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Avoid reading out of buffer limits in readWordFromBytes #402

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Avoid reading out of buffer limits in readWordFromBytes #402

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6527a34
Expr: avoid wraparound in readWordFromBytes
zoep Oct 3, 2023
4e02ac6
Compute the wraparound
zoep Oct 9, 2023
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14 changes: 11 additions & 3 deletions src/EVM/Expr.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -279,9 +279,17 @@ readWord i b = readWordFromBytes i b
-- returns an abstract ReadWord expression if a concrete word cannot be constructed
readWordFromBytes :: Expr EWord -> Expr Buf -> Expr EWord
readWordFromBytes (Lit idx) (ConcreteBuf bs) =
case toInt idx of
Nothing -> Lit 0
Just i -> Lit $ word $ padRight 32 $ BS.take 32 $ BS.drop i bs
if idx <= (maxBound :: W256) - 31
-- the word we are trying to read is within the address space of the buffer
then case toInt idx of
Nothing -> Lit 0
Just i -> Lit $ word $ padRight 32 $ BS.take 32 $ BS.drop i bs
-- if the word we are trying to read goes outside of bounds, then
-- wraparound and take the remaing bytes from the beginning of the
-- buffer
else case toInt ((maxBound :: W256) - idx + 1) of
Nothing -> Lit 0 -- should never reach here because maxBound - idx + 1 < 32
Just i -> Lit $ word $ padLeft i $ BS.take (32 - i) bs
readWordFromBytes i@(Lit idx) buf = let
bytes = [readByte (Lit i') buf | i' <- [idx .. idx + 31]]
in if Prelude.and . (fmap isLitByte) $ bytes
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