slip: remove slip_hangup() to fix use-after-free in slip_receive_buf()

Jaeyoung Chung and Eulgyu Kim reported a slab-use-after-free read
in slip_receive_buf() when racing against tty hangup.

tty_ldisc_hangup() calls ld->ops->hangup() while holding only
a read lock on tty->ldisc_sem (via tty_ldisc_ref()).
Because slip_hangup() simply called slip_close(), it ran concurrently
with reader functions such as slip_receive_buf().

slip_close() unregisters and frees the net device and its private
struct slip, causing concurrent reader threads in slip_receive_buf()
to dereference freed memory.

Line discipline close() is already guaranteed to be called under
the write lock of tty->ldisc_sem during hangup processing
(in tty_ldisc_reinit() or tty_ldisc_kill()).

Remove slip_hangup() so teardown is serialized cleanly by slip_close().

Fixes: 5342b77c41 ("slip: Clean up create and destroy")
Reported-by: Jaeyoung Chung <[email protected]>
Reported-by: Eulgyu Kim <[email protected]>
Closes: https://lore.kernel.org/netdev/[email protected]/
Cc: Qingfang Deng <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

drivers/net/slip/slip.c

@@ -886,8 +886,6 @@ err_exit:
  * Close down a SLIP channel.
  * This means flushing out any pending queues, and then returning. This
  * call is serialized against other ldisc functions.
- *
- * We also use this method fo a hangup event
  */
 
 static void slip_close(struct tty_struct *tty)
@@ -915,11 +913,6 @@ static void slip_close(struct tty_struct *tty)
 	unregister_netdev(sl->dev);
 	/* sl_uninit() has dropped the slip_devs[] entry by now */
 }
-
-static void slip_hangup(struct tty_struct *tty)
-{
-	slip_close(tty);
-}
  /************************************************************************
   *			STANDARD SLIP ENCAPSULATION		  	 *
   ************************************************************************/
@@ -1280,7 +1273,6 @@ static struct tty_ldisc_ops sl_ldisc = {
 	.name 		= "slip",
 	.open 		= slip_open,
 	.close	 	= slip_close,
-	.hangup	 	= slip_hangup,
 	.ioctl		= slip_ioctl,
 	.receive_buf	= slip_receive_buf,
 	.write_wakeup	= slip_write_wakeup,