netdev - Re: [PATCH net-next v2 0/5] bpf: BPF for lightweight tunnel encapsulation

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Date:   Tue, 1 Nov 2016 12:11:28 -0700
From:   Thomas Graf <tgraf@...g.ch>
To:     Tom Herbert <tom@...bertland.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        roopa <roopa@...ulusnetworks.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 0/5] bpf: BPF for lightweight tunnel encapsulation

On 1 November 2016 at 11:51, Tom Herbert <tom@...bertland.com> wrote:
> On Tue, Nov 1, 2016 at 11:20 AM, Thomas Graf <tgraf@...g.ch> wrote:
>> The headers cannot be extended or reduced so the offsets always remain
>> correct. What can happen is that the header contains invalid data.
>>
> If we can't add/remove headers then doesn't that really limit the
> utility of these patches? My assumption was that BPF+LWT is needed to
> allow users to define and implement their own encapsulations, EH
> insertion, packet modification, etc.

I thought you were specifically referring to lwtunnel_output(). You
can extend headers at lwtunnel_xmit() but you can't for
lwtunnel_output() and lwtunnel_input(). The xmit hook is in
ip_finish_output2() where it is fine to do so. We have existing code
doing this there.

>> You keep saying this "middle in the stack" but the point is exactly
>> the same as a raw socket with IPPROTO_RAW and hdrincl, see
>> rawv6_sendmsg() and rawv6_send_hdrincl(). An IPv6 raw socket can feed
>> arbitrary garbage into dst_output(). IPv4 does some minimal sanity
>> checks.
>>
> What I mean is that an admin can create a BPF program that run on any
> user packets (for instance default route could be set). This would be
> in the path of TCP, UDP, and other protocols tightly integrated with
> the stack. Packets being routed may be encapsulated, VLAN, have
> checksum offload, GORed set etc. They also might be looped back in
> which case the settings in skbuff become receive parameters.

You can do the same with act_pedit or cls_bpf at dev_queue_xmit()
before or after you go into the encapsulation device. This is a tool
for root, if you install a drop all ssh rule then you won't be able to
log into the box anymore. If you modify the packet and render it
invalid, the packet will be dropped.
If you attach a VLAN header while VLAN offload is already set up, then
the hardware will add another VLAN header, this is what I would
expect. I truly hope that we don't have code which crashes if we
dev_queue_xmit() garbage into any device. That would be horrible.

Do you have a specific example that could be a problem?