netdev - Re: [PATCH net] nfp: bpf: Add an MTU check before offloading BPF

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Message-ID: <20210930075959.587f9905@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 30 Sep 2021 07:59:59 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Niklas Söderlund 
        <niklas.soderlund@...igine.com>
Cc:     Simon Horman <simon.horman@...igine.com>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        oss-drivers@...igine.com, Yu Xiao <yu.xiao@...igine.com>,
        Yinjun Zhang <yinjun.zhang@...igine.com>,
        Louis Peens <louis.peens@...igine.com>
Subject: Re: [PATCH net] nfp: bpf: Add an MTU check before offloading BPF

On Thu, 30 Sep 2021 16:46:34 +0200 Niklas Söderlund wrote:
> When the MTU is changed after the program is offloaded the check in 
> nfp_bpf_check_mtu() is consulted and as it checks the MTU differently 
> and fails the change. Maybe we should align this the other way around 
> and update the check in nfp_bpf_check_mtu() to match the one in 
> nfp_net_bpf_load()?

That sounds reasonable. Although I don't remember how reliable the
max_pkt_offset logic is in practice (whether it's actually capable 
of finding the max offset for realistic programs or it's mostly going
to be set to MAX).

> On a side note the check in nfp_net_bpf_load() allows for BPF programs 
> to be offloaded that do access data beyond the CMT size limit provided 
> the MTU is set below the CMT threshold value.

Right, because of variable length offsets verifier will not be able to
estimate max_pkt_offset.

> There should be no real harm in this as the verifier forces bounds
> check so with a MTU small enough it should never happen. But maybe we
> should add a check for this too to prevent such a program to be
> loaded in the first place.