netdev - Re: [PATCH v2 1/2] net: check if protocol extracted by virtio_net_hdr_set

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Date:   Mon, 8 Mar 2021 11:01:35 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Balazs Nemeth <bnemeth@...hat.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        David Miller <davem@...emloft.net>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v2 1/2] net: check if protocol extracted by
 virtio_net_hdr_set_proto is correct

On Mon, Mar 8, 2021 at 5:32 AM Balazs Nemeth <bnemeth@...hat.com> 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 <bnemeth@...hat.com>

Going forward, please mark your the patch as targeting the net tree
using [PATCH net]

> ---
>  include/linux/virtio_net.h | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index e8a924eeea3d..6c478eee0452 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -79,8 +79,14 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
>                 if (gso_type && skb->network_header) {
>                         struct flow_keys_basic keys;
>
> -                       if (!skb->protocol)
> +                       if (!skb->protocol) {
> +                               const struct ethhdr *eth = skb_eth_hdr(skb);

eth is no longer used.

> +                               __be16 etype = dev_parse_header_protocol(skb);

nit: customary to call this protocol. etype, I guess short for
EtherType, makes sense, but is not commonly used in the kernel.

> +
>                                 virtio_net_hdr_set_proto(skb, hdr);
> +                               if (etype && etype != skb->protocol)
> +                                       return -EINVAL;
> +                       }
>  retry:
>                         if (!skb_flow_dissect_flow_keys_basic(NULL, skb, &keys,
>                                                               NULL, 0, 0, 0,
> --
> 2.29.2
>