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 .)
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 c9d38aa..da1e431 100644
--- a/Modules/_operator.c
+++ b/Modules/_operator.c
@@ -749,7 +749,7 @@
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;