[PATCH V2] mISDN: fix use-after-free bugs in l1oip timer handlers

[Duoming Zhou]

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 use del_timer_sync(&hc->keep_tl)
and cancel_work_sync(&hc->keep_tl) twice to stop keep_tl timer
and hc->workq. Then, we add del_timer_sync(&hc->timeout_tl)
behind l1oip_socket_close() to stop timeout_tl timer.

Fixes: 3712b42d4b1b ("Add layer1 over IP support")
Signed-off-by: Duoming Zhou <duoming@xxxxxxxxxx>
---
Changes in v2:
  - Solve the problem that timers could be restarted by other threads.

 drivers/isdn/mISDN/l1oip_core.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/isdn/mISDN/l1oip_core.c b/drivers/isdn/mISDN/l1oip_core.c
index 2c40412466e..7b89d98a781 100644
--- a/drivers/isdn/mISDN/l1oip_core.c
+++ b/drivers/isdn/mISDN/l1oip_core.c
@@ -1232,17 +1232,16 @@ release_card(struct l1oip *hc)
 {
 	int	ch;
 
-	if (timer_pending(&hc->keep_tl))
-		del_timer(&hc->keep_tl);
-
-	if (timer_pending(&hc->timeout_tl))
-		del_timer(&hc->timeout_tl);
-
+	del_timer_sync(&hc->keep_tl);
+	cancel_work_sync(&hc->workq);
+	del_timer_sync(&hc->keep_tl);
 	cancel_work_sync(&hc->workq);
 
 	if (hc->socket_thread)
 		l1oip_socket_close(hc);
 
+	del_timer_sync(&hc->timeout_tl);
+
 	if (hc->registered && hc->chan[hc->d_idx].dch)
 		mISDN_unregister_device(&hc->chan[hc->d_idx].dch->dev);
 	for (ch = 0; ch < 128; ch++) {
-- 
2.17.1