mISDN: fix use-after-free bugs in l1oip timer handlers

The l1oip_cleanup() traverses the l1oip_ilist and calls
release_card() to cleanup module and stack. However,
release_card() calls del_timer() to delete the timers
such as keep_tl and timeout_tl. If the timer handler is
running, the del_timer() will not stop it and result in
UAF bugs. One of the processes is shown below:

    (cleanup routine)          |        (timer handler)
release_card()                 | l1oip_timeout()
 ...                           |
 del_timer()                   | ...
 ...                           |
 kfree(hc) //FREE              |
                               | hc->timeout_on = 0 //USE

Fix by calling del_timer_sync() in release_card(), which
makes sure the timer handlers have finished before the
resources, such as l1oip and so on, have been deallocated.

What's more, the hc->workq and hc->socket_thread can kick
those timers right back in. We add a bool flag to show
if card is released. Then, check this flag in hc->workq
and hc->socket_thread.

Fixes: 3712b42d ("Add layer1 over IP support")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/mISDN/l1oip.h

@@ -59,6 +59,7 @@ struct l1oip {
 	int			bundle;		/* bundle channels in one frm */
 	int			codec;		/* codec to use for transmis. */
 	int			limit;		/* limit number of bchannels */
+	bool			shutdown;	/* if card is released */
 
 	/* timer */
 	struct timer_list	keep_tl;

drivers/isdn/mISDN/l1oip_core.c

@@ -275,7 +275,7 @@ l1oip_socket_send(struct l1oip *hc, u8 localcodec, u8 channel, u32 chanmask,
 	p = frame;
 
 	/* restart timer */
-	if (time_before(hc->keep_tl.expires, jiffies + 5 * HZ))
+	if (time_before(hc->keep_tl.expires, jiffies + 5 * HZ) && !hc->shutdown)
 		mod_timer(&hc->keep_tl, jiffies + L1OIP_KEEPALIVE * HZ);
 	else
 		hc->keep_tl.expires = jiffies + L1OIP_KEEPALIVE * HZ;
@@ -601,7 +601,9 @@ l1oip_socket_parse(struct l1oip *hc, struct sockaddr_in *sin, u8 *buf, int len)
 		goto multiframe;
 
 	/* restart timer */
-	if (time_before(hc->timeout_tl.expires, jiffies + 5 * HZ) || !hc->timeout_on) {
+	if ((time_before(hc->timeout_tl.expires, jiffies + 5 * HZ) ||
+	     !hc->timeout_on) &&
+	    !hc->shutdown) {
 		hc->timeout_on = 1;
 		mod_timer(&hc->timeout_tl, jiffies + L1OIP_TIMEOUT * HZ);
 	} else /* only adjust timer */
@@ -1232,11 +1234,10 @@ release_card(struct l1oip *hc)
 {
 	int	ch;
 
-	if (timer_pending(&hc->keep_tl))
-		del_timer(&hc->keep_tl);
+	hc->shutdown = true;
 
-	if (timer_pending(&hc->timeout_tl))
-		del_timer(&hc->timeout_tl);
+	del_timer_sync(&hc->keep_tl);
+	del_timer_sync(&hc->timeout_tl);
 
 	cancel_work_sync(&hc->workq);