Matej Cepl
debb82ab6f
OBS-URL: https://build.opensuse.org/package/show/devel:languages:python:Factory/python?expand=0&rev=386
36 lines
1.8 KiB
Diff
36 lines
1.8 KiB
Diff
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
|
|
@@ -259,7 +259,7 @@ _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;
|