CVE-2022-1198: Use After Free (CWE-416)
value: A use-after-free vulnerabilitity was discovered in drivers/net/hamradio/6pack.c of linux that allows an attacker to crash linux kernel by simulating ax25 device using 6pack driver from user space.
CWE description: The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
CVSS v3 base score: 5.5
Published: 2022-08-29T15:15Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: drivers/net/hamradio/6pack.c
Commit date: 2022-02-18 10:58:17 +0000
Commit message: drivers: hamradio: 6pack: fix UAF bug caused by mod_timer()

When a 6pack device is detaching, the sixpack_close() will act to cleanup
necessary resources. Although del_timer_sync() in sixpack_close()
won't return if there is an active timer, one could use mod_timer() in
sp_xmit_on_air() to wake up timer again by calling userspace syscall such
as ax25_sendmsg(), ax25_connect() and ax25_ioctl().

This unexpected waked handler, sp_xmit_on_air(), realizes nothing about
the undergoing cleanup and may still call pty_write() to use driver layer
resources that have already been released.

One of the possible race conditions is shown below:

      (USE)                      |      (FREE)
ax25_sendmsg()                   |
 ax25_queue_xmit()               |
  ...                            |
  sp_xmit()                      |
   sp_encaps()                   | sixpack_close()
    sp_xmit_on_air()             |  del_timer_sync(&sp->tx_t)
     mod_timer(&sp->tx_t,...)    |  ...
                                 |  unregister_netdev()
                                 |  ...
     (wait a while)              | tty_release()
                                 |  tty_release_struct()
                                 |   release_tty()
    sp_xmit_on_air()             |    tty_kref_put(tty_struct) //FREE
     pty_write(tty_struct) //USE |    ...

The corresponding fail log is shown below:
===============================================================
BUG: KASAN: use-after-free in __run_timers.part.0+0x170/0x470
Write of size 8 at addr ffff88800a652ab8 by task swapper/2/0
...
Call Trace:
  ...
  queue_work_on+0x3f/0x50
  pty_write+0xcd/0xe0pty_write+0xcd/0xe0
  sp_xmit_on_air+0xb2/0x1f0
  call_timer_fn+0x28/0x150
  __run_timers.part.0+0x3c2/0x470
  run_timer_softirq+0x3b/0x80
  __do_softirq+0xf1/0x380
  ...

This patch reorders the del_timer_sync() after the unregister_netdev()
to avoid UAF bugs. Because the unregister_netdev() is well synchronized,
it flushs out any pending queues, waits the refcount of net_device
decreases to zero and removes net_device from kernel. There is not any
running routines after executing unregister_netdev(). Therefore, we could
not arouse timer from userspace again.

Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Reviewed-by: Lin Ma <linma@zju.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

Diff: diff --git a/drivers/net/hamradio/6pack.c b/drivers/net/hamradio/6pack.c
index b1fc153125d9..45c3c4a1101b 100644
--- a/drivers/net/hamradio/6pack.c
+++ b/drivers/net/hamradio/6pack.c
@@ -666,15 +666,15 @@ static void sixpack_close(struct tty_struct *tty)
 	 * on the free buffers. The sp->dead completion is not sufficient
 	 * to protect us from sp->xbuff access.
 	 */
 	netif_stop_queue(sp->dev);
 
+	unregister_netdev(sp->dev);
+
 	del_timer_sync(&sp->tx_t);
 	del_timer_sync(&sp->resync_t);
 
-	unregister_netdev(sp->dev);
-
 	/* Free all 6pack frame buffers after unreg. */
 	kfree(sp->rbuff);
 	kfree(sp->xbuff);
 
 	free_netdev(sp->dev);

Vulnerable code: 	unregister_netdev(sp->dev);

Fixed code: 	unregister_netdev(sp->dev);