iwlwifi: mvm: fix a NULL pointer dereference of error in recovery

Sometimes, we can have an firmware crash while trying to
recover from a previous firmware problem.
When that happens, lots of things can go wrong. For example
the stations don't get added properly to mvm->fw_id_to_mac_id.

Mac80211 tries to stop A-MPDU upon reconfig but in case of
a firmware crash we will bail out fairly early and in the
end, we won't delete the A-MPDU Rx timeout.
When that timer expired after a double firmware crash,
we end up dereferencing mvm->fw_id_to_mac_id[sta_id]
which is NULL.

Fixes: 10b2b201 ("iwlwifi: mvm: add infrastructure for tracking BA session in driver")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>

drivers/net/wireless/intel/iwlwifi/mvm/sta.c

@@ -277,6 +277,18 @@ static void iwl_mvm_rx_agg_session_expired(unsigned long data)
 
 	/* Timer expired */
 	sta = rcu_dereference(ba_data->mvm->fw_id_to_mac_id[ba_data->sta_id]);
+
+	/*
+	 * sta should be valid unless the following happens:
+	 * The firmware asserts which triggers a reconfig flow, but
+	 * the reconfig fails before we set the pointer to sta into
+	 * the fw_id_to_mac_id pointer table. Mac80211 can't stop
+	 * A-MDPU and hence the timer continues to run. Then, the
+	 * timer expires and sta is NULL.
+	 */
+	if (!sta)
+		goto unlock;
+
 	mvm_sta = iwl_mvm_sta_from_mac80211(sta);
 	ieee80211_stop_rx_ba_session_offl(mvm_sta->vif,
 					  sta->addr, ba_data->tid);