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

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Date:   Thu, 18 Jan 2018 00:20:09 -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 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))))

Note that the above commit also only had these checks in the TSO
branch

  if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) {

but the case where this condition fails has to be protected just as
well, so a revert of that patch + new tests in all handlers added
since is not sufficient.

>> Link:http://lkml.kernel.org/r/<001a1137452496ffc305617e5fe0@...gle.com>
>> Reported-by:syzbot+fee64147a25aecd48055@...kaller.appspotmail.com
>> Signed-off-by: Willem de Bruijn<willemb@...gle.com>
>> ---
>>   net/ipv4/af_inet.c     | 21 +++++++++++++++++++++
>>   net/ipv6/ip6_offload.c | 23 ++++++++++++++++++++++-
>>   2 files changed, 43 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
>> index f00499a46927..d5a36827f7b1 100644
>> --- a/net/ipv4/af_inet.c
>> +++ b/net/ipv4/af_inet.c
>> @@ -1220,6 +1220,25 @@ int inet_sk_rebuild_header(struct sock *sk)
>>   }
>>   EXPORT_SYMBOL(inet_sk_rebuild_header);
>>   +static bool inet_gso_validate_dodgy(struct sk_buff *skb, int ipproto)
>> +{
>> +       unsigned int gso_type = skb_shinfo(skb)->gso_type;
>> +
>> +       if (gso_type & SKB_GSO_DODGY) {
>> +               switch (gso_type & ~SKB_GSO_DODGY) {
>> +               case SKB_GSO_TCPV4:
>> +               case SKB_GSO_TCPV4 | SKB_GSO_TCP_ECN:
>> +                       return ipproto == IPPROTO_TCP;
>> +               case SKB_GSO_UDP:
>> +                       return ipproto == IPPROTO_UDP;
>> +               default:
>> +                       return false;
>> +               }
>> +       }
>> +
>> +       return true;
>> +}
>
>
> This seems more strict than what was removed by 5c7cdf339af5. Any reason for
> this? I'm asking since this probably work for virito-net but I'm not sure it
> works for other sources.

All sources are limited to the same VIRTIO_NET_HDR_GSO_.. types that
virtio_net_hdr_to_skb supports, as far as I know.