8000 bpo-40791: Make compare_digest more constant-time. (GH-23438) (GH-23767) · python/cpython@8bef9eb · GitHub
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bpo-40791: Make compare_digest more constant-time. (GH-23438) (GH-23767)
The existing volatile `left`/`right` pointers guarantee that the reads will all occur, but does not guarantee that they will be _used_. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency between `result` and the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization. (This is change GH-1 from https://bugs.python.org/issue40791 .) (cherry picked from commit 3172936) Co-authored-by: Devin Jeanpierre <jeanpierreda@google.com>
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Add ``volatile`` to the accumulator variable in ``hmac.compare_digest``, making constant-time-defeating optimizations less likely.

Modules/_operator.c

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@@ -184,7 +184,7 @@ _tscmp(const unsigned char *a, const unsigned char *b,
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volatile const unsigned char *left;
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volatile const unsigned char *right;
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Py_ssize_t i;
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unsigned char result;
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volatile unsigned char result;
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/* loop count depends on length of b */
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length = len_b;

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