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

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Date:   Thu, 23 Jan 2020 22:36:55 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Luigi Rizzo <lrizzo@...gle.com>,
        Toke Høiland-Jørgensen 
        <toke@...hat.com>
Cc:     netdev@...r.kernel.org, Jesper Dangaard Brouer <hawk@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, sameehj@...zon.com
Subject: Re: [PATCH] net-xdp: netdev attribute to control xdpgeneric skb
 linearization

On 1/23/20 7:06 PM, Luigi Rizzo wrote:
> On Thu, Jan 23, 2020 at 10:01 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>> Luigi Rizzo <lrizzo@...gle.com> writes:
>>> On Thu, Jan 23, 2020 at 8:14 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>>>> Daniel Borkmann <daniel@...earbox.net> writes:
>>>>> On 1/23/20 10:53 AM, Toke Høiland-Jørgensen wrote:
>>>>>> Luigi Rizzo <lrizzo@...gle.com> 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).

Thanks,
Daniel