iwl3945: fix possible il->txq NULL pointer dereference in delayed works

commit 210787e8 upstream.

On il3945_down procedure we free tx queue data and nullify il->txq
pointer. After that we drop mutex and then cancel delayed works. There
is possibility, that after drooping mutex and before the cancel, some
delayed work will start and crash while trying to send commands to
the device. For example, here is reported crash in
il3945_bg_reg_txpower_periodic():
https://bugzilla.kernel.org/show_bug.cgi?id=42766#c10

Patch fix problem by adding il->txq check on works that send commands,
hence utilize tx queue.
Reported-by: Clemens Eisserer <linuxhippy@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlegacy/iwl-3945.c

@@ -1872,11 +1872,12 @@ static void iwl3945_bg_reg_txpower_periodic(struct work_struct *work)
 	struct iwl_priv *priv = container_of(work, struct iwl_priv,
 					     _3945.thermal_periodic.work);
 
-	if (test_bit(STATUS_EXIT_PENDING, &priv->status))
-		return;
-
 	mutex_lock(&priv->mutex);
+	if (test_bit(STATUS_EXIT_PENDING, &priv->status) || priv->txq == NULL)
+		goto out;
+
 	iwl3945_reg_txpower_periodic(priv);
+out:
 	mutex_unlock(&priv->mutex);
 }
 

drivers/net/wireless/iwlegacy/iwl3945-base.c

@@ -2763,7 +2763,7 @@ static void iwl3945_bg_alive_start(struct work_struct *data)
 	    container_of(data, struct iwl_priv, alive_start.work);
 
 	mutex_lock(&priv->mutex);
-	if (test_bit(STATUS_EXIT_PENDING, &priv->status))
+	if (test_bit(STATUS_EXIT_PENDING, &priv->status) || priv->txq == NULL)
 		goto out;
 
 	iwl3945_alive_start(priv);