netdev - Re: Misalignment, MIPS, and ip

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Date:   Sun, 11 Dec 2016 15:50:31 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-mips@...ux-mips.org, Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Dan Lüdtke <mail@...rl.com>,
        Willy Tarreau <w@....eu>,
        Måns Rullgård <mans@...sr.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        WireGuard mailing list <wireguard@...ts.zx2c4.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Felix Fietkau <nbd@....name>, Jiri Benc <jbenc@...hat.com>,
        David Miller <davem@...emloft.net>
Subject: Re: Misalignment, MIPS, and ip_hdr(skb)->version

Hey guys,

Thanks for the extremely detailed answers. The main take-away from
this is that passing unaligned packets to the networking stack kills
kittens. So now it's a question of mitigation. I have three options:

1. Copy the plaintext to three bytes before the start of the cipher
text, overwriting parts of the header that aren't actually required.
Pros: no changes required, MTU stays small.
Cons: scatterwalk's fast paths aren't hit, which means two page table
mappings are taken instead of one. I have no idea if this actually
matters or will slow down anything relavent.

2. Add 3 bytes to the plaintext header, set to zero, marked for future use.
Pros: satisfies IETF mantras and makes unaligned in-place decryption
straightforward.
Cons: lowers MTU, additional unauthenticated cleartext bits in the
header are of limited utility in protocol.

3. Add 3 bytes of padding, set to zero, to the encrypted section just
before the IP header, marked for future use.
Pros: satisfies IETF mantras, can use those extra bits in the future
for interesting protocol extensions for authenticated peers.
Cons: lowers MTU, marginally more difficult to implement but still
probably just one or two lines of code.

Of these, I'm leaning toward (3).

Anyway, thanks a lot for the input. "Doing nothing" is no longer under
serious consideration, thanks to your messages.

Thanks,
Jason