[PATCH] 6pack: fix free memory scribbles

[Alan <gnomes@lxorguk.ukuu.org.uk>]

commit acf673a3187edf72068ee2f92f4dc47d66baed47 fixed a user triggerable free
memory scribble but in doing so replaced it with a different one that allows
the user to control the data and scribble even more.

sixpack_close is called by the tty layer in tty context. The tty context is
protected by sp_get() and sp_put(). However network layer activity via
sp_xmit() is not protected this way. We must therefore stop the queue
otherwise the user gets to dump a buffer mostly of their choice into freed
kernel pages.

Signed-off-by: Alan Cox <alan@linux.intel.com>
---
 drivers/net/hamradio/6pack.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/net/hamradio/6pack.c b/drivers/net/hamradio/6pack.c
index 9f0b1c3..b3ba6a8 100644
--- a/drivers/net/hamradio/6pack.c
+++ b/drivers/net/hamradio/6pack.c
@@ -683,6 +683,12 @@ static void sixpack_close(struct tty_struct *tty)
 	if (!atomic_dec_and_test(&sp->refcnt))
 		down(&sp->dead_sem);
 
+	/* We must stop the queue to avoid potentially scribbling
+	   on the free buffers. The sp->dead_sem is not sufficient
+	   to protect us from sp->xbuff access */
+
+	netif_stop_queue(sp->dev);
+
 	del_timer_sync(&sp->tx_t);
 	del_timer_sync(&sp->resync_t);