mac80211: fix txq null pointer dereference

If the interface type is P2P_DEVICE or NAN, read the file of
'/sys/kernel/debug/ieee80211/phyx/netdev:wlanx/aqm' will get a
NULL pointer dereference. As for those interface type, the
pointer sdata->vif.txq is NULL.

Unable to handle kernel NULL pointer dereference at virtual address 00000011
CPU: 1 PID: 30936 Comm: cat Not tainted 4.14.104 #1
task: ffffffc0337e4880 task.stack: ffffff800cd20000
PC is at ieee80211_if_fmt_aqm+0x34/0xa0 [mac80211]
LR is at ieee80211_if_fmt_aqm+0x34/0xa0 [mac80211]
[...]
Process cat (pid: 30936, stack limit = 0xffffff800cd20000)
[...]
[<ffffff8000b7cd00>] ieee80211_if_fmt_aqm+0x34/0xa0 [mac80211]
[<ffffff8000b7c414>] ieee80211_if_read+0x60/0xbc [mac80211]
[<ffffff8000b7ccc4>] ieee80211_if_read_aqm+0x28/0x30 [mac80211]
[<ffffff80082eff94>] full_proxy_read+0x2c/0x48
[<ffffff80081eef00>] __vfs_read+0x2c/0xd4
[<ffffff80081ef084>] vfs_read+0x8c/0x108
[<ffffff80081ef494>] SyS_read+0x40/0x7c
Signed-off-by: Miaoqing Pan <miaoqing@codeaurora.org>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/1569549796-8223-1-git-send-email-miaoqing@codeaurora.org
[trim useless data from commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/debugfs_netdev.c

@@ -487,9 +487,14 @@ static ssize_t ieee80211_if_fmt_aqm(
 	const struct ieee80211_sub_if_data *sdata, char *buf, int buflen)
 {
 	struct ieee80211_local *local = sdata->local;
-	struct txq_info *txqi = to_txq_info(sdata->vif.txq);
+	struct txq_info *txqi;
 	int len;
 
+	if (!sdata->vif.txq)
+		return 0;
+
+	txqi = to_txq_info(sdata->vif.txq);
+
 	spin_lock_bh(&local->fq.lock);
 	rcu_read_lock();
 
@@ -658,7 +663,9 @@ static void add_common_files(struct ieee80211_sub_if_data *sdata)
 	DEBUGFS_ADD(rc_rateidx_vht_mcs_mask_5ghz);
 	DEBUGFS_ADD(hw_queues);
 
-	if (sdata->local->ops->wake_tx_queue)
+	if (sdata->local->ops->wake_tx_queue &&
+	    sdata->vif.type != NL80211_IFTYPE_P2P_DEVICE &&
+	    sdata->vif.type != NL80211_IFTYPE_NAN)
 		DEBUGFS_ADD(aqm);
 }