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

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Date:   Wed, 28 Sep 2022 21:35:36 +0800 (GMT+08:00)
From:   duoming@....edu.cn
To:     "Leon Romanovsky" <leon@...nel.org>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        isdn@...ux-pingi.de, kuba@...nel.org
Subject: Re: [PATCH V2] mISDN: fix use-after-free bugs in l1oip timer
 handlers

Hello,

On Wed, 28 Sep 2022 12:46:10 +0300 Leon Romanovsky wrote:

> On Fri, Sep 23, 2022 at 10:25:14PM +0800, Duoming Zhou wrote:
> > 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@....edu.cn>
> > ---
> > 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);
> 
> It is racy, the call twice to del_timer_sync and cancel_work_sync
> doesn't solve "the problem that timers could be restarted by other
> threads."

Thank you for your reply! I will add a bool flag in struct l1oip to judge
whether the card is released and check this flag in other threads.

Best regards,
Duoming Zhou