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fix fuzz 18: modexp - handling of infinitely right-padded inputs leading to buffer overflow or stack overflow (#264)
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Mamy Ratsimbazafy
GitHub
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@ -120,7 +120,7 @@ func eth_evm_ecadd*(r: var openArray[byte], inputs: openarray[byte]): CttEVMStat
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# Auto-pad with zero
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var padded: array[128, byte]
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padded.rawCopy(0, inputs, 0, min(inputs.len, 128))
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padded.rawCopy(0, inputs, 0, min(inputs.len, padded.len))
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var P{.noInit.}, Q{.noInit.}, R{.noInit.}: ECP_ShortW_Jac[Fp[BN254_Snarks], G1]
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@ -173,8 +173,8 @@ func eth_evm_ecmul*(r: var openArray[byte], inputs: openarray[byte]): CttEVMStat
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return cttEVM_InvalidOutputSize
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# Auto-pad with zero
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var padded: array[128, byte]
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padded.rawCopy(0, inputs, 0, min(inputs.len, 128))
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var padded: array[96, byte]
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padded.rawCopy(0, inputs, 0, min(inputs.len, padded.len))
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var P{.noInit.}: ECP_ShortW_Jac[Fp[BN254_Snarks], G1]
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@ -401,10 +401,15 @@ func eth_evm_modexp*(r: var openArray[byte], inputs: openArray[byte]): CttEVMSta
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# Input parse sizes
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# -----------------
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# Auto-pad with zero
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var paddedLengths: array[96, byte]
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paddedLengths.rawCopy(0, inputs, 0, min(inputs.len, paddedLengths.len))
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let
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bL = BigInt[256].unmarshal(inputs.toOpenArray(0, 31), bigEndian)
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eL = BigInt[256].unmarshal(inputs.toOpenArray(32, 63), bigEndian)
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mL = BigInt[256].unmarshal(inputs.toOpenArray(64, 95), bigEndian)
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bL = BigInt[256].unmarshal(paddedLengths.toOpenArray(0, 31), bigEndian)
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eL = BigInt[256].unmarshal(paddedLengths.toOpenArray(32, 63), bigEndian)
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mL = BigInt[256].unmarshal(paddedLengths.toOpenArray(64, 95), bigEndian)
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maxSize = BigInt[256].fromUint(high(uint)) # A CPU can only address up to high(uint)
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@ -433,13 +438,20 @@ func eth_evm_modexp*(r: var openArray[byte], inputs: openArray[byte]): CttEVMSta
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# Special cases
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# ----------------------
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if paddedLengths.len + baseByteLen + exponentByteLen >= inputs.len:
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# Modulus value is in the infinitely right padded zeros input, hence is zero.
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r.setZero()
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return cttEVM_Success
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if modulusByteLen == 0:
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r.setZero()
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return cttEVM_Success
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if exponentByteLen == 0:
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r.setZero()
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r[r.len-1] = byte 1 # 0^0 = 1 and x^0 = 1
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return cttEVM_Success
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if baseByteLen == 0:
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r.setZero()
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return cttEVM_Success
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@ -448,25 +460,42 @@ func eth_evm_modexp*(r: var openArray[byte], inputs: openArray[byte]): CttEVMSta
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# ---------------------
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# Inclusive stops
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let baseStart = 96
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# Due to special-case checks and early returns,
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# only the modulus can require right-padding with zeros here
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# inputs[expStop] cannot buffer overflow
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let baseStart = paddedLengths.len
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let baseStop = baseStart+baseByteLen-1
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let expStart = baseStop+1
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let expStop = expStart+exponentByteLen-1
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let modStart = expStop+1
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let modStop = modStart+modulusByteLen-1
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# We assume that gas checks prevent numbers too big for stack allocation.
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var baseBuf = allocStackArray(SecretWord, baseWordLen)
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var modulusBuf = allocStackArray(SecretWord, modulusWordLen)
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var outputBuf = allocStackArray(SecretWord, modulusWordLen)
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template base(): untyped = baseBuf.toOpenArray(0, baseWordLen-1)
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template exponent(): untyped = inputs.toOpenArray(expStart, expStop)
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template modulus(): untyped = modulusBuf.toOpenArray(0, modulusWordLen-1)
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template output(): untyped = outputBuf.toOpenArray(0, modulusWordLen-1)
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# Base deserialization
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base.toOpenArray(0, baseWordLen-1).unmarshal(inputs.toOpenArray(baseStart, baseStop), WordBitWidth, bigEndian)
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modulus.toOpenArray(0, modulusWordLen-1).unmarshal(inputs.toOpenArray(modStart, modStop), WordBitWidth, bigEndian)
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template exponent(): untyped =
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inputs.toOpenArray(expStart, expStop)
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# Modulus deserialization
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let realLen = paddedLengths.len + baseByteLen + exponentByteLen + modulusByteLen
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let overflowLen = realLen - inputs.len
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if overflowLen > 0:
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let physLen = inputs.len-modStart # Length of data physically present (i.e. excluding padded zeros)
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var paddedModBuf = allocStackArray(byte, modulusByteLen)
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template paddedMod(): untyped = paddedModBuf.toOpenArray(0, modulusByteLen-1)
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paddedMod.rawCopy(0, inputs, modStart, physLen)
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zeroMem(paddedMod[physLen].addr, overflowLen)
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modulus.unmarshal(paddedMod, WordBitWidth, bigEndian)
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else:
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modulus.unmarshal(inputs.toOpenArray(modStart, modStop), WordBitWidth, bigEndian)
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# Computation
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# ---------------------
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@ -75,4 +75,40 @@ suite "EVM ModExp precompile (EIP-198)":
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var r = newSeq[byte](56)
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let status = r.eth_evm_modexp(input)
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doAssert status == cttEVM_Success
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test "Audit #18 - Handling of inputs infinitely right-padded with zeros (read past buffers or stack overflow for temporaries)":
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let input = [
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# Base length
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uint8 0x00,
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x08,
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# Exponent length
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00, 0x00, 0xab, 0xa8, 0xfd,
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# Modulus length
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01,
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0xc1, 0x00, 0x00, 0x00, 0x51, 0x00, 0x9b, 0x9b,
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0x9b, 0x9b, 0x9b, 0x9b, 0x9b, 0x9b, 0x9b, 0x9b,
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0x9b, 0x9b, 0x9b, 0x9b, 0x00, 0x50, 0x50, 0x50,
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0x50, 0x50, 0x9b, 0x9b, 0x9b, 0x9b, 0x9b, 0x9b,
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0x9b, 0x9b, 0x9b, 0x9b, 0x9b, 0x9b, 0x9b, 0x9b,
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0x9b, 0x9b, 0x9b, 0x9b, 0x9b, 0x9b, 0x9b, 0x9b,
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0x9b, 0x9b, 0xbc, 0x9b, 0xa0, 0x9b, 0x9b, 0x9b,
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0x9b, 0x9b, 0x9b, 0x9b, 0x9b, 0x9b, 0x9b, 0x9b,
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0x9b, 0x9b, 0x9b, 0x9b, 0x00, 0x50, 0x50, 0x50,
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0x50, 0x50, 0x50, 0x50, 0x50, 0x50, 0x50, 0x50,
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0x50, 0x50, 0x50, 0x50, 0x50, 0x50, 0x50, 0x50,
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0x50, 0x50, 0x50, 0x50, 0x50, 0x00, 0x00, 0x00,
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0xa0]
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var r = newSeq[byte](1)
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let status = r.eth_evm_modexp(input)
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doAssert status == cttEVM_Success
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