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EVM modexp precompile: Fuzz fix unitialized mem #280

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Oct 10, 2023
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EVM modexp precompile: Fuzz fix unitialized mem #280

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18 changes: 9 additions & 9 deletions constantine/math_arbitrary_precision/arithmetic/bigints_views.nim
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Original file line number Diff line number Diff line change
Expand Up @@ -170,28 +170,28 @@ func powMod_vartime*(
# https://cetinkayakoc.net/docs/j34.pdf

let qBits = mBits-ctz
let pBits = 1+ctz
let kBits = 1+ctz
let qWords = qBits.wordsRequired()
let pWords = pBits.wordsRequired()
let kWords = kBits.wordsRequired()

var qBuf = allocStackArray(SecretWord, qWords)
var a1Buf = allocStackArray(SecretWord, qWords)
var a2Buf = allocStackArray(SecretWord, pWords)
var yBuf = allocStackArray(SecretWord, pWords)
var qInv2kBuf = allocStackArray(SecretWord, pWords)
var a2Buf = allocStackArray(SecretWord, kWords)
var yBuf = allocStackArray(SecretWord, kWords)
var qInv2kBuf = allocStackArray(SecretWord, kWords)

template q: untyped = qBuf.toOpenArray(0, qWords-1)
template a1: untyped = a1Buf.toOpenArray(0, qWords-1)
template a2: untyped = a2Buf.toOpenArray(0, pWords-1)
template y: untyped = yBuf.toOpenArray(0, pWords-1)
template qInv2k: untyped = qInv2kBuf.toOpenArray(0, pWords-1)
template a2: untyped = a2Buf.toOpenArray(0, kWords-1)
template y: untyped = yBuf.toOpenArray(0, kWords-1)
template qInv2k: untyped = qInv2kBuf.toOpenArray(0, kWords-1)

q.shiftRight_vartime(M, ctz)

a1.powOddMod_vartime(a, exponent, q, window)
a2.powMod2k_vartime(a, exponent, k = uint ctz)

qInv2k.invMod2k_vartime(qBuf.toOpenArray(0, qWords-1), uint ctz)
qInv2k.invMod2k_vartime(q, uint ctz)
y.submod2k_vartime(a2, a1, uint ctz)
y.mulmod2k_vartime(y, qInv2k, uint ctz)

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5 changes: 4 additions & 1 deletion constantine/math_arbitrary_precision/arithmetic/limbs_mod2k.nim
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Expand Up @@ -145,9 +145,12 @@ func powMod2k_vartime*(
var sBuf = allocStackArray(SecretWord, r.len)
template s: untyped = sBuf.toOpenArray(0, r.len-1)

for i in 0 ..< min(r.len, a.len):
let truncLen = min(r.len, a.len)
for i in 0 ..< truncLen:
# range [r.len, a.len) will be truncated (mod 2ᵏ)
sBuf[i] = a[i]
for i in truncLen ..< r.len:
sBuf[i] = Zero

# TODO: sliding/fixed window exponentiation
for i in countdown(exponent.len-1, 0):
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35 changes: 34 additions & 1 deletion tests/t_ethereum_evm_modexp.nim
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Expand Up @@ -73,7 +73,7 @@ suite "EVM ModExp precompile (EIP-198)":
doAssert status == cttEVM_Success
doAssert r[0] == 0, ". Result was " & $r[0]

test "Audit #5-3 - temp buffer extra unintialized word":
test "Audit #5-3 - temp buffer extra uninitialized word":
let input = [

# Length of base (1)
Expand Down Expand Up @@ -104,6 +104,39 @@ suite "EVM ModExp precompile (EIP-198)":
doAssert status == cttEVM_Success
doAssert r == @[byte 0, 0, 1, 45, 106, 227, 225, 162, 136], ". Result was " & $r

test "Audit #5-4 - temp buffer extra uninitialized word (2)":
var input = [
# Length of base
uint8 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x0c,

# Length of exponent
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x08,

# Length of modulus
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x2b,

# Base
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,


# Exponent
0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xe0,

# Modulus
0x17, 0xc6, 0xab, 0xaa, 0x3f, 0x00, 0xe5, 0xc0, 0x5b, 0x75, 0x74, 0xcb,
0xcf, 0x2a, 0x44, 0xd4, 0x3a, 0xca, 0x4a, 0xc0, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
]
var r = newSeq[byte](0x2b)
let status = eth_evm_modexp(r, input)
doAssert status == cttEVM_Success
doAssert r == @[byte 10, 141, 74, 46, 2, 18, 2, 37, 247, 220, 246, 65, 109, 246, 7, 144, 85, 202, 194, 191, 255, 255, 255, 0, 0, 0, 0, 0, 0, 0, 32, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1], ". Result was " & $r

test "Audit #8 - off-by-1 buffer overflow - ptr + length exclusive vs openArray(lo, hi) inclusive":
let input = [
# Length of base (24)
Expand Down