Re: [PATCH 2/2] can: j1939: avoid possible use-after-free when j1939_can_rx_register fails

[Fedor Pchelkin]

On Sat, May 27, 2023 at 07:57:04AM +0200, Oleksij Rempel wrote:
> Hi Fedor,
> 
> On Fri, May 26, 2023 at 09:50:26PM +0300, Fedor Pchelkin wrote:
> > Hi Oleksij,
> > 
> > thanks for the reply!
> > 
> > On Fri, May 26, 2023 at 08:15:00PM +0200, Oleksij Rempel wrote:
> > > Hi Fedor,
> > > 
> > > On Fri, May 26, 2023 at 08:19:10PM +0300, Fedor Pchelkin wrote:
> > > 
> > > 
> > > Thank you for your investigation. How about this change?
> > > --- a/net/can/j1939/main.c
> > > +++ b/net/can/j1939/main.c
> > > @@ -285,8 +285,7 @@ struct j1939_priv *j1939_netdev_start(struct net_device *ndev)
> > >                  */
> > >                 kref_get(&priv_new->rx_kref);
> > >                 spin_unlock(&j1939_netdev_lock);
> > > -               dev_put(ndev);
> > > -               kfree(priv);
> > > +               j1939_priv_put(priv);
> > 
> > I don't think that's good because the priv which is directly freed here is
> > still local to the thread, and parallel threads don't have any access to
> > it. j1939_priv_create() has allocated a fresh priv and called dev_hold()
> > so dev_put() and kfree() here are okay.
> > 
> > >                 return priv_new;
> > >         }
> > >         j1939_priv_set(ndev, priv);
> > > @@ -300,8 +299,7 @@ struct j1939_priv *j1939_netdev_start(struct net_device *ndev)
> > >  
> > >   out_priv_put:
> > >         j1939_priv_set(ndev, NULL);
> > > -       dev_put(ndev);
> > > -       kfree(priv);
> > > +       j1939_priv_put(priv);
> > >  
> > >         return ERR_PTR(ret);
> > >  }
> > > 
> > > If I see it correctly, the problem is kfree() which is called without respecting
> > > the ref counting. If CPU1 has priv_new, refcounting is increased. The priv will
> > > not be freed on this place.
> > 
> > With your suggestion, I think it doesn't work correctly if
> > j1939_can_rx_register() fails and we go to out_priv_put. The priv is kept
> > but the parallel thread which may have already grabbed it thinks that
> > j1939_can_rx_register() has succeeded when actually it hasn't succeed.
> > Moreover, j1939_priv_set() makes it NULL on error path so that priv cannot
> > be accessed from ndev.
> > 
> > I also considered the alternatives where we don't have to serialize access
> > to j1939_can_rx_register() and subsequently introduce mutex. But with
> > current j1939_netdev_start() implementation I can't see how to fix the
> > racy bug without it.
>  
> Ok, it make sense.
> 
> I'll try to do some testing next week. If i'll forget it, please feel
> free to ping me.

Got it, thank you.

> 
> Regards,
> Oleksij
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