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

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Date:   Sat, 25 Dec 2021 02:00:11 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     tytso@....edu, linux-kernel@...r.kernel.org
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>
Subject: [PATCH] random: do not sign extend bytes for rotation when mixing

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>
---
 drivers/char/random.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 99cce575a79c..82db125aaed7 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -546,7 +546,7 @@ static void _mix_pool_bytes(struct entropy_store *r, const void *in,
 	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;
-- 
2.34.1