[PATCH v2] mac80211: fix NULL pointer dereference in monitor mode

[dhyaan19022009-hue <dhyan19022009@gmail.com>]

From: Dhyan K Prajapati <dhyaan19022009@gmail.com>

When switching an interface to monitor mode on drivers that set the
IEEE80211_HW_WANT_MONITOR_VIF flag, ieee80211_link_info_change_notify()
can pass BSS change notifications to the driver callback even though
monitor interfaces have no associated BSS context (link->conf->bss is NULL)
This triggers a NULL pointer dereference in drivers like iwldvm when
they access BSS-specific fields without checking for NULL.
Example crash on Intel Centrino Advanced-N 6205 (iwldvm):
  BUG: kernel NULL pointer dereference, address: 0000000000000000 [ 5295.135825]
fixed the issue by adding a NULL check for link->conf->bss before proceeding to
driver notification
Tested on Intel Centrino Advanced-N 6205 [Taylor Peak] (rev 34)
Signed-off-by: Dhyan K Prajapati <dhyaan19022009@gmail.com>
---
 net/mac80211/main.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/mac80211/main.c b/net/mac80211/main.c
index b05e313c7..190222c26 100644
--- a/net/mac80211/main.c
+++ b/net/mac80211/main.c
@@ -416,6 +416,8 @@ void ieee80211_link_info_change_notify(struct ieee80211_sub_if_data *sdata,
 	case NL80211_IFTYPE_MONITOR:
 		if (!ieee80211_hw_check(&local->hw, WANT_MONITOR_VIF))
 			return;
+		if (!link->conf->bss)
+			return;
 		break;
 	default:
 		break;
-- 
2.43.0