mac80211 crash in ieee80211_sta_scan_work

[Larry Finger <Larry.Finger@lwfinger.net>]

Johannes,

With the latest wireless-2.6 git tree on my x86_64 system, I am getting a GPF in
ieee80211_sta_scan_work. I tracked it down to the following astatement:

                if (!sband ||
                    (local->scan_channel_idx >= sband->n_channels &&
                     local->scan_band >= IEEE80211_NUM_BANDS)) {

Specifically, it is the "local->scan_channel_idx >= sband->n_channels" part of the if test. When I
added test prints of local->scan_channel_idx, local->scan_band, and sband, I got the following:

mac80211: scan_channel_idx = 0, scan_band = 0, sband = ffffffff882c2f10
mac80211: scan_channel_idx = 1, scan_band = 0, sband = ffffffff882c2f10
...
...
mac80211: scan_channel_idx = 13, scan_band = 0, sband = ffffffff882c2f10
mac80211: scan_channel_idx = 0, scan_band = 2, sband = dead4ead00000001
general protection fault: 0000 [1] SMP

As can be seen, "sband" is some kind of magic number and is an invalid pointer when scan_band is
larger than IEEE80211_NUM_BANDS, which causes the GPF.

With the following patch, it works:

Index: wireless-2.6/net/mac80211/ieee80211_sta.c
===================================================================
--- wireless-2.6.orig/net/mac80211/ieee80211_sta.c
+++ wireless-2.6/net/mac80211/ieee80211_sta.c
@@ -3237,8 +3237,7 @@ void ieee80211_sta_scan_work(struct work
 		}

 		if (!sband ||
-		    (local->scan_channel_idx >= sband->n_channels &&
-		     local->scan_band >= IEEE80211_NUM_BANDS)) {
+		     local->scan_band >= IEEE80211_NUM_BANDS) {
 			ieee80211_scan_completed(local_to_hw(local));
 			return;
 		}

It seems to me that it should be OK to skip the scan_chan_idx >= sband->n_channels part of the test
as scan_band won't get to be >= to IEEE80211_NUM_BANDS until all the channels have been tested in
the legal bands.

Larry