netdev - Re: [PATCH net] gso: validate gso_type if SKB_GSO

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Date:   Thu, 18 Jan 2018 01:04:48 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net] gso: validate gso_type if SKB_GSO_DODGY

On Thu, Jan 18, 2018 at 12:20 AM, Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
> On Wed, Jan 17, 2018 at 10:48 PM, Jason Wang <jasowang@...hat.com> wrote:
>>
>>
>> On 2018年01月18日 07:11, Willem de Bruijn wrote:
>>>
>>> From: Willem de Bruijn<willemb@...gle.com>
>>>
>>> Validate gso_type of untrusted SKB_GSO_DODGY packets during
>>> segmentation.
>>>
>>> Untrusted user packets are limited to a small set of gso types in
>>> virtio_net_hdr_to_skb. But segmentation occurs on packet contents.
>>> Syzkaller was able to enter gso callbacks that are not hardened
>>> against untrusted user input.
>>>
>>> Fixes: f43798c27684 ("tun: Allow GSO using virtio_net_hdr")
>>
>>
>> This commit is suspicious, I guess it should be 5c7cdf339af5 ("gso: Remove
>> arbitrary checks for unsupported GSO")
>
> The specific SCTP path was introduced with commit 90017accff61 ("sctp:
> add GSO support"). But the main issue that packets can be delivered to
> gso handlers different from their gso_type goes back further.
>
> The commit you reference is actually older than the sctp gso patch, so
> it makes sense that it did not have a check in the sctp_gso_segment.
>
> I still think that we should check in inet_gso_segment when we have
> the proto, instead of in each {tcp, sctp, udp, esp, ...} handler having
> a check of the form.
>
>       !(type & (SKB_GSO_TCPV4 |
>         SKB_GSO_TCPV6))))

Unless we can create packets that legitimate combine
SKB_GSO_DODGY with tunnel headers.

virtio_net_hdr_to_skb does not accept tunneled gso types.

But a tun device can be bridged with a gre tunnel in the
host, creating a path that will call gre_gso_segment.

If that is possible, then this patch is indeed too strict and
we do need checks in the individual handlers.