netdev - Re: [PATCH 5/6] net: add and use helpers when adjusting gso

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Message-Id: <20180228.105622.1054351882596152165.davem@davemloft.net>
Date:   Wed, 28 Feb 2018 10:56:22 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     dja@...ens.net
Cc:     netdev@...r.kernel.org, daniel@...earbox.net,
        alexei.starovoitov@...il.com
Subject: Re: [PATCH 5/6] net: add and use helpers when adjusting gso_size

From: Daniel Axtens <dja@...ens.net>
Date: Wed, 28 Feb 2018 00:04:06 +1100

> An audit of users of gso_size reveals that an eBPF tc action on a
> veth pair can be passed an SCTP GSO skb, which has gso_size of
> GSO_BY_FRAGS.
> 
> If that action calls bpf_skb_change_proto(), bpf_skb_net_grow()
> or bpf_skb_net_shrink(), the gso_size will be unconditionally
> incremented or decremented to some nonsense value.
> 
> Add helpers that WARN if attempting to change a gso_size of a
> GSO_BY_FRAGS skb (and leave the value unchanged).
> 
> Signed-off-by: Daniel Axtens <dja@...ens.net>

We can't really do this.

It means that a user loaded eBPF program can trigger logs full of
warnings merely by using this eBPF helper and generating GSO'd SCTP
traffic.

Daniel and Alexei, this is a serious problem.  The eBPF helpers
mentioned here cannot handle SCTP GSO packets properly, and in fact
corrupt them if they adjust the gso_size.

SCTP GSO packets use the GSO_BY_FRAGS scheme and cannot be treated
the same way we treat normal GSO packets.