From 4229ad00c19966fe365f4a945cae9d6af8b2f094 Mon Sep 17 00:00:00 2001 From: Devin Jeanpierre Date: Sat, 21 Nov 2020 01:55:23 -0700 Subject: [PATCH] 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 31729366e2bc09632e78f3896dbce0ae64914f28) Co-authored-by: Devin Jeanpierre --- .../next/Security/2020-05-28-06-06-47.bpo-40791.QGZClX.rst | 1 + Modules/_operator.c | 2 +- 2 files changed, 2 insertions(+), 1 deletion(-) create mode 100644 Misc/NEWS.d/next/Security/2020-05-28-06-06-47.bpo-40791.QGZClX.rst diff --git a/Misc/NEWS.d/next/Security/2020-05-28-06-06-47.bpo-40791.QGZClX.rst b/Misc/NEWS.d/next/Security/2020-05-28-06-06-47.bpo-40791.QGZClX.rst new file mode 100644 index 00000000000000..69b9de1beae0d1 --- /dev/null +++ b/Misc/NEWS.d/next/Security/2020-05-28-06-06-47.bpo-40791.QGZClX.rst @@ -0,0 +1 @@ +Add ``volatile`` to the accumulator variable in ``hmac.compare_digest``, making constant-time-defeating optimizations less likely. \ No newline at end of file diff --git a/Modules/_operator.c b/Modules/_operator.c index 51daa1ff6cfb30..7fff654ca610be 100644 --- a/Modules/_operator.c +++ b/Modules/_operator.c @@ -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; pFad - Phonifier reborn

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