hamradio: defer 6pack kfree after unregister_netdev

There is a possible race condition (use-after-free) like below

 (USE)                       |  (FREE)
  dev_queue_xmit             |
   __dev_queue_xmit          |
    __dev_xmit_skb           |
     sch_direct_xmit         | ...
      xmit_one               |
       netdev_start_xmit     | tty_ldisc_kill
        __netdev_start_xmit  |  6pack_close
         sp_xmit             |   kfree
          sp_encaps          |
                             |

According to the patch "defer ax25 kfree after unregister_netdev", this
patch reorder the kfree after the unregister_netdev to avoid the possible
UAF as the unregister_netdev() is well synchronized and won't return if
there is a running routine.

Signed-off-by: Lin Ma <linma@zju.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/hamradio/6pack.c b/drivers/net/hamradio/6pack.c
index f4e8793..fb0a382 100644 (file)
--- a/drivers/net/hamradio/6pack.c
+++ b/drivers/net/hamradio/6pack.c
@@ -672,11 +672,13 @@ static void sixpack_close(struct tty_struct *tty)
        del_timer_sync(&sp->tx_t);
        del_timer_sync(&sp->resync_t);
 
-       /* Free all 6pack frame buffers. */
+       unregister_netdev(sp->dev);
+
+       /* Free all 6pack frame buffers after unreg. */
        kfree(sp->rbuff);
        kfree(sp->xbuff);
 
-       unregister_netdev(sp->dev);
+       free_netdev(sp->dev);
 }
 
 /* Perform I/O control on an active 6pack channel. */