From 3423012bcd1f598f62e02e3e319486ef19d45ded Mon Sep 17 00:00:00 2001 From: Devin Jeanpierre Date: Wed, 27 May 2020 06:53:49 -0700 Subject: [PATCH 1/2] 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 .) --- Modules/_operator.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/Modules/_operator.c b/Modules/_operator.c index 19026b6c38e603..10468997bb9356 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; From f4c395e7b62faf6595e097380edf45a8f808aea5 Mon Sep 17 00:00:00 2001 From: "blurb-it[bot]" <43283697+blurb-it[bot]@users.noreply.github.com> Date: Thu, 28 May 2020 06:06:48 +0000 Subject: [PATCH 2/2] =?UTF-8?q?=F0=9F=93=9C=F0=9F=A4=96=20Added=20by=20blu?= =?UTF-8?q?rb=5Fit.?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- .../next/Security/2020-05-28-06-06-47.bpo-40791.QGZClX.rst | 1 + 1 file changed, 1 insertion(+) 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 pFad - Phonifier reborn

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