On Thu, Jan 18, 2018 at 12:20 AM, Willem de Bruijn
<willemdebruijn.ker...@gmail.com> wrote:
> On Wed, Jan 17, 2018 at 10:48 PM, Jason Wang <jasow...@redhat.com> wrote:
>>
>>
>> On 2018年01月18日 07:11, Willem de Bruijn wrote:
>>>
>>> From: Willem de Bruijn<will...@google.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.
