cfg80211: Address some corner cases in scan result channel updating

cfg80211_get_bss_channel() is used to update the RX channel based on the
available frame payload information (channel number from DSSS Parameter
Set element or HT Operation element). This is needed on 2.4 GHz channels
where frames may be received on neighboring channels due to overlapping
frequency range.

This might of some use on the 5 GHz band in some corner cases, but
things are more complex there since there is no n:1 or 1:n mapping
between channel numbers and frequencies due to multiple different
starting frequencies in different operating classes. This could result
in ieee80211_channel_to_frequency() returning incorrect frequency and
ieee80211_get_channel() returning incorrect channel information (or
indication of no match). In the previous implementation, this could
result in some scan results being dropped completely, e.g., for the 4.9
GHz channels. That prevented connection to such BSSs.

Fix this by using the driver-provided channel pointer if
ieee80211_get_channel() does not find matching channel data for the
channel number in the frame payload and if the scan is done with 5 MHz
or 10 MHz channel bandwidth. While doing this, also add comments
describing what the function is trying to achieve to make it easier to
understand what happens here and why.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/scan.c

@@ -1058,13 +1058,23 @@ cfg80211_bss_update(struct cfg80211_registered_device *rdev,
 	return NULL;
 }
 
+/*
+ * Update RX channel information based on the available frame payload
+ * information. This is mainly for the 2.4 GHz band where frames can be received
+ * from neighboring channels and the Beacon frames use the DSSS Parameter Set
+ * element to indicate the current (transmitting) channel, but this might also
+ * be needed on other bands if RX frequency does not match with the actual
+ * operating channel of a BSS.
+ */
 static struct ieee80211_channel *
 cfg80211_get_bss_channel(struct wiphy *wiphy, const u8 *ie, size_t ielen,
-			 struct ieee80211_channel *channel)
+			 struct ieee80211_channel *channel,
+			 enum nl80211_bss_scan_width scan_width)
 {
 	const u8 *tmp;
 	u32 freq;
 	int channel_number = -1;
+	struct ieee80211_channel *alt_channel;
 
 	tmp = cfg80211_find_ie(WLAN_EID_DS_PARAMS, ie, ielen);
 	if (tmp && tmp[1] == 1) {
@@ -1078,16 +1088,45 @@ cfg80211_get_bss_channel(struct wiphy *wiphy, const u8 *ie, size_t ielen,
 		}
 	}
 
-	if (channel_number < 0)
+	if (channel_number < 0) {
+		/* No channel information in frame payload */
 		return channel;
+	}
 
 	freq = ieee80211_channel_to_frequency(channel_number, channel->band);
-	channel = ieee80211_get_channel(wiphy, freq);
-	if (!channel)
-		return NULL;
-	if (channel->flags & IEEE80211_CHAN_DISABLED)
+	alt_channel = ieee80211_get_channel(wiphy, freq);
+	if (!alt_channel) {
+		if (channel->band == NL80211_BAND_2GHZ) {
+			/*
+			 * Better not allow unexpected channels when that could
+			 * be going beyond the 1-11 range (e.g., discovering
+			 * BSS on channel 12 when radio is configured for
+			 * channel 11.
+			 */
+			return NULL;
+		}
+
+		/* No match for the payload channel number - ignore it */
+		return channel;
+	}
+
+	if (scan_width == NL80211_BSS_CHAN_WIDTH_10 ||
+	    scan_width == NL80211_BSS_CHAN_WIDTH_5) {
+		/*
+		 * Ignore channel number in 5 and 10 MHz channels where there
+		 * may not be an n:1 or 1:n mapping between frequencies and
+		 * channel numbers.
+		 */
+		return channel;
+	}
+
+	/*
+	 * Use the channel determined through the payload channel number
+	 * instead of the RX channel reported by the driver.
+	 */
+	if (alt_channel->flags & IEEE80211_CHAN_DISABLED)
 		return NULL;
-	return channel;
+	return alt_channel;
 }
 
 /* Returned bss is reference counted and must be cleaned up appropriately. */
@@ -1112,7 +1151,8 @@ cfg80211_inform_bss_data(struct wiphy *wiphy,
 		    (data->signal < 0 || data->signal > 100)))
 		return NULL;
 
-	channel = cfg80211_get_bss_channel(wiphy, ie, ielen, data->chan);
+	channel = cfg80211_get_bss_channel(wiphy, ie, ielen, data->chan,
+					   data->scan_width);
 	if (!channel)
 		return NULL;
 
@@ -1210,7 +1250,7 @@ cfg80211_inform_bss_frame_data(struct wiphy *wiphy,
 		return NULL;
 
 	channel = cfg80211_get_bss_channel(wiphy, mgmt->u.beacon.variable,
-					   ielen, data->chan);
+					   ielen, data->chan, data->scan_width);
 	if (!channel)
 		return NULL;