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. > > Can you please test it? > > Regards, > Oleksij > -- > Pengutronix e.K. | | > Steuerwalder Str. 21 | http://www.pengutronix.de/ | > 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | > Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |
