linux-kernel - [PATCH 1/2] thunderbolt: Compare HMAC values in constant time

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Message-ID: <20250731192545.29869-2-ebiggers@kernel.org>
Date: Thu, 31 Jul 2025 12:25:44 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: linux-usb@...r.kernel.org,
	Andreas Noever <andreas.noever@...il.com>,
	Michael Jamet <michael.jamet@...el.com>,
	Mika Westerberg <westeri@...nel.org>,
	Yehezkel Bernat <YehezkelShB@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-crypto@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Eric Biggers <ebiggers@...nel.org>
Subject: [PATCH 1/2] thunderbolt: Compare HMAC values in constant time

To prevent timing attacks, HMAC value comparison needs to be constant
time.  Replace the memcmp() with the correct function, crypto_memneq().

Fixes: f67cf491175a ("thunderbolt: Add support for Internal Connection Manager (ICM)")
Signed-off-by: Eric Biggers <ebiggers@...nel.org>
---
 drivers/thunderbolt/domain.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/thunderbolt/domain.c b/drivers/thunderbolt/domain.c
index 45239703745e5..7e0eb3c07f1c7 100644
--- a/drivers/thunderbolt/domain.c
+++ b/drivers/thunderbolt/domain.c
@@ -11,10 +11,11 @@
 #include <linux/module.h>
 #include <linux/pm_runtime.h>
 #include <linux/slab.h>
 #include <linux/random.h>
 #include <crypto/hash.h>
+#include <crypto/utils.h>
 
 #include "tb.h"
 
 static DEFINE_IDA(tb_domain_ida);
 
@@ -746,11 +747,11 @@ int tb_domain_challenge_switch_key(struct tb *tb, struct tb_switch *sw)
 	ret = crypto_shash_digest(shash, challenge, sizeof(hmac), hmac);
 	if (ret)
 		goto err_free_shash;
 
 	/* The returned HMAC must match the one we calculated */
-	if (memcmp(response, hmac, sizeof(hmac))) {
+	if (crypto_memneq(response, hmac, sizeof(hmac))) {
 		ret = -EKEYREJECTED;
 		goto err_free_shash;
 	}
 
 	crypto_free_shash(tfm);
-- 
2.50.1