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

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Message-ID: <87blqui1zu.fsf@toke.dk>
Date:   Thu, 23 Jan 2020 17:14:29 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Luigi Rizzo <lrizzo@...gle.com>, netdev@...r.kernel.org
Cc:     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

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?

> In tc/BPF for direct packet access we have bpf_skb_pull_data() helper
> which can pull in up to X bytes into linear section on demand. I guess
> something like this could be done for XDP context as well, e.g.
> generic XDP would pull when non-linear and native XDP would have
> nothing todo (though in this case you end up writing the prog
> specifically for generic XDP with slowdown when you'd load it on
> native XDP where it's linear anyway, but that could/should be
> documented if so).

Yeah, I really don't think this is a good idea; there are enough gotchas
with the difference between generic and native XDP as it is... :/

-Toke