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

On Tue, Oct 6, 2015 at 10:56 AM, Eric Dumazet [off-list ref] wrote:

On Tue, 2015-10-06 at 10:50 -0700, Alexei Starovoitov wrote:

quoted

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