David Miller [off-list ref] wrote:

From: Jesper Dangaard Brouer <redacted>
Date: Wed, 27 Jun 2012 08:32:13 +0200

quoted

Using it as default, might be "dangerous" and open an attack vector
on SYN cookies in Linux.

If it's dangerous for syncookies then it's just as dangerous for
the routing hash and the socket hashes where we use it already.
Therefore, this sounds like a baseless claim to me.

I doubt using jhash is safe for syncookies.

There a several differences to other uses in kernel:
- all hash input except u32 cookie_secret[2] is known
- we transmit hash result (i.e, its visible to 3rd party)
- we do not re-seed the secret, ever

it should be quite easy to recompute cookie_secret[] from known syncookie
values?