Re: [PATCH net] net: rose: fix UAF bug caused by rose_t0timer_expiry

[Eric Dumazet]

On Thu, Jun 30, 2022 at 5:08 PM <duoming@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx>
> > > ---
> > >  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);

This will rearm the timer.  del_timer_sync() will not help.

Please read the comment in front of del_timer_sync(), in kernel/time/timer.c

> }
>
> Best regards,
> Duoming Zhou