Re: [PATCH V3] net: rose: Prevent the use of freed digipeat

[Lizhi Xu <lizhi.xu@windriver.com>]

On Mon, 27 Oct 2025 10:40:34 -0700, Kuniyuki Iwashima <kuniyu@google.com> wrote:
> On Sat, Oct 25, 2025 at 12:53 AM Lizhi Xu <lizhi.xu@windriver.com> wrote:
> >
> > On Sat, 25 Oct 2025 00:15:51 -0700, Kuniyuki Iwashima <kuniyu@google.com> wrote:
> > > On Fri, Oct 24, 2025 at 11:46 PM Lizhi Xu <lizhi.xu@windriver.com> wrote:
> > > >
> > > > On Fri, 24 Oct 2025 21:25:20 -0700, Kuniyuki Iwashima <kuniyu@google.com> wrote:
> > > > > On Fri, Oct 24, 2025 at 8:51 PM Lizhi Xu <lizhi.xu@windriver.com> wrote:
> > > > > >
> > > > > > On Fri, 24 Oct 2025 19:18:46 -0700, Kuniyuki Iwashima <kuniyu@google.com> wrote:
> > > > > > > On Fri, Oct 24, 2025 at 2:39 AM Lizhi Xu <lizhi.xu@windriver.com> wrote:
> > > > > > > >
> > > > > > > > There is no synchronization between the two timers, rose_t0timer_expiry
> > > > > > > > and rose_timer_expiry.
> > > > > > > > rose_timer_expiry() puts the neighbor when the rose state is ROSE_STATE_2.
> > > > > > > > However, rose_t0timer_expiry() does initiate a restart request on the
> > > > > > > > neighbor.
> > > > > > > > When rose_t0timer_expiry() accesses the released neighbor member digipeat,
> > > > > > > > a UAF is triggered.
> > > > > > > >
> > > > > > > > To avoid this UAF, defer the put operation to rose_t0timer_expiry() and
> > > > > > > > stop restarting t0timer after putting the neighbor.
> > > > > > > >
> > > > > > > > When putting the neighbor, set the neighbor to NULL. Setting neighbor to
> > > > > > > > NULL prevents rose_t0timer_expiry() from restarting t0timer.
> > > > > > > >
> > > > > > > > syzbot reported a slab-use-after-free Read in ax25_find_cb.
> > > > > > > > BUG: KASAN: slab-use-after-free in ax25_find_cb+0x3b8/0x3f0 net/ax25/af_ax25.c:237
> > > > > > > > Read of size 1 at addr ffff888059c704c0 by task syz.6.2733/17200
> > > > > > > > Call Trace:
> > > > > > > >  ax25_find_cb+0x3b8/0x3f0 net/ax25/af_ax25.c:237
> > > > > > > >  ax25_send_frame+0x157/0xb60 net/ax25/ax25_out.c:55
> > > > > > > >  rose_send_frame+0xcc/0x2c0 net/rose/rose_link.c:106
> > > > > > > >  rose_transmit_restart_request+0x1b8/0x240 net/rose/rose_link.c:198
> > > > > > > >  rose_t0timer_expiry+0x1d/0x150 net/rose/rose_link.c:83
> > > > > > > >
> > > > > > > > Freed by task 17183:
> > > > > > > >  kfree+0x2b8/0x6d0 mm/slub.c:6826
> > > > > > > >  rose_neigh_put include/net/rose.h:165 [inline]
> > > > > > > >  rose_timer_expiry+0x537/0x630 net/rose/rose_timer.c:183
> > > > > > > >
> > > > > > > > Fixes: d860d1faa6b2 ("net: rose: convert 'use' field to refcount_t")
> > > > > > > > Reported-by: syzbot+caa052a0958a9146870d@syzkaller.appspotmail.com
> > > > > > > > Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
> > > > > > > > ---
> > > > > > > > V1 -> V2: Putting the neighbor stops t0timer from automatically starting
> > > > > > > > V2 -> V3: add rose_neigh_putex for set rose neigh to NULL
> > > > > > > >
> > > > > > > >  include/net/rose.h   | 12 ++++++++++++
> > > > > > > >  net/rose/rose_link.c |  5 +++++
> > > > > > > >  2 files changed, 17 insertions(+)
> > > > > > > >
> > > > > > > > diff --git a/include/net/rose.h b/include/net/rose.h
> > > > > > > > index 2b5491bbf39a..33de310ba778 100644
> > > > > > > > --- a/include/net/rose.h
> > > > > > > > +++ b/include/net/rose.h
> > > > > > > > @@ -167,6 +167,18 @@ static inline void rose_neigh_put(struct rose_neigh *rose_neigh)
> > > > > > > >         }
> > > > > > > >  }
> > > > > > > >
> > > > > > > > +static inline void rose_neigh_putex(struct rose_neigh **roseneigh)
> > > > > > > > +{
> > > > > > > > +       struct rose_neigh *rose_neigh = *roseneigh;
> > > > > > > > +       if (refcount_dec_and_test(&rose_neigh->use)) {
> > > > > > > > +               if (rose_neigh->ax25)
> > > > > > > > +                       ax25_cb_put(rose_neigh->ax25);
> > > > > > > > +               kfree(rose_neigh->digipeat);
> > > > > > > > +               kfree(rose_neigh);
> > > > > > > > +               *roseneigh = NULL;
> > > > > > > > +       }
> > > > > > > > +}
> > > > > > > > +
> > > > > > > >  /* af_rose.c */
> > > > > > > >  extern ax25_address rose_callsign;
> > > > > > > >  extern int  sysctl_rose_restart_request_timeout;
> > > > > > > > diff --git a/net/rose/rose_link.c b/net/rose/rose_link.c
> > > > > > > > index 7746229fdc8c..334c8cc0876d 100644
> > > > > > > > --- a/net/rose/rose_link.c
> > > > > > > > +++ b/net/rose/rose_link.c
> > > > > > > > @@ -43,6 +43,9 @@ void rose_start_ftimer(struct rose_neigh *neigh)
> > > > > > > >
> > > > > > > >  static void rose_start_t0timer(struct rose_neigh *neigh)
> > > > > > > >  {
> > > > > > > > +       if (!neigh)
> > > > > > > > +               return;
> > > > > > > > +
> > > > > > > >         timer_delete(&neigh->t0timer);
> > > > > > > >
> > > > > > > >         neigh->t0timer.function = rose_t0timer_expiry;
> > > > > > > > @@ -80,10 +83,12 @@ static void rose_t0timer_expiry(struct timer_list *t)
> > > > > > > >  {
> > > > > > > >         struct rose_neigh *neigh = timer_container_of(neigh, t, t0timer);
> > > > > > > >
> > > > > > >
> > > > > > > What prevents rose_timer_expiry() from releasing the
> > > > > > > last refcnt here ?
> > > > > > The issue reported by syzbot is that rose_t0timer_expiry() is triggered
> > > > > > first, followed by rose_timer_expiry().
> > > > >
> > > > > I don't see how you read that ordering from the report.
> > > > > https://syzkaller.appspot.com/bug?extid=caa052a0958a9146870d
> > > > Here's my understanding: See the two calltraces below.
> > >
> > > The same question still applies.
> > >
> > > What prevents rose_timer_expiry() from releasing the last
> > > refcnt before [1] ?
> > @@ -80,10 +83,12 @@ static void rose_t0timer_expiry(struct timer_list *t)
> >  {
> >         struct rose_neigh *neigh = timer_container_of(neigh, t, t0timer);
> >
> > +       rose_neigh_hold(neigh); // [3] This prevents rose_timer_expiry() from putting neigh.
> 
> If you ask yourself the same question once more here,
> you will notice the fix is broken.
> 
> What prevents rose_timer_expiry() from releasing the
> last refcnt before rose_neigh_hold() ?
The UAF issue reported by syzbot is shown below:
	CPU0				CPU1
	====				====
 rose_t0timer_expiry()
 rose_transmit_restart_request()
 rose_send_frame()
 ax25_send_frame()			rose_timer_expiry()
 					rose_neigh_put()
					kfree(neigh)
 ax25_find_cb()

My patch calls rose_neigh_hold() before executing rose_transmit_restart_request()
in rose_t0timer_expiry(). It then calls rose_neigh_putex() to release and
set neigh to NULL before executing rose_start_t0timer(). This also prevents
timer0 from restarting.

I think the only questionable part of the patch is the expiration time of
rose_timer. I don't know the expiration time because I don't have a reproducer.
If the value is very small, the result may be different.