Out-of-bounds read in Eq and Ord on numbers with extreme exponents

scientific-0.3.9.0 reimplemented Eq and Ord without normalising their arguments. The new implementation short-circuits on integerLog10' c + e computed in Int, which overflows when the base-10 exponent is close to maxBound :: Int. The overflowed comparison then falls through to magnitude (e1 - e2), where the exponent difference overflows to a negative Int as well. magnitude indexed its cached power-of-ten array without a bounds check, so the negative index performed an out-of-bounds read and the process died with SIGSEGV.

With scientific-0.3.9.0 (GHC 9.10.3, x86_64-linux), as reported in basvandijk/scientific#120:

import Data.Scientific

main :: IO ()
main = print (scientific 10 maxBound == scientific 1 minBound)

$ ./repro; echo "exit=$?"
exit=139

The issue was discovered when Stackage nightly test suites (genvalidity-hspec-aeson, genvalidity-sydtest-aeson) started segfaulting after upgrading to scientific-0.3.9.0: their property tests round-trip aeson Values, which produces Scientifics with extreme exponents.

Versions before 0.3.9.0 did not crash, but Eq/Ord returned invalid results there instead (the normalisation-based implementation overflows the exponent, reporting scientific 10 maxBound == scientific 1 minBound as True). The same unchecked read was also reachable through (+) and (-) on older versions, guarded only by the documented warning about the potential 10^e blowup — reading outside the cache is not.

Resolution

scientific-0.3.10.0 computes the logarithms using Integer, fixing both the crash and the invalid comparison results, and bounds-checks magnitude, so a negative exponent now raises an error instead of reading out of bounds. All entry points (Eq, Ord, (+), (-)) are covered; upgrade to scientific-0.3.10.0 or later.

Note that printing values with exponents close to maxBound :: Int remains unreliable due to the inherent Int exponent overflow in normalize; this limitation is documented upstream and postponed to the next major release.

Credits

Reported by Tom Sydney Kerckhove (@NorfairKing); fixed by Oleg Grenrus (@phadej).