8000 [3.8] bpo-40791: Make compare_digest more constant-time. (GH-20444) by miss-islington · Pull Request #23437 · python/cpython · GitHub
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[3.8] bpo-40791: Make compare_digest more constant-time. (GH-20444) #23437

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Nov 21, 2020
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[3.8] bpo-40791: Make compare_digest more constant-time. (GH-20444) #23437

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bpo-40791: Make compare_digest more constant-time. (GH-20444) 
* bpo-40791: Make compare_digest more constant-time.

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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@ssbr @miss-islington
ssbr authored and miss-islington committed Nov 21, 2020
commit d823614c7e074313cedf4227e485194b6509c6ce
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Security/2020-05-28-06-06-47.bpo-40791.QGZClX.rst
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@@ -0,0 +1 @@
Add ``volatile`` to the accumulator variable in ``hmac.compare_digest``, making constant-time-defeating optimizations less likely.
2 changes: 1 addition & 1 deletion Modules/_operator.c
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Expand Up @@ -735,7 +735,7 @@ _tscmp(const unsigned char *a, const unsigned char *b,
volatile const unsigned char *left;
volatile const unsigned char *right;
Py_ssize_t i;
unsigned char result;
volatile unsigned char result;

/* loop count depends on length of b */
length = len_b;
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