linux-kernel - [PATCH 5.4 040/240] random: do not sign extend bytes for rotation when mixing

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Message-Id: <20220620124739.101869255@linuxfoundation.org>
Date:   Mon, 20 Jun 2022 14:49:01 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>
Subject: [PATCH 5.4 040/240] random: do not sign extend bytes for rotation when mixing

From: "Jason A. Donenfeld" <Jason@...c4.com>

commit 0d9488ffbf2faddebc6bac055bfa6c93b94056a3 upstream.

By using `char` instead of `unsigned char`, certain platforms will sign
extend the byte when `w = rol32(*bytes++, input_rotate)` is called,
meaning that bit 7 is overrepresented when mixing. This isn't a real
problem (unless the mixer itself is already broken) since it's still
invertible, but it's not quite correct either. Fix this by using an
explicit unsigned type.

Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/char/random.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -548,7 +548,7 @@ static void _mix_pool_bytes(struct entro
 	unsigned long i, tap1, tap2, tap3, tap4, tap5;
 	int input_rotate;
 	int wordmask = r->poolinfo->poolwords - 1;
-	const char *bytes = in;
+	const unsigned char *bytes = in;
 	__u32 w;

 	tap1 = r->poolinfo->tap1;