hamradio: defer ax25 kfree after unregister_netdev
authorLin Ma <linma@zju.edu.cn>
Mon, 8 Nov 2021 10:37:21 +0000 (18:37 +0800)
committerDavid S. Miller <davem@davemloft.net>
Tue, 9 Nov 2021 11:48:39 +0000 (11:48 +0000)
commit3e0588c291d6ce225f2b891753ca41d45ba42469
treeb7cc68c35dcab1f3a89b7c4c26291d683d46d698
parent54f0bad6686cdc50a3f4c5f7c4252c5018511459
hamradio: defer ax25 kfree after unregister_netdev

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>
drivers/net/hamradio/mkiss.c