[PATCH 3.16 060/192] ath9k: fix tx99 use after free

[Ben Hutchings]

3.16.49-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Miaoqing Pan <miaoqing@xxxxxxxxxxxxxx>

commit cf8ce1ea61b75712a154c93e40f2a5af2e4dd997 upstream.

One scenario that could lead to UAF is two threads writing
simultaneously to the "tx99" debug file. One of them would
set the "start" value to true and follow to ath9k_tx99_init().
Inside the function it would set the sc->tx99_state to true
after allocating sc->tx99skb. Then, the other thread would
execute write_file_tx99() and call ath9k_tx99_deinit().
sc->tx99_state would be freed. After that, the first thread
would continue inside ath9k_tx99_init() and call
r = ath9k_tx99_send(sc, sc->tx99_skb, &txctl);
that would make use of the freed sc->tx99_skb memory.

Signed-off-by: Miaoqing Pan <miaoqing@xxxxxxxxxxxxxx>
Signed-off-by: Kalle Valo <kvalo@xxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/net/wireless/ath/ath9k/tx99.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

--- a/drivers/net/wireless/ath/ath9k/tx99.c
+++ b/drivers/net/wireless/ath/ath9k/tx99.c
@@ -184,22 +184,27 @@ static ssize_t write_file_tx99(struct fi
 	if (strtobool(buf, &start))
 		return -EINVAL;
 
+	mutex_lock(&sc->mutex);
+
 	if (start == sc->tx99_state) {
 		if (!start)
-			return count;
+			goto out;
 		ath_dbg(common, XMIT, "Resetting TX99\n");
 		ath9k_tx99_deinit(sc);
 	}
 
 	if (!start) {
 		ath9k_tx99_deinit(sc);
-		return count;
+		goto out;
 	}
 
 	r = ath9k_tx99_init(sc);
-	if (r)
+	if (r) {
+		mutex_unlock(&sc->mutex);
 		return r;
-
+	}
+out:
+	mutex_unlock(&sc->mutex);
 	return count;
 }