mac80211: Fix possible sband related NULL pointer de-reference

Existing API 'ieee80211_get_sdata_band' returns default 2 GHz band even
if the channel context configuration is NULL. This crashes for chipsets
which support 5 Ghz alone when it tries to access members of 'sband'.
Channel context configuration can be NULL in multivif case and when
channel switch is in progress (or) when it fails. Fix this by replacing
the API 'ieee80211_get_sdata_band' with  'ieee80211_get_sband' which
returns a NULL pointer for sband when the channel configuration is NULL.

An example scenario is as below:

In multivif mode (AP + STA) with drivers like ath10k, when we do a
channel switch in the AP vif (which has a number of clients connected)
and a STA vif which is connected to some other AP, when the channel
switch in AP vif fails, while the STA vifs tries to connect to the
other AP, there is a window where the channel context is NULL/invalid
and this results in a crash  while the clients connected to the AP vif
tries to reconnect and this race is very similar to the one investigated
by Michal in https://patchwork.kernel.org/patch/3788161/ and this does
happens with hardware that supports 5Ghz alone after long hours of
testing with continuous channel switch on the AP vif

ieee80211 phy0: channel context reservation cannot be finalized because
some interfaces aren't switching
wlan0: failed to finalize CSA, disconnecting
wlan0-1: deauthenticating from 8c:fd:f0:01:54:9c by local choice
	(Reason: 3=DEAUTH_LEAVING)

	WARNING: CPU: 1 PID: 19032 at net/mac80211/ieee80211_i.h:1013 sta_info_alloc+0x374/0x3fc [mac80211]
	[<bf77272c>] (sta_info_alloc [mac80211])
	[<bf78776c>] (ieee80211_add_station [mac80211]))
	[<bf73cc50>] (nl80211_new_station [cfg80211])

	Unable to handle kernel NULL pointer dereference at virtual
	address 00000014
	pgd = d5f4c000
	Internal error: Oops: 17 [#1] PREEMPT SMP ARM
	PC is at sta_info_alloc+0x380/0x3fc [mac80211]
	LR is at sta_info_alloc+0x37c/0x3fc [mac80211]
	[<bf772738>] (sta_info_alloc [mac80211])
	[<bf78776c>] (ieee80211_add_station [mac80211])
	[<bf73cc50>] (nl80211_new_station [cfg80211]))

Cc: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qti.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/net/mac80211/ibss.c b/net/mac80211/ibss.c
index 9d0f6100942b..6db09fa18269 100644
--- a/net/mac80211/ibss.c
+++ b/net/mac80211/ibss.c
@@ -992,7 +992,7 @@ static void ieee80211_update_sta_info(struct ieee80211_sub_if_data *sdata,
         enum nl80211_band band = rx_status->band;
         enum nl80211_bss_scan_width scan_width;
         struct ieee80211_local *local = sdata->local;
-        struct ieee80211_supported_band *sband = local->hw.wiphy->bands[band];
+        struct ieee80211_supported_band *sband;
         bool rates_updated = false;
         u32 supp_rates = 0;
 
@@ -1002,6 +1002,10 @@ static void ieee80211_update_sta_info(struct ieee80211_sub_if_data *sdata,
         if (!ether_addr_equal(mgmt->bssid, sdata->u.ibss.bssid))
                 return;
 
+        sband = local->hw.wiphy->bands[band];
+        if (WARN_ON(!sband))
+                return;
+
         rcu_read_lock();
         sta = sta_info_get(sdata, mgmt->sa);