wireguard: noise: read preshared key while taking lock

wireguard: noise: read preshared key while taking lock

Prior we read the preshared key after dropping the handshake lock, which
isn't an actual crypto issue if it races, but it's still not quite
correct. So copy that part of the state into a temporary like we do with
the rest of the handshake state variables. Then we can release the lock,
operate on the temporary, and zero it out at the end of the function. In
performance tests, the impact of this was entirely unnoticable, probably
because those bytes are coming from the same cacheline as other things
that are being copied out in the same manner.

Reported-by: Matt Dunwoodie <ncon@noconroy.net>
Fixes: e7096c131e51 ("net: WireGuard secure network tunnel")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>