Re: [PATCH net-next v2 1/7] net: Don't set transport offset to invalid value

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Fri, Feb 22, 2019 at 7:30 AM Maxim Mikityanskiy <maximmi@mellanox.com> wrote:
>
> > -----Original Message-----
> > From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> > Sent: 21 February, 2019 19:28
> > To: Maxim Mikityanskiy <maximmi@mellanox.com>
> > Cc: David S. Miller <davem@davemloft.net>; Saeed Mahameed
> > <saeedm@mellanox.com>; Willem de Bruijn <willemb@google.com>; Jason Wang
> > <jasowang@redhat.com>; Eric Dumazet <edumazet@google.com>;
> > netdev@vger.kernel.org; Eran Ben Elisha <eranbe@mellanox.com>; Tariq Toukan
> > <tariqt@mellanox.com>
> > Subject: Re: [PATCH net-next v2 1/7] net: Don't set transport offset to
> > invalid value
> >
> > On Thu, Feb 21, 2019 at 7:40 AM Maxim Mikityanskiy <maximmi@mellanox.com>
> > wrote:
> > >
> > > If the socket was created with socket(AF_PACKET, SOCK_RAW, 0),
> > > skb->protocol will be unset, __skb_flow_dissect() will fail, and
> > > skb_probe_transport_header() will fall back to the offset_hint, making
> > > the resulting skb_transport_offset incorrect.
> > >
> > > If, however, there is no transport header in the packet,
> > > transport_header shouldn't be set to an arbitrary value.
> > >
> > > Fix it by leaving the transport offset unset if it couldn't be found, to
> > > be explicit rather than to fill it with some wrong value. It changes the
> > > behavior, but if some code relied on the old behavior, it would be
> > > broken anyway, as the old one is incorrect.
> > >
> > > Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
> >
> > qdisc_pkt_len_init also expects skb_transport_header(skb) to always be
> > set for gso packets.
> >
> > Once net is merged into net-next, commit d5be7f632bad ("net: validate
>
> This commit is already in net-next, isn't it?

It is now yes.

> > untrusted gso packets without csum offload") will ensure that packets
> > that fail flow dissection do not make it into the stack. But we have
> > to skip dissection in some cases, like tun [1].
>
> OK, got you. However, is everything OK with patch [1]? It fixes false
> positives, when a packet was dropped because network_header had not been
> set yet for dissection to succeed, but what about evil packets that have
> no network_offset at the moment of calling virtio_net_hdr_to_skb? Why
> are all of them considered valid?

They are not considered valid. But it is the cost of avoiding false
positives. We cannot break legitimate traffic.

The two patches as is restrict a large class of illegal packets
from packet sockets. Which always set network header, so
this cannot be worked around.

Extending these checks to tun will require an independent
additional fix.

> > I think we need to add a check in qdisc_pkt_len_init to skip the gso
> > size estimation branch if !skb_transport_header_was_set(skb).
> >
> > Otherwise this patch set looks good to me. To avoid resubmitting
> > everything we can fix up the qdisc_pkt_len_init in a follow-up, in
> > which case I'm happy to add my Acked-by to this series.
>
> I'll add this check and submit the patch soon. Thanks for reviewing!
>
> > [1] http://patchwork.ozlabs.org/patch/1044429/

Looks good, thanks.