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

[Fedor Pchelkin]

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
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