linux-kernel - [PATCH v1 2/2] hamradio: defer 6pack kfree after unregister

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Date:   Mon,  8 Nov 2021 18:37:59 +0800
From:   Lin Ma <linma@....edu.cn>
To:     netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuba@...nel.org, jirislaby@...nel.org,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        Lin Ma <linma@....edu.cn>
Subject: [PATCH v1 2/2] 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@....edu.cn>
---
 drivers/net/hamradio/6pack.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/net/hamradio/6pack.c b/drivers/net/hamradio/6pack.c
index 49f10053a794..bfdf89e54752 100644
--- 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. */
-- 
2.33.1