From 0b9111922b1f399aba6ed1e1b8f2079c3da1aed8 Mon Sep 17 00:00:00 2001
From: Lin Ma <linma@zju.edu.cn>
Date: Mon, 8 Nov 2021 18:37:59 +0800
Subject: [PATCH] hamradio: defer 6pack kfree after unregister_netdev
Git-commit: 0b9111922b1f399aba6ed1e1b8f2079c3da1aed8
Patch-mainline: v5.16-rc1
References: CVE-2022-1195 bsc#1198029

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

---
 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 f4e8793e995d..fb0a3825edd0 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. */
-- 
2.31.1