netdev - Re: [PATCH bpf-next V6 2/7] bpf: fix bpf_fib_lookup helper MTU check for SKB ctx

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Date:   Fri, 20 Nov 2020 14:15:52 +0100
From:   Carlo Carraro <colrack@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        maze@...gle.com, lmb@...udflare.com, shaun@...era.io,
        Lorenzo Bianconi <lorenzo@...nel.org>, marek@...udflare.com,
        John Fastabend <john.fastabend@...il.com>,
        Jakub Kicinski <kuba@...nel.org>, eyal.birger@...il.com
Subject: Re: [PATCH bpf-next V6 2/7] bpf: fix bpf_fib_lookup helper MTU check
 for SKB ctx

Hi

I report here the issue with the previous patch.
The code is now checking against params->tot_len but then it is still
using is_skb_forwardable.
Consider this case where I shrink the packet:
skb->len == 1520
dev->mtu == 1500
params->tot_len == 1480
So the incoming pkt has len 1520, and the out interface has mtu 1500.
In this case fragmentation is not needed because params->tot_len < dev->mtu.
However the code calls is_skb_forwardable and may return false because
skb->len > dev->mtu, resulting in BPF_FIB_LKUP_RET_FRAG_NEEDED.
What I propose is using params->tot_len only if provided, without
falling back to use is_skb_forwardable when provided.
Something like this:

if (params->tot_len > 0) {
  if (params->tot_len > mtu)
    rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
} else if (!is_skb_forwardable(dev, skb)) {
  rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
}

However, doing so we are skipping more relaxed MTU checks inside
is_skb_forwardable, so I'm not sure about this.
Please comment

Il giorno ven 20 nov 2020 alle ore 09:26 Jesper Dangaard Brouer
<brouer@...hat.com> ha scritto:
>
> On Wed, 18 Nov 2020 16:29:35 +0100
> Jesper Dangaard Brouer <brouer@...hat.com> wrote:
>
> > BPF end-user on Cilium slack-channel (Carlo Carraro) wants to use
> > bpf_fib_lookup for doing MTU-check, but *prior* to extending packet size,
> > by adjusting fib_params 'tot_len' with the packet length plus the
> > expected encap size. (Just like the bpf_check_mtu helper supports). He
> > discovered that for SKB ctx the param->tot_len was not used, instead
> > skb->len was used (via MTU check in is_skb_forwardable()).
> >
> > Fix this by using fib_params 'tot_len' for MTU check.  If not provided
> > (e.g. zero) then keep existing behaviour intact.
>
> Carlo pointed out (in slack) that the logic is not correctly
> implemented in this patch.
>
> I will send a V7.
>
>
> > Fixes: 4c79579b44b1 ("bpf: Change bpf_fib_lookup to return lookup status")
> > Reported-by: Carlo Carraro <colrack@...il.com>
> > Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> > ---
> >  net/core/filter.c |   12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 1ee97fdeea64..ae1fe8e6069a 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -5567,10 +5567,20 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
> >
> >       if (!rc) {
> >               struct net_device *dev;
> > +             u32 mtu;
> >
> >               dev = dev_get_by_index_rcu(net, params->ifindex);
> > -             if (!is_skb_forwardable(dev, skb))
> > +             mtu = dev->mtu;
> > +
> > +             /* Using tot_len for L3 MTU check if provided by user. Notice at
> > +              * this TC cls_bpf level skb->len contains L2 size, but
> > +              * is_skb_forwardable takes that into account.
> > +              */
> > +             if (params->tot_len > mtu) {
> >                       rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
> > +             } else if (!is_skb_forwardable(dev, skb)) {
> > +                     rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;
> > +             }
> >       }
> >
> >       return rc;
>
> --
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer
>