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| author | Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com> | 2020-11-21 09:18:41 (GMT) |
|---|---|---|
| committer | GitHub <noreply@github.com> | 2020-11-21 09:18:41 (GMT) |
| commit | c1bbca5b004b3f74d240ef8a76ff445cc1a27efb (patch) | |
| tree | 902e56c276906522951a81baf8232ddbde0f3e0c | |
| parent | ed0201805c036e4bc26670ec7ccffefb8e219f78 (diff) | |
| download | cpython-c1bbca5b004b3f74d240ef8a76ff445cc1a27efb.zip cpython-c1bbca5b004b3f74d240ef8a76ff445cc1a27efb.tar.gz cpython-c1bbca5b004b3f74d240ef8a76ff445cc1a27efb.tar.bz2 | |
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 <jeanpierreda@google.com>
| -rw-r--r-- | Misc/NEWS.d/next/Security/2020-05-28-06-06-47.bpo-40791.QGZClX.rst | 1 | ||||
| -rw-r--r-- | Modules/_operator.c | 2 |
2 files changed, 2 insertions, 1 deletions
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 0000000..69b9de1 --- /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 8a54829..6f8f68f 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; |