[PATCH 4.19 03/12] hamradio: defer 6pack kfree after unregister_netdev

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Lin Ma <linma@zju.edu.cn>

commit 0b9111922b1f399aba6ed1e1b8f2079c3da1aed8 upstream.

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>
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/net/hamradio/6pack.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/drivers/net/hamradio/6pack.c
+++ b/drivers/net/hamradio/6pack.c
@@ -679,9 +679,11 @@ static void sixpack_close(struct tty_str
 	del_timer_sync(&sp->tx_t);
 	del_timer_sync(&sp->resync_t);
 
-	/* Free all 6pack frame buffers. */
+	/* Free all 6pack frame buffers after unreg. */
 	kfree(sp->rbuff);
 	kfree(sp->xbuff);
+
+	free_netdev(sp->dev);
 }
 
 /* Perform I/O control on an active 6pack channel. */