[PATCH 4.19 049/338] drivers: hamradio: 6pack: fix UAF bug caused by mod_timer()

Greg Kroah-Hartman
2022-04-14T13:19:28Z

From: Duoming Zhou <duoming@zju.edu.cn>

commit efe4186e6a1b54bf38b9e05450d43b0da1fd7739 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/net/hamradio/6pack.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/net/hamradio/6pack.c
+++ b/drivers/net/hamradio/6pack.c
@@ -674,14 +674,14 @@ static void sixpack_close(struct tty_str
*/
netif_stop_queue(sp->dev);

+ unregister_netdev(sp->dev);
+
del_timer_sync(&sp->tx_t);
del_timer_sync(&sp->resync_t);

/* Free all 6pack frame buffers. */
kfree(sp->rbuff);
kfree(sp->xbuff);
-
- unregister_netdev(sp->dev);
}

/* Perform I/O control on an active 6pack channel. */