Re: [PATCH] net: check if protocol extracted by virtio_net_hdr_set_proto is correct

[Willem de Bruijn]

On Tue, Feb 23, 2021 at 8:48 AM Balazs Nemeth <bnemeth@xxxxxxxxxx> wrote:
>
> On Mon, 2021-02-22 at 11:39 +0800, Jason Wang wrote:
> >
> > On 2021/2/19 10:55 下午, Willem de Bruijn wrote:
> > > On Fri, Feb 19, 2021 at 3:53 AM Jason Wang <jasowang@xxxxxxxxxx>
> > > wrote:
> > > >
> > > > On 2021/2/18 11:50 下午, Willem de Bruijn wrote:
> > > > > On Thu, Feb 18, 2021 at 10:01 AM Balazs Nemeth <
> > > > > bnemeth@xxxxxxxxxx> wrote:
> > > > > > For gso packets, virtio_net_hdr_set_proto sets the protocol
> > > > > > (if it isn't
> > > > > > set) based on the type in the virtio net hdr, but the skb
> > > > > > could contain
> > > > > > anything since it could come from packet_snd through a raw
> > > > > > socket. If
> > > > > > there is a mismatch between what virtio_net_hdr_set_proto
> > > > > > sets and
> > > > > > the actual protocol, then the skb could be handled
> > > > > > incorrectly later
> > > > > > on by gso.
> > > > > >
> > > > > > The network header of gso packets starts at 14 bytes, but a
> > > > > > specially
> > > > > > crafted packet could fool the call to
> > > > > > skb_flow_dissect_flow_keys_basic
> > > > > > as the network header offset in the skb could be incorrect.
> > > > > > Consequently, EINVAL is not returned.
> > > > > >
> > > > > > There are even packets that can cause an infinite loop. For
> > > > > > example, a
> > > > > > packet with ethernet type ETH_P_MPLS_UC (which is unnoticed
> > > > > > by
> > > > > > virtio_net_hdr_to_skb) that is sent to a geneve interface
> > > > > > will be
> > > > > > handled by geneve_build_skb. In turn, it calls
> > > > > > udp_tunnel_handle_offloads which then calls
> > > > > > skb_reset_inner_headers.
> > > > > > After that, the packet gets passed to mpls_gso_segment. That
> > > > > > function
> > > > > > calculates the mpls header length by taking the difference
> > > > > > between
> > > > > > network_header and inner_network_header. Since the two are
> > > > > > equal
> > > > > > (due to the earlier call to skb_reset_inner_headers), it will
> > > > > > calculate
> > > > > > a header of length 0, and it will not pull any headers. Then,
> > > > > > it will
> > > > > > call skb_mac_gso_segment which will again call
> > > > > > mpls_gso_segment, etc...
> > > > > > This leads to the infinite loop.
> > > >
> > > > I remember kernel will validate dodgy gso packets in gso ops. I
> > > > wonder
> > > > why not do the check there? The reason is that virtio/TUN is not
> > > > the
> > > > only source for those packets.
> > > It is? All other GSO packets are generated by the stack itself,
> > > either
> > > locally or through GRO.
> >
> >
> > Something like what has been done in tcp_tso_segment()?
> >
> >      if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) {
> >                  /* Packet is from an untrusted source, reset
> > gso_segs. */
> >
> >          skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss);
> >
> >          segs = NULL;
> >                  goto out;
> >          }
> >
> > My understanding of the header check logic is that it tries to dealy
> > the
> > check as much as possible, so for device that has GRO_ROBUST, there's
> > even no need to do that.
> >
> >
> > >
> > > But indeed some checks are better performed in the GSO layer. Such
> > > as
> > > likely the 0-byte mpls header length.
> > >
> > > If we cannot trust virtio_net_hdr.gso_type passed from userspace,
> > > then
> > > we can also not trust the eth.h_proto coming from the same source.
> >
> >
> > I agree.
> >
> I'll add a check in the GSO layer as well.
> >
> > > But
> > > it makes sense to require them to be consistent. There is a
> > > dev_parse_header_protocol that may return the link layer type in a
> > > more generic fashion than casting to skb_eth_hdr.
> > >
> > > Question remains what to do for the link layer types that do not
> > > implement
> > > header_ops->parse_protocol, and so we cannot validate the packet's
> > > network protocol. Drop will cause false positives, accepts will
> > > leave a
> > > potential path, just closes it for Ethernet.
> > >
> > > This might call for multiple fixes, both on first ingest and inside
> > > the stack?
> >
> Given that this is related to dodgy packets and that we can't trust
> eth.h_proto, wouldn't it make sense to always drop packets (with
> potential false positives), erring on the side of caution, if
> header_ops->parse_protocol isn't implemented for the dev in question?

Unfortunately, that might break applications somewhere out there.