diff options
authorMiss Islington (bot) <31488909+miss-islington@users.noreply.github.com>2020-11-22 09:33:09 -0800
committerMichał Górny <mgorny@gentoo.org>2020-12-14 13:08:00 +0100
commitbfc498a6c971c7393d37c25bdcf5f892afb16ed2 (patch)
parentbpo-41944: No longer call eval() on content received via HTTP in the CJK code... (diff)
bpo-40791: Make compare_digest more constant-time. (GH-23438)
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> Rebased for Python 2.7 by Michał Górny <mgorny@gentoo.org>
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 0000000000..69b9de1bea
--- /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 7ddd123f40..67011a6a82 100644
--- a/Modules/operator.c
+++ b/Modules/operator.c
@@ -259,7 +259,7 @@ _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;