can: isotp: isotp_sendmsg(): fix return error on FC timeout on TX path

BugLink: https://bugs.launchpad.net/bugs/1952136

commit d674a8f upstream.

When the a large chunk of data send and the receiver does not send a
Flow Control frame back in time, the sendmsg() does not return a error
code, but the number of bytes sent corresponding to the size of the
packet.

If a timeout occurs the isotp_tx_timer_handler() is fired, sets
sk->sk_err and calls the sk->sk_error_report() function. It was
wrongly expected that the error would be propagated to user space in
every case. For isotp_sendmsg() blocking on wait_event_interruptible()
this is not the case.

This patch fixes the problem by checking if sk->sk_err is set and
returning the error to user space.

Fixes: e057dd3 ("can: add ISO 15765-2:2016 transport protocol")
Link: hartkopp/can-isotp#42
Link: hartkopp/can-isotp#43
Link: https://lore.kernel.org/all/[email protected]
Cc: [email protected]
Reported-by: Sottas Guillaume (LMB) <[email protected]>
Tested-by: Oliver Hartkopp <[email protected]>
Signed-off-by: Marc Kleine-Budde <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Kamal Mostafa <[email protected]>
Signed-off-by: Kleber Sacilotto de Souza <[email protected]>

net/can/isotp.c

@@ -958,6 +958,9 @@ static int isotp_sendmsg(struct socket *sock, struct msghdr *msg, size_t size)
 	if (wait_tx_done) {
 		/* wait for complete transmission of current pdu */
 		wait_event_interruptible(so->wait, so->tx.state == ISOTP_IDLE);
+
+		if (sk->sk_err)
+			return -sk->sk_err;
 	}
 
 	return size;