Re: [PATCH net v3 1/2] net: check if protocol extracted by virtio_net_hdr_set_proto is correct

On Tue, Mar 9, 2021 at 6:32 AM Balazs Nemeth [off-list ref] 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.

An example where this poses an issue is with the subsequent call to
skb_flow_dissect_flow_keys_basic which relies on skb->protocol being set
correctly. A specially crafted packet could fool
skb_flow_dissect_flow_keys_basic preventing EINVAL to be returned.

Avoid blindly trusting the information provided by the virtio net header
by checking that the protocol in the packet actually matches the
protocol set by virtio_net_hdr_set_proto. Note that since the protocol
is only checked if skb->dev implements header_ops->parse_protocol,
packets from devices without the implementation are not checked at this
stage.

Fixes: 9274124f023b ("net: stricter validation of untrusted gso packets")
Signed-off-by: Balazs Nemeth <redacted>

Acked-by: Willem de Bruijn <willemb@google.com>

This still relies entirely on data from the untrusted process. But it
adds the constraint that the otherwise untrusted data at least has to
be consistent, closing one loophole.

As responded in v2, we may want to look at the (few) callers and make
sure that they initialize skb->protocol before the call to
virtio_net_hdr_to_skb where possible. That will avoid this entire
branch.