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

[ Upstream commit 79f5962 ]

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: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Link: https://lore.kernel.org/r/20210628123246.2070558-1-johan.almbladh@anyfinetworks.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

net/mac80211/iface.c

@@ -1985,9 +1985,16 @@ 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) {