Two derivations for the same peer can overlap, for example a rotation and
a Signal re-bootstrap. The generation guard checked and recorded the
generation under a lock but called SetPresharedKey after releasing it, so an
older apply already inside SetPresharedKey could finish after a newer one
and leave WireGuard on the stale key, splitting the peers until the next
exchange (reported in the security review of #7098).
Hold the lock across the generation check and the SetPresharedKey call. The
lock only serialises PSK applies, which are quick local calls, so this does
not reach into the manager's own locking.
riccardom
e9a622c895