Re: [PATCH net] net: validate untrusted gso packets without csum offload

[David Miller <davem@davemloft.net>]

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: Fri, 15 Feb 2019 12:15:47 -0500

> From: Willem de Bruijn <willemb@google.com>
> 
> Syzkaller again found a path to a kernel crash through bad gso input.
> By building an excessively large packet to cause an skb field to wrap.
> 
> If VIRTIO_NET_HDR_F_NEEDS_CSUM was set this would have been dropped in
> skb_partial_csum_set.
> 
> GSO packets that do not set checksum offload are suspicious and rare.
> Most callers of virtio_net_hdr_to_skb already pass them to
> skb_probe_transport_header.
> 
> Move that test forward, change it to detect parse failure and drop
> packets on failure as those cleary are not one of the legitimate
> VIRTIO_NET_HDR_GSO types.
> 
> Fixes: bfd5f4a3d605 ("packet: Add GSO/csum offload support.")
> Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr")
> Reported-by: syzbot <syzkaller@googlegroups.com>
> Signed-off-by: Willem de Bruijn <willemb@google.com>

Applied and queued up for -stable, thanks Willem.

> This captures a variety of bad gso packets, but to tighten further:
> 
> - drop SKB_GSO_DODGY packets with ipip/sit/.. , which cannot be legal.
>     by ipip_gso_segment wrappers around inet_gso_segment
>     expands on 121d57af308d ("gso: validate gso_type in GSO handlers")
> 
> - limit the number of ipv6 exthdrs allowed from dodgy sources.
>     not sure where to draw the line. but not at 64K ;)
> 
> - validate the network and transport protocol returned in
>     skb_probe_transport_header against the VIRTIO_NET_HDR_GSO type
> 
> - probe all dodgy GSO packets, also those that set checksum offload.
>     this will have a performance impact, discussed previously in
>     http://patchwork.ozlabs.org/patch/861874/
>     but it would have blocked this latest bug as well
> 
> All but the last one seem pretty uncontroversial to me. If no one
> objects I plan to send those to net-next.

No objections from me.