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

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

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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().
Therefore, in rose_t0timer_expiry(), the reference count of neigh is
increased before entering rose_transmit_restart_request() to prevent
neigh from being put in rose_timer_expiry(). Then, in rose_t0timer_expiry(),
neigh is put before executing rose_start_t0timer() and the neigh value is
set to NULL to prevent t0timer restarts.

The case where rose_timer_expiry() is triggered before rose_t0timer_expiry()
is not considered at this time.
> 
> The t0timer could be triggered even after that happens.
> 
> 
> > +       rose_neigh_hold(neigh);
> >         rose_transmit_restart_request(neigh);
> >
> >         neigh->dce_mode = 0;
> >
> > +       rose_neigh_putex(&neigh);
> >         rose_start_t0timer(neigh);
> >  }
> >
> > --
> > 2.43.0
> >