mac80211: Fix monitor MTU limit so that A-MSDUs get through

The maximum MTU was set to 2304, which is the maximum MSDU size. While
this is valid for normal WLAN interfaces, it is too low for monitor
interfaces. A monitor interface may receive and inject MPDU frames, and
the maximum MPDU frame size is larger than 2304. The MPDU may also
contain an A-MSDU frame, in which case the size may be much larger than
the MTU limit. Since the maximum size of an A-MSDU depends on the PHY
mode of the transmitting STA, it is not possible to set an exact MTU
limit for a monitor interface. Now the maximum MTU for a monitor
interface is unrestricted.
Signed-off-by: NJohan Almbladh <johan.almbladh@anyfinetworks.com>
Link: https://lore.kernel.org/r/20210628123246.2070558-1-johan.almbladh@anyfinetworks.comSigned-off-by: NJohannes Berg <johannes.berg@intel.com>

net/mac80211/iface.c

@@ -2001,9 +2001,16 @@ int ieee80211_if_add(struct ieee80211_local *local, const char *name,
 
 		netdev_set_default_ethtool_ops(ndev, &ieee80211_ethtool_ops);
 
-		/* MTU range: 256 - 2304 */
+		/* MTU range is normally 256 - 2304, where the upper limit is
+		 * the maximum MSDU size. Monitor interfaces send and receive
+		 * MPDU and A-MSDU frames which may be much larger so we do
+		 * not impose an upper limit in that case.
+		 */
 		ndev->min_mtu = 256;
-		ndev->max_mtu = local->hw.max_mtu;
+		if (type == NL80211_IFTYPE_MONITOR)
+			ndev->max_mtu = 0;
+		else
+			ndev->max_mtu = local->hw.max_mtu;
 
 		ret = cfg80211_register_netdevice(ndev);
 		if (ret) {