tty: n_gsm: fix wrong command retry handling

n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010.
See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516
The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to
the newer 27.010 here. Chapter 5.7.3 states that the valid range for the
maximum number of retransmissions (N2) is from 0 to 255 (both including).
gsm_config() fails to limit this range correctly. Furthermore,
gsm_control_retransmit() handles this number incorrectly by performing
N2 - 1 retransmission attempts. Setting N2 to zero results in more than 255
retransmission attempts.
Fix the range check in gsm_config() and the value handling in
gsm_control_send() and gsm_control_retransmit() to comply with 3GPP 27.010.

Fixes: e1eaea46 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: NDaniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220414094225.4527-11-daniel.starke@siemens.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -1354,7 +1354,6 @@ static void gsm_control_retransmit(struct timer_list *t)
 	spin_lock_irqsave(&gsm->control_lock, flags);
 	ctrl = gsm->pending_cmd;
 	if (ctrl) {
-		gsm->cretries--;
 		if (gsm->cretries == 0) {
 			gsm->pending_cmd = NULL;
 			ctrl->error = -ETIMEDOUT;
@@ -1363,6 +1362,7 @@ static void gsm_control_retransmit(struct timer_list *t)
 			wake_up(&gsm->event);
 			return;
 		}
+		gsm->cretries--;
 		gsm_control_transmit(gsm, ctrl);
 		mod_timer(&gsm->t2_timer, jiffies + gsm->t2 * HZ / 100);
 	}
@@ -1403,7 +1403,7 @@ static struct gsm_control *gsm_control_send(struct gsm_mux *gsm,
 
 	/* If DLCI0 is in ADM mode skip retries, it won't respond */
 	if (gsm->dlci[0]->mode == DLCI_MODE_ADM)
-		gsm->cretries = 1;
+		gsm->cretries = 0;
 	else
 		gsm->cretries = gsm->n2;
 
@@ -2343,7 +2343,7 @@ static int gsm_config(struct gsm_mux *gsm, struct gsm_config *c)
 	/* Check the MRU/MTU range looks sane */
 	if (c->mru > MAX_MRU || c->mtu > MAX_MTU || c->mru < 8 || c->mtu < 8)
 		return -EINVAL;
-	if (c->n2 < 3)
+	if (c->n2 > 255)
 		return -EINVAL;
 	if (c->encapsulation > 1)	/* Basic, advanced, no I */
 		return -EINVAL;