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

[Eric Dumazet]

On Thu, Jun 30, 2022 at 5:51 PM <duoming@xxxxxxxxxx> wrote:
>
> Hello,
>
> On Thu, 30 Jun 2022 17:17:10 +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.
>
> Thank you for your time, but I don't think so.
>
> > Please read the comment in front of del_timer_sync(), in kernel/time/timer.c
>
> I wrote a kernel module to test whether del_timer_sync() could finish a timer handler
> that use mod_timer() to rewind itself. The following is the result.
>
> # insmod del_timer_sync.ko
> [  929.374405] my_timer will be create.
> [  929.374738] the jiffies is :4295595572
> [  930.411581] In my_timer_function
> [  930.411956] the jiffies is 4295596609
> [  935.466643] In my_timer_function
> [  935.467505] the jiffies is 4295601665
> [  940.586538] In my_timer_function
> [  940.586916] the jiffies is 4295606784
> [  945.706579] In my_timer_function
> [  945.706885] the jiffies is 4295611904
>
> #
> # rmmod del_timer_sync.ko
> [  948.507692] the del_timer_sync is :1
> [  948.507692]
> #
> #
>
> The result of the experiment shows that the timer handler could
> be killed after we execute del_timer_sync(), even if the timer could
> rewind itself.


This is not enough to run an experiment to determine a comment is obsolete.

Especially if you are not running the code from interrupts, like rose
protocol might...

If you think the comment is obsolete, please send a patch to amend it.