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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Thu, 23 Jan 2020 19:01:11 +0100

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? :)

-Toke