can: isotp: convert struct tpcon::{idx,len} to unsigned int

In isotp_rcv_ff() 32 bit of data received over the network is assigned
to struct tpcon::len. Later in that function the length is checked for
the maximal supported length against MAX_MSG_LENGTH.

As struct tpcon::len is an "int" this check does not work, if the
provided length overflows the "int".

Later on struct tpcon::idx is compared against struct tpcon::len.

To fix this problem this patch converts both struct tpcon::{idx,len}
to unsigned int.

Fixes: e057dd3fc2 ("can: add ISO 15765-2:2016 transport protocol")
Link: https://lore.kernel.org/all/20220105132429.1170627-1-mkl@pengutronix.de
Cc: stable@vger.kernel.org
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Reported-by: syzbot+4c63f36709a642f801c5@syzkaller.appspotmail.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

net/can/isotp.c

@@ -119,8 +119,8 @@ enum {
 };
 
 struct tpcon {
-	int idx;
-	int len;
+	unsigned int idx;
+	unsigned int len;
 	u32 state;
 	u8 bs;
 	u8 sn;