util/syspolicy/source: fix data race between Reader.Close and reload

Reader.Close set r.store to nil without holding r.mu, while reload read
r.store while holding r.mu. If a policy store is closed while a
concurrent reload is in flight, reload could observe a nil store and
crash tailscaled with a nil interface method call in
readPolicySettingValue.

Nil out r.store only while holding r.mu, and make reload return the
last known policy once the reader is closing instead of reading from
a store that may no longer exist.

Fixes tailscale/corp#45548
Fixes tailscale/triage#394

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I494cfe9ea1df67b563bc061db9e6944f87b42a4e

util/syspolicy/source/policy_reader.go

@@ -125,6 +125,11 @@ func (r *Reader) ReadSettings() (*setting.Snapshot, error) {
 func (r *Reader) reload(force bool) (*setting.Snapshot, error) {
 	r.mu.Lock()
 	defer r.mu.Unlock()
+	if r.closing {
+		// The reader is closing (or already closed) and r.store may be nil.
+		// Return the last known policy instead of reading from the store.
+		return r.lastPolicy, nil
+	}
 	if r.upToDate && !force {
 		return r.lastPolicy, nil
 	}
@@ -267,12 +272,13 @@ func (r *Reader) Close() error {
 			return err
 		}
 	}
-	r.store = nil
-
 	close(r.doneCh)
 
 	r.mu.Lock()
 	defer r.mu.Unlock()
+	// Nil out the store only while holding r.mu; reload reads r.store
+	// under the same lock, so writing it unlocked would be a data race.
+	r.store = nil
 	for _, c := range r.sessions {
 		c.closeInternal()
 	}