Misalignment, MIPS, and ip_hdr(skb)->version

Jason A. Donenfeld Jason at zx2c4.com
Sun Dec 11 15:50:31 CET 2016

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
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.


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