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Date:   Tue, 28 Jan 2020 10:00:47 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        syzbot <syzkaller@...glegroups.com>,
        Steffen Klassert <steffen.klassert@...unet.com>
Subject: Re: [PATCH net] udp: segment looped gso packets correctly

On Tue, Jan 28, 2020 at 6:27 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Mon, 2020-01-27 at 15:40 -0500, Willem de Bruijn wrote:
> > From: Willem de Bruijn <willemb@...gle.com>
> >
> > Multicast and broadcast packets can be looped from egress to ingress
> > pre segmentation with dev_loopback_xmit. That function unconditionally
> > sets ip_summed to CHECKSUM_UNNECESSARY.
> >
> > udp_rcv_segment segments gso packets in the udp rx path. Segmentation
> > usually executes on egress, and does not expect packets of this type.
> > __udp_gso_segment interprets !CHECKSUM_PARTIAL as CHECKSUM_NONE. But
> > the offsets are not correct for gso_make_checksum.
> >
> > UDP GSO packets are of type CHECKSUM_PARTIAL, with their uh->check set
> > to the correct pseudo header checksum. Reset ip_summed to this type.
> > (CHECKSUM_PARTIAL is allowed on ingress, see comments in skbuff.h)
> >
> > Reported-by: syzbot <syzkaller@...glegroups.com>
> > Fixes: cf329aa42b66 ("udp: cope with UDP GRO packet misdirection")
> > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> > ---
> >  include/net/udp.h | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/include/net/udp.h b/include/net/udp.h
> > index bad74f7808311..8f163d674f072 100644
> > --- a/include/net/udp.h
> > +++ b/include/net/udp.h
> > @@ -476,6 +476,9 @@ static inline struct sk_buff *udp_rcv_segment(struct sock *sk,
> >       if (!inet_get_convert_csum(sk))
> >               features |= NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM;
> >
> > +     if (skb->pkt_type == PACKET_LOOPBACK)
> > +             skb->ip_summed = CHECKSUM_PARTIAL;
> > +
> >       /* the GSO CB lays after the UDP one, no need to save and restore any
> >        * CB fragment
> >        */
>
> LGTM, Thanks!
>
> Acked-by: Paolo Abeni <pabeni@...hat.com>
>
> Out of sheer curiosity, do you know what was the kernel behaviour
> before the 'fixes' commit ? GSO packet delivered to user-space stillaggregated ?!?

Yes.. I had missed this entire ip_mc_output path when adding
segmentation (clearly).

Your patch fixed that. The other options would be segmenting in
dev_loopback_xmit or outright failing the call in udp_send_skb.
Neither of which is appealing.

Thanks for reviewing!

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