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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 #1 from https://bugs.python.org/issue40791 .)
This commit is contained in:
Devin Jeanpierre 2020-11-21 01:55:23 -07:00 committed by GitHub
parent aa3a3521ce
commit 31729366e2
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2 changed files with 2 additions and 1 deletions
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1
Misc/NEWS.d/next/Security/2020-05-28-06-06-47.bpo-40791.QGZClX.rst Normal file
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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
Modules/_operator.c
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@ -749,7 +749,7 @@ _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;