netdev - Re: [PATCH net] macsec: avoid use-after-free in macsec_handle

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Message-ID: <e9c835401b45da48559f4f0c9347b60c2b9c0911.camel@redhat.com>
Date:   Wed, 07 Oct 2020 16:45:51 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] macsec: avoid use-after-free in
 macsec_handle_frame()

On Wed, 2020-10-07 at 16:31 +0200, Eric Dumazet wrote:
> On Wed, Oct 7, 2020 at 4:09 PM Paolo Abeni <pabeni@...hat.com> wrote:
> > Hi,
> > 
> > On Wed, 2020-10-07 at 01:42 -0700, Eric Dumazet wrote:
> > > @@ -1232,9 +1233,10 @@ static rx_handler_result_t macsec_handle_frame(struct sk_buff **pskb)
> > >       macsec_rxsc_put(rx_sc);
> > > 
> > >       skb_orphan(skb);
> > > +     len = skb->len;
> > >       ret = gro_cells_receive(&macsec->gro_cells, skb);
> > >       if (ret == NET_RX_SUCCESS)
> > > -             count_rx(dev, skb->len);
> > > +             count_rx(dev, len);
> > >       else
> > >               macsec->secy.netdev->stats.rx_dropped++;
> > 
> > I'm sorry I'm low on coffee, but I can't see the race?!? here we are in
> > a BH section, and the above code dereference the skb only if it's has
> > been enqueued into the gro_cells napi. It could be dequeued/dropped
> > only after we leave this section ?!?
> 
> We should think of this as an alias for napi_gro_receive(), and not
> make any assumptions.
> Semantically the skb has been given to another layer.
> netif_rx() can absolutely queue the skb to another cpu backlog (RPS,
> RFS...), and the other cpu might have consumed the skb right away.

Ah! I completely missed that code path in gro_cells_receive()!
Thank you for pointing that out!

Acked-by: Paolo Abeni <pabeni@...hat.com>