netdev - Re: [PATCH net-next 1/2] bpf: enable non-root eBPF programs

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Message-ID: <CALCETrXwYoVMnsPJZxFtgGaD5YXzKMBYG=EJAQTbwGdKwmdb5g@mail.gmail.com>
Date:	Tue, 6 Oct 2015 11:05:21 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Alexei Starovoitov <ast@...mgrid.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Ingo Molnar <mingo@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Kees Cook <keescook@...omium.org>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 1/2] bpf: enable non-root eBPF programs

On Tue, Oct 6, 2015 at 10:56 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2015-10-06 at 10:50 -0700, Alexei Starovoitov wrote:
>
>> was also thinking that we can do it only in paths that actually
>> have multiple protocol layers, since today bpf is mainly used with
>> tcpdump(raw_socket) and new af_packet fanout both have cb cleared
>> on RX, because it just came out of alloc_skb and no layers were called,
>> and on TX we can clear 20 bytes in dev_queue_xmit_nit().
>> af_unix/netlink also have clean skb. Need to analyze tun and sctp...
>> but it feels overly fragile to save a branch in sk_filter,
>> so planning to go with
>> if(unlikely(prog->cb_access)) memset in sk_filter().
>>
>
> This will break TCP use of sk_filter().
> skb->cb[] contains useful data in TCP layer.
>
>

Since I don't know too much about the networking details:

1. Does "skb->cb" *ever* contain anything useful for an unprivileged user?

2. Does sbk->cb form a stable ABI?

Unless both answers are solid yesses, then maybe the right solution is
to just deny access entirely to unprivileged users.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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