
pragma solidity =0.8.17;
interface MemoryMaster {
function memorize(uint256 n) external;
function recall() external view returns (uint256);
}
contract Challenge {
function test(MemoryMaster m, uint256 n) external returns (bool) {
m.memorize(n);
uint256 recalled = m.recall();
return recalled == n;
}
}
牛刀小试¶
pragma solidity =0.8.17;
interface MemoryMaster {
function memorize(uint16 n) external;
function recall() external view returns (uint16);
}
contract Challenge {
function test(MemoryMaster m, uint16 n) external returns (bool) {
try this.memorize_revert(m, n) {
} catch (bytes memory) {
}
uint16 recalled = m.recall();
return recalled == n;
}
function memorize_revert(MemoryMaster m, uint16 n) external {
m.memorize(n);
revert();
}
}
终极挑战¶
pragma solidity =0.8.17;
interface MemoryMaster {
function memorize(uint256 n) external view;
function recall() external view returns (uint256);
}
contract Challenge {
function test(MemoryMaster m, uint256 n) external returns (bool) {
m.memorize(n);
uint256 recalled = m.recall();
return recalled == n;
}
}
解题思路¶
需要编写包含函数 memorize 和函数 recall 的合约 MemoryMaster，合约 Challenge 将首先调用函数 memorize 并传入参数 n，随后调用函数 recall 并期望返回 n。
记忆练习¶
本题没有对函数 memorize 和函数 recall 进行任何限制，因而可以直接借助状态变量。
pragma solidity 0.8.17;
contract MemoryMaster {
uint256 n;
function memorize(uint256 _n) public {
n = _n;
}
function recall() public view returns (uint256) {
return n;
}
}
Flag¶
flag{Y0u_Ar3_n0w_f4M1l1ar_W1th_S0l1dity_st0rage_dd0d6977ef}
牛刀小试¶
- 函数
memorize被调用后即revert，尽管由于try/catch的存在，不影响后续操作，但无法再使用状态变量来传递n值 - 便想到可以借助
gasleft()，汽油的消耗不受回滚的影响，并且n的类型也由uint256调整为了uint16，不过实施起来就没那么简单了 :( - 梳理一下已知的信息
- 交易的初始汽油量受
geth的--rpc.gascap控制，默认为 \(50000000\)1 - 每次函数调用会传入剩余汽油的 63/64
- 交易的初始汽油量受
- 若函数
memorize故意消耗掉x汽油，那么传入recall的汽油量为(50000000 - k - x) * 63 / 64。\(50000000\) 映射到 \(2^{16}\)，每个区间约 \(763\) 汽油，考虑到63/64，可以以 \(720\) 为一个单位gasleft()包含GAS操作码，获取该操作执行结束后剩余的汽油量2
-
首先可利用
revert获得执行到函数recall的剩余汽油量，并计算出kpragma solidity 0.8.17; import "@openzeppelin/contracts/utils/Strings.sol"; contract MemoryMaster { function memorize(uint16 n) public { uint256 g = gasleft(); while (gasleft() > g - 720 * uint256(n)) gasleft(); } function recall() public view returns (uint16) { uint256 n = gasleft(); revert(Strings.toString(n)); return uint16(n); } } -
提交字节码到服务器，由此可大致算出
k = 30040((50000000 - k - 27688 * 720) * 63 / 64 = 29565309)Testing 1/10... n = 27688 ... web3.exceptions.ContractLogicError: execution reverted: 29565309
Exploit¶
pragma solidity 0.8.17;
contract MemoryMaster {
function memorize(uint16 n) public {
uint256 g = gasleft();
while (gasleft() > g - 720 * uint256(n)) gasleft();
}
function recall() public view returns (uint16) {
return uint16((50000000 - 30040 - gasleft() * 64 / 63) / 720);
}
}
Flag¶
flag{Gas_gAs_gaS_c4n_b3_us3d_aS_s1de_ChaNNel_5a01148fd5}
终极挑战¶
- 函数
memorize添加了view修饰符，因而不能修改状态变量，而n的类型又恢复为uint256，上一题的策略也不能再使用 - 一部分操作码，如
SSTORE、SLOAD，消耗的汽油量与访问的位置是否是初次访问有关，冷访问要消耗更多的汽油 -
可以通过故意访问一些特定的位置来向
recall传递n的值，可以使用SLOAD，访问冷/热存储位置的开销分别为2100/100，或借助于其它ADDRESS_TOUCHING_OPCODES、STORAGE_TOUCHING_OPCODESpragma solidity 0.8.17; contract MemoryMaster { mapping(uint16 => bool) access; function memorize(uint256 n) external view { for (uint16 i = 0; i < 256; i ++) { if ((n >> i) & 1 != 0) access[i]; } } function recall() external view returns (uint256) { uint256 n = 0; for (uint16 i = 256; i > 0; i --) { // i 减到 -1 会导致 revert n <<= 1; uint256 g = gasleft(); access[i - 1]; if (g - gasleft() < 1000) n += 1; } return n; } } -
状态回滚包括地址和存储位置的冷热状态
Flag¶
flag{EVM_1s_c0mPl1c4ted_bUt_Rea11y_FuN_T0_d1g_Deeper_9d3b7f6932}
参考资料¶
最后更新:
2022年12月19日 00:22:49
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