linux-kernel - Re: [PATCH net] net: check the minimum value of gso size in virtio_net_hdr_to

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Message-ID: <CACGkMEvAWj5CFPwXx=zWjvZnMUYBORuXm-mMQe89P8xdBRid5w@mail.gmail.com>
Date: Mon, 28 Jul 2025 11:21:21 +0800
From: Jason Wang <jasowang@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Wang Liang <wangliang74@...wei.com>, mst@...hat.com, xuanzhuo@...ux.alibaba.com, 
	eperezma@...hat.com, pabeni@...hat.com, davem@...emloft.net, 
	willemb@...gle.com, atenart@...nel.org, yuehaibing@...wei.com, 
	zhangchangzhong@...wei.com, netdev@...r.kernel.org, 
	virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	steffen.klassert@...unet.com, tobias@...ongswan.org
Subject: Re: [PATCH net] net: check the minimum value of gso size in virtio_net_hdr_to_skb()

On Mon, Jul 28, 2025 at 1:16 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> Willem de Bruijn wrote:
> > Wang Liang wrote:
> > >
> > > 在 2025/7/24 21:29, Willem de Bruijn 写道:
> > > > Wang Liang wrote:
> > > >> When sending a packet with virtio_net_hdr to tun device, if the gso_type
> > > >> in virtio_net_hdr is SKB_GSO_UDP and the gso_size is less than udphdr
> > > >> size, below crash may happen.
> > > >>
> > > > gso_size is the size of the segment payload, excluding the transport
> > > > header.
> > > >
> > > > This is probably not the right approach.
> > > >
> > > > Not sure how a GSO skb can be built that is shorter than even the
> > > > transport header. Maybe an skb_dump of the GSO skb can be elucidating.
> > > >>                          return -EINVAL;
> > > >>
> > > >>                  /* Too small packets are not really GSO ones. */
> > > >> --
> > > >> 2.34.1
> > > >>
> > >
> > > Thanks for your review!
> >
> > Thanks for the dump and repro.
> >
> > I can indeed reproduce, only with the UDP_ENCAP_ESPINUDP setsockopt.
> >
> > > Here is the skb_dump result:
> > >
> > >      skb len=4 headroom=98 headlen=4 tailroom=282
> > >      mac=(64,14) mac_len=14 net=(78,20) trans=98
> > >      shinfo(txflags=0 nr_frags=0 gso(size=0 type=0 segs=0))
> > >      csum(0x8c start=140 offset=0 ip_summed=1 complete_sw=0 valid=1 level=0)
> >
> > So this is as expected not the original GSO skb, but a segment,
> > after udp_rcv_segment from udp_queue_rcv_skb.
> >
> > It is a packet with skb->data pointing to the transport header, and
> > only 4B length. So this is an illegal UDP packet with length shorter
> > than sizeof(struct udphdr).
> >
> > The packet does not enter xfrm4_gro_udp_encap_rcv, so we can exclude
> > that.
> >
> > It does enter __xfrm4_udp_encap_rcv, which will return 1 because the
> > pskb_may_pull will fail. There is a negative integer overflow just
> > before that:
> >
> >         len = skb->len - sizeof(struct udphdr);
> >         if (!pskb_may_pull(skb, sizeof(struct udphdr) + min(len, 8)))
> >                 return 1;
> >
> > This is true for all the segments btw, not just the last one. On
> > return of 1 here, the packet does not enter encap_rcv but gets
> > passed to the socket as a normal UDP packet:
> >
> >       /* If it's a keepalive packet, then just eat it.
> >        * If it's an encapsulated packet, then pass it to the
> >        * IPsec xfrm input.
> >        * Returns 0 if skb passed to xfrm or was dropped.
> >        * Returns >0 if skb should be passed to UDP.
> >        * Returns <0 if skb should be resubmitted (-ret is protocol)
> >        */
> >       int xfrm4_udp_encap_rcv(struct sock *sk, struct sk_buff *skb)
> >
> > But so the real bug, an skb with 4B in the UDP layer happens before
> > that.
> >
> > An skb_dump in udp_queue_rcv_skb of the GSO skb shows
> >
> > [  174.151409] skb len=190 headroom=64 headlen=190 tailroom=66
> > [  174.151409] mac=(64,14) mac_len=14 net=(78,20) trans=98
> > [  174.151409] shinfo(txflags=0 nr_frags=0 gso(size=4 type=65538 segs=0))
> > [  174.151409] csum(0x8c start=140 offset=0 ip_summed=3 complete_sw=0 valid=1 level=0)
> > [  174.151409] hash(0x0 sw=0 l4=0) proto=0x0800 pkttype=2 iif=8
> > [  174.151409] priority=0x0 mark=0x0 alloc_cpu=1 vlan_all=0x0
> > [  174.151409] encapsulation=0 inner(proto=0x0000, mac=0, net=0, trans=0)
> > [  174.152101] dev name=tun0 feat=0x00002000000048c1
> >
> > And of segs[0] after segmentation
> >
> > [  103.081442] skb len=38 headroom=64 headlen=38 tailroom=218
> > [  103.081442] mac=(64,14) mac_len=14 net=(78,20) trans=98
> > [  103.081442] shinfo(txflags=0 nr_frags=0 gso(size=0 type=0 segs=0))
> > [  103.081442] csum(0x8c start=140 offset=0 ip_summed=1 complete_sw=0 valid=1 level=0)
> > [  103.081442] hash(0x0 sw=0 l4=0) proto=0x0800 pkttype=2 iif=8
> > [  103.081442] priority=0x0 mark=0x0 alloc_cpu=0 vlan_all=0x0
> > [  103.081442] encapsulation=0 inner(proto=0x0000, mac=0, net=0, trans=0)
> >
> > So here translen is already 38 - (98-64) == 38 - 34 == 4.
> >
> > So the bug happens in segmentation.
> >
> > [ongoing ..]
>
> Oh of course, this is udp fragmentation offload (UFO):
> VIRTIO_NET_HDR_GSO_UDP.
>
> So only the first packet has an UDP header, and that explains why the
> other packets are only 4B.
>
> They are not UDP packets, but they have already entered the UDP stack
> due to this being GSO applied in udp_queue_rcv_skb.
>
> That was never intended to be used for UFO. Only for GRO, which does
> not build such packets.
>
> Maybe we should just drop UFO (SKB_GSO_UDP) packets in this code path?
>

Just to make sure I understand this. Did you mean to disable UFO for
guest -> host path? If yes, it seems can break some appllication.

Thanks