TLS session key re-negotiation

[chucklever]

Session keys created by a TLS handshake are small and should be thrown out after a while (usually a certain number of payload bytes transferred within the session) to mitigate an attacker's ability to guess them.

The current handshake upcall does not support renegotiation. Instead, it treats a key reneg request as an error that is fatal to a session, then sets up a fresh session. This is typically not a significant problem because RPC connections/sessions are long-lived.

We would like the netlink upcall protocol and tlshd eventually to handle renegotiation. In this case, the upcall would pass a socket that already has a TLS session on it, and tlshd would redirect that to the library so that a reneg handshake can be done. The result should be that a fresh session key is plugged into the socket via setsockopt(3).

[hreinecke]

Hmm. Not sure if that works out with the current in-kernel crypto implementation:

        /* Currently we don't support set crypto info more than one time */
        if (TLS_CRYPTO_INFO_READY(crypto_info))
                return -EBUSY;