From 3e0588c291d6ce225f2b891753ca41d45ba42469 Mon Sep 17 00:00:00 2001
From: Lin Ma <linma@zju.edu.cn>
Date: Mon, 8 Nov 2021 18:37:21 +0800
Subject: [PATCH] hamradio: defer ax25 kfree after unregister_netdev
Git-commit: 3e0588c291d6ce225f2b891753ca41d45ba42469
Patch-mainline: v5.16-rc1
References: git-fixes

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>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/net/hamradio/mkiss.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/net/hamradio/mkiss.c b/drivers/net/hamradio/mkiss.c
index 867252a0247b..e2b332b54f06 100644
--- a/drivers/net/hamradio/mkiss.c
+++ b/drivers/net/hamradio/mkiss.c
@@ -792,13 +792,14 @@ 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);
 }
 
-- 
2.31.1