hamradio: defer ax25 kfree after unregister_netdev

commit 3e0588c291d6ce225f2b891753ca41d45ba42469 upstream.

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

 (USE)                       |  (FREE)
ax25_sendmsg                 |
 ax25_queue_xmit             |
  dev_queue_xmit             |
   __dev_queue_xmit          |
    __dev_xmit_skb           |
     sch_direct_xmit         | ...
      xmit_one               |
       netdev_start_xmit     | tty_ldisc_kill
        __netdev_start_xmit  |  mkiss_close
         ax_xmit             |   kfree
          ax_encaps          |
                             |

Even though there are two synchronization primitives before the kfree:
1. wait_for_completion(&ax->dead). This can prevent the race with
routines from mkiss_ioctl. However, it cannot stop the routine coming
from upper layer, i.e., the ax25_sendmsg.

2. netif_stop_queue(ax->dev). It seems that this line of code aims to
halt the transmit queue but it fails to stop the routine that already
being xmit.

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/net/hamradio/mkiss.c b/drivers/net/hamradio/mkiss.c
index 078eb110381c2bee14fcc5700f79915fce890497..f0de35ac01cf0ca1718d7a965c800420e9116e9f 100644 (file)
--- a/drivers/net/hamradio/mkiss.c
+++ b/drivers/net/hamradio/mkiss.c
@@ -803,13 +803,14 @@ static void mkiss_close(struct tty_struct *tty)
         */
        netif_stop_queue(ax->dev);
 
-       /* Free all AX25 frame buffers. */
-       kfree(ax->rbuff);
-       kfree(ax->xbuff);
-
        ax->tty = NULL;
 
        unregister_netdev(ax->dev);
+
+       /* Free all AX25 frame buffers after unreg. */
+       kfree(ax->rbuff);
+       kfree(ax->xbuff);
+
        free_netdev(ax->dev);
 }