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

From: Greg Kroah-Hartman
Date: Mon Jun 20 2022 - 09:32:05 EST

From: "Jason A. Donenfeld" <Jason@xxxxxxxxx>

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@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
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;