API introduced in a927a66e has two cases of state determination
happening out of critical section for the state value:
(1) expiredSession loads sessionExpiresNano, then releases all locks
and calls noteSessionState(Expired). So a concurrent refresh that
lands in that gap gets clobbered by a stale Expired -- and sticks
until the next re-key.
(2) Likewise in noteSessionHandshakeStopped, hasKeyMaterial check
happens out of the session state lock and races with ZeroAndFlushAll.
Both lead to a wrong state emitted via the device.sessionState.fn,
but are otherwise benign.
This moves the expiry timestamp under a lock to address the former,
and provides a noteSessionStateLocked helper for the latter.
Also changes API semantics to serialize events per-peer, to avoid
sharing a single lock for all timestamps.
Updates tailscale/corp#42874
Signed-off-by: Alex Valiushko <alexvaliushko@tailscale.com>
Change-Id: Iee2cdf135375519e58a8e84362349d966a6a6964
15b912c1c0