[Bluetooth] Correct RFCOMM channel MTU for broken implementations

Some Bluetooth RFCOMM implementations try to negotiate a bigger channel
MTU than we can support for a particular session. The maximum MTU for
a RFCOMM session is limited through the L2CAP layer. So if the other
side proposes a channel MTU that is bigger than the underlying L2CAP
MTU, we should reduce it to the L2CAP MTU of the session minus five
bytes for the RFCOMM headers.
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/rfcomm/core.c

@@ -55,6 +55,7 @@
 #define VERSION "1.8"
 
 static int disable_cfc = 0;
+static int channel_mtu = -1;
 static unsigned int l2cap_mtu = RFCOMM_MAX_L2CAP_MTU;
 
 static struct task_struct *rfcomm_thread;
@@ -812,6 +813,9 @@ static int rfcomm_send_pn(struct rfcomm_session *s, int cr, struct rfcomm_dlc *d
 		pn->credits   = 0;
 	}
 
+	if (cr && channel_mtu >= 0)
+		pn->mtu = htobs(channel_mtu);
+	else
 		pn->mtu = htobs(d->mtu);
 
 	*ptr = __fcs(buf); ptr++;
@@ -1243,7 +1247,10 @@ static int rfcomm_apply_pn(struct rfcomm_dlc *d, int cr, struct rfcomm_pn *pn)
 
 	d->priority = pn->priority;
 
-	d->mtu = s->mtu = btohs(pn->mtu);
+	d->mtu = btohs(pn->mtu);
+
+	if (cr && d->mtu > s->mtu)
+		d->mtu = s->mtu;
 
 	return 0;
 }
@@ -1770,6 +1777,11 @@ static inline void rfcomm_accept_connection(struct rfcomm_session *s)
 	s = rfcomm_session_add(nsock, BT_OPEN);
 	if (s) {
 		rfcomm_session_hold(s);
+
+		/* We should adjust MTU on incoming sessions.
+		 * L2CAP MTU minus UIH header and FCS. */
+		s->mtu = min(l2cap_pi(nsock->sk)->omtu, l2cap_pi(nsock->sk)->imtu) - 5;
+
 		rfcomm_schedule(RFCOMM_SCHED_RX);
 	} else
 		sock_release(nsock);
@@ -2087,6 +2099,9 @@ module_exit(rfcomm_exit);
 module_param(disable_cfc, bool, 0644);
 MODULE_PARM_DESC(disable_cfc, "Disable credit based flow control");
 
+module_param(channel_mtu, int, 0644);
+MODULE_PARM_DESC(channel_mtu, "Default MTU for the RFCOMM channel");
+
 module_param(l2cap_mtu, uint, 0644);
 MODULE_PARM_DESC(l2cap_mtu, "Default MTU for the L2CAP connection");