Subject: Re: [PATCH] net-xdp: netdev attribute to control xdpgeneric skb linearization

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 24 Jan 2020 10:57:10 +0100

Daniel Borkmann <[email-masked]> writes:

> On 1/23/20 7:06 PM, Luigi Rizzo wrote:
>> On Thu, Jan 23, 2020 at 10:01 AM Toke Høiland-Jørgensen <[email-masked]> wrote:
>>> Luigi Rizzo <[email-masked]> writes:
>>>> On Thu, Jan 23, 2020 at 8:14 AM Toke Høiland-Jørgensen <[email-masked]> wrote:
>>>>> Daniel Borkmann <[email-masked]> writes:
>>>>>> On 1/23/20 10:53 AM, Toke Høiland-Jørgensen wrote:
>>>>>>> Luigi Rizzo <[email-masked]> writes:
>>>>>>>
>>>>>>>> Add a netdevice flag to control skb linearization in generic xdp mode.
>>>>>>>> Among the various mechanism to control the flag, the sysfs
>>>>>>>> interface seems sufficiently simple and self-contained.
>>>>>>>> The attribute can be modified through
>>>>>>>>      /sys/class/net/<DEVICE>/xdp_linearize
>>>>>>>> The default is 1 (on)
>>>>>>
>>>>>> Needs documentation in Documentation/ABI/testing/sysfs-class-net.
>>>>>>
>>>>>>> Erm, won't turning off linearization break the XDP program's ability to
>>>>>>> do direct packet access?
>>>>>>
>>>>>> Yes, in the worst case you only have eth header pulled into linear
>>>>>> section. :/
>>>>>
>>>>> In which case an eBPF program could read/write out of bounds since the
>>>>> verifier only verifies checks against xdp->data_end. Right?
>>>>
>>>> Why out of bounds? Without linearization we construct xdp_buff as follows:
>>>>
>>>> mac_len = skb->data - skb_mac_header(skb);
>>>> hlen = skb_headlen(skb) + mac_len;
>>>> xdp->data = skb->data - mac_len;
>>>> xdp->data_end = xdp->data + hlen;
>>>> xdp->data_hard_start = skb->data - skb_headroom(skb);
>>>>
>>>> so we shouldn't go out of bounds.
>>>
>>> Hmm, right, as long as it's guaranteed that the bit up to hlen is
>>> already linear; is it? :)
>> 
>> honest question: that would be skb->len - skb->data_len, isn't that
>> the linear part by definition ?
>
> Yep, that's the linear part by definition. Generic XDP with ->data/->data_end is in
> this aspect no different from tc/BPF where we operate on skb context. Only linear part
> can be covered from skb (unless you pull in more via helper for the
> latter).

OK, but then why are we linearising in the first place? Just to get
sufficient headroom?

-Toke