python/CVE-2022-48566-compare_digest-more-constant.patch

From 8bef9ebb1b88cfa4b2a38b93fe4ea22015d8254a Mon Sep 17 00:00:00 2001
From: "Miss Islington (bot)"
 <31488909+miss-islington@users.noreply.github.com>
Date: Mon, 14 Dec 2020 09:04:57 -0800
Subject: [PATCH] bpo-40791: Make compare_digest more constant-time. (GH-23438)
 (GH-23767)

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>
---
 Misc/NEWS.d/next/Security/2020-05-28-06-06-47.bpo-40791.QGZClX.rst |    1 +
 Modules/_operator.c                                                |    2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Security/2020-05-28-06-06-47.bpo-40791.QGZClX.rst

--- /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
--- a/Modules/_operator.c
+++ b/Modules/_operator.c
@@ -182,7 +182,7 @@ _tscmp(const unsigned char *a, const uns
     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;