netdev - Re: [PATCH net] net: rose: fix UAF bug caused by rose_t0timer

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Date:   Thu, 30 Jun 2022 23:08:41 +0800 (GMT+08:00)
From:   duoming@....edu.cn
To:     "Eric Dumazet" <edumazet@...gle.com>
Cc:     linux-hams@...r.kernel.org, netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Paolo Abeni" <pabeni@...hat.com>,
        "Jakub Kicinski" <kuba@...nel.org>,
        "David Miller" <davem@...emloft.net>,
        "Ralf Baechle" <ralf@...ux-mips.org>
Subject: Re: [PATCH net] net: rose: fix UAF bug caused by
 rose_t0timer_expiry

Hello,

On Thu, 30 Jun 2022 16:44:29 +0200 Eric Dumazet wrote:

> > There are UAF bugs caused by rose_t0timer_expiry(). The
> > root cause is that del_timer() could not stop the timer
> > handler that is running and there is no synchronization.
> > One of the race conditions is shown below:
> >
> >     (thread 1)             |        (thread 2)
> >                            | rose_device_event
> >                            |   rose_rt_device_down
> >                            |     rose_remove_neigh
> > rose_t0timer_expiry        |       rose_stop_t0timer(rose_neigh)
> >   ...                      |         del_timer(&neigh->t0timer)
> >                            |         kfree(rose_neigh) //[1]FREE
> >   neigh->dce_mode //[2]USE |
> >
> > The rose_neigh is deallocated in position [1] and use in
> > position [2].
> >
> > The crash trace triggered by POC is like below:
> >
> > BUG: KASAN: use-after-free in expire_timers+0x144/0x320
> > Write of size 8 at addr ffff888009b19658 by task swapper/0/0
> > ...
> > Call Trace:
> >  <IRQ>
> >  dump_stack_lvl+0xbf/0xee
> >  print_address_description+0x7b/0x440
> >  print_report+0x101/0x230
> >  ? expire_timers+0x144/0x320
> >  kasan_report+0xed/0x120
> >  ? expire_timers+0x144/0x320
> >  expire_timers+0x144/0x320
> >  __run_timers+0x3ff/0x4d0
> >  run_timer_softirq+0x41/0x80
> >  __do_softirq+0x233/0x544
> >  ...
> >
> > This patch changes del_timer() in rose_stop_t0timer() and
> > rose_stop_ftimer() to del_timer_sync() in order that the
> > timer handler could be finished before the resources such as
> > rose_neigh and so on are deallocated. As a result, the UAF
> > bugs could be mitigated.
> >
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Signed-off-by: Duoming Zhou <duoming@....edu.cn>
> > ---
> >  net/rose/rose_link.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/rose/rose_link.c b/net/rose/rose_link.c
> > index 8b96a56d3a4..9734d1264de 100644
> > --- a/net/rose/rose_link.c
> > +++ b/net/rose/rose_link.c
> > @@ -54,12 +54,12 @@ static void rose_start_t0timer(struct rose_neigh *neigh)
> >
> >  void rose_stop_ftimer(struct rose_neigh *neigh)
> >  {
> > -       del_timer(&neigh->ftimer);
> > +       del_timer_sync(&neigh->ftimer);
> >  }
> 
> Are you sure this is safe ?
> 
> del_timer_sync() could hang if the caller holds a lock that the timer
> function would need to acquire.

I think this is safe. The rose_ftimer_expiry() is an empty function that is
shown below:

static void rose_ftimer_expiry(struct timer_list *t)
{
}

> >
> >  void rose_stop_t0timer(struct rose_neigh *neigh)
> >  {
> > -       del_timer(&neigh->t0timer);
> > +       del_timer_sync(&neigh->t0timer);
> >  }
> 
> Same here, please explain why it is safe.

The rose_stop_t0timer() may hold "rose_node_list_lock" and "rose_neigh_list_lock",
but the timer handler rose_t0timer_expiry() that is shown below does not need
these two locks.

static void rose_t0timer_expiry(struct timer_list *t)
{
	struct rose_neigh *neigh = from_timer(neigh, t, t0timer);

	rose_transmit_restart_request(neigh);

	neigh->dce_mode = 0;

	rose_start_t0timer(neigh);
}

Best regards,
Duoming Zhou