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 (file)
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 c9d38aa3423667367f5c0ad47236c1933445a19e..da1e43158ee0ac2e30ada9d03a9eea89ea6a075f 100644 (file)
--- a/Modules/_operator.c
+++ b/Modules/_operator.c
@@ -749,7 +749,7 @@ _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;