netdev - Re: [PATCH net-next] flow_dissector: Add IPv6 flow label to symmetric keys

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Date:   Wed, 5 Aug 2020 10:26:37 +0200
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Tom Herbert <tom@...bertland.com>
Cc:     David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] flow_dissector: Add IPv6 flow label to symmetric keys

On Tue, Aug 4, 2020 at 5:57 PM Tom Herbert <tom@...bertland.com> wrote:
>
> The definition for symmetric keys does not include the flow label so
> that when symmetric keys is used a non-zero IPv6 flow label is not
> extracted. Symmetric keys are used in functions to compute the flow
> hash for packets, and these functions also set "stop at flow label".
> The upshot is that for IPv6 packets with a non-zero flow label, hashes
> are only based on the address two tuple and there is no input entropy
> from transport layer information. This patch fixes this bug.

If this is a bug fix, it should probably target net and have a Fixes tag.

Should the actual fix be to remove the
FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL argument from
__skb_get_hash_symmetric?

The original commit mentions the symmetric key flow dissector to
compute a flat symmetric hash over only the protocol, addresses and
ports.

Autoflowlabel uses symmetric __get_hash_from_flowi6 to derive a flow
label, but this cannot be generally relied on. RFC 6437 suggests
even a PRNG as input, for instance.