lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:   Mon, 21 Sep 2020 11:37:18 +0100
From:   Lorenz Bauer <>
To:     Maciej ┼╗enczykowski <>
Cc:     Jesper Dangaard Brouer <>,
        Saeed Mahameed <>,
        Daniel Borkmann <>,
        Alexei Starovoitov <>,
        BPF-dev-list <>,
        "" <>,
        Lorenzo Bianconi <>,
        John Fastabend <>,
        Jakub Kicinski <>,
        Shaun Crampton <>,
        David Miller <>,
        Marek Majkowski <>
Subject: Re: BPF redirect API design issue for BPF-prog MTU feedback?

On Sat, 19 Sep 2020 at 00:06, Maciej ┼╗enczykowski <> wrote:
> > This is a good point.  As bpf_skb_adjust_room() can just be run after
> > bpf_redirect() call, then a MTU check in bpf_redirect() actually
> > doesn't make much sense.  As clever/bad BPF program can then avoid the
> > MTU check anyhow.  This basically means that we have to do the MTU
> > check (again) on kernel side anyhow to catch such clever/bad BPF
> > programs.  (And I don't like wasting cycles on doing the same check two
> > times).
> If you get rid of the check in bpf_redirect() you might as well get
> rid of *all* the checks for excessive mtu in all the helpers that
> adjust packet size one way or another way.  They *all* then become
> useless overhead.
> I don't like that.  There may be something the bpf program could do to
> react to the error condition (for example in my case, not modify
> things and just let the core stack deal with things - which will
> probably just generate packet too big icmp error).
> btw. right now our forwarding programs first adjust the packet size
> then call bpf_redirect() and almost immediately return what it
> returned.
> but this could I think easily be changed to reverse the ordering, so
> we wouldn't increase packet size before the core stack was informed we
> would be forwarding via a different interface.

We do the same, except that we also use XDP_TX when appropriate. This
complicates the matter, because there is no helper call we could
return an error from.

My preference would be to have three helpers: get MTU for a device,
redirect ctx to a device (with MTU check), resize ctx (without MTU
check) but that doesn't work with XDP_TX. Your idea of doing checks in
redirect and adjust_room is pragmatic and seems easier to implement.

Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

Powered by blists - more mailing lists