ssh/tailssh: fix data race on conn auth state in OnPolicyChange

OnPolicyChange can observe a conn in activeConns before authentication
completes. The previous `c.info == nil` guard was itself a data race
against clientAuth writing c.info, and even when c.info appeared
non-nil, c.localUser could still be nil, causing a nil pointer
dereference at c.localUser.Username.

Add an authCompleted atomic.Bool to conn, stored true after all auth
fields are written in clientAuth. OnPolicyChange checks this atomic
instead of c.info, which provides the memory barrier guaranteeing all
prior writes are visible to the concurrent reader.

Updates tailscale/corp#36268 (fixes, but we might want to cherry-pick)

Co-authored-by: Gesa Stupperich <gesa@tailscale.com>
Change-Id: I4c69843541f5f9f04add9bf431e320c65a203a39
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ssh/tailssh/tailssh_test.go

@@ -31,6 +31,7 @@ import (
 	"sync"
 	"sync/atomic"
 	"testing"
+	"testing/synctest"
 	"time"
 
 	gossh "golang.org/x/crypto/ssh"
@@ -1111,6 +1112,7 @@ func TestSSH(t *testing.T) {
 	}
 	sc.action0 = &tailcfg.SSHAction{Accept: true}
 	sc.finalAction = sc.action0
+	sc.authCompleted.Store(true)
 
 	sc.Handler = func(s ssh.Session) {
 		sc.newSSHSession(s).run()
@@ -1320,6 +1322,79 @@ func TestStdOsUserUserAssumptions(t *testing.T) {
 	}
 }
 
+func TestOnPolicyChangeSkipsPreAuthConns(t *testing.T) {
+	tests := []struct {
+		name       string
+		sshRule    *tailcfg.SSHRule
+		wantCancel bool
+	}{
+		{
+			name:       "accept-after-auth",
+			sshRule:    newSSHRule(&tailcfg.SSHAction{Accept: true}),
+			wantCancel: false,
+		},
+		{
+			name:       "reject-after-auth",
+			sshRule:    newSSHRule(&tailcfg.SSHAction{Reject: true}),
+			wantCancel: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			synctest.Test(t, func(t *testing.T) {
+				srv := &server{
+					logf: tstest.WhileTestRunningLogger(t),
+					lb: &localState{
+						sshEnabled:   true,
+						matchingRule: tt.sshRule,
+					},
+				}
+				c := &conn{
+					srv: srv,
+					info: &sshConnInfo{
+						sshUser: "alice",
+						src:     netip.MustParseAddrPort("1.2.3.4:30343"),
+						dst:     netip.MustParseAddrPort("100.100.100.102:22"),
+					},
+					localUser: &userMeta{User: user.User{Username: currentUser}},
+				}
+				srv.activeConns = map[*conn]bool{c: true}
+				ctx, cancel := context.WithCancelCause(context.Background())
+				ss := &sshSession{ctx: ctx, cancelCtx: cancel}
+				c.sessions = []*sshSession{ss}
+
+				// Before authCompleted is set, OnPolicyChange should skip
+				// the conn entirely — no goroutine spawned.
+				srv.OnPolicyChange()
+				synctest.Wait()
+				select {
+				case <-ctx.Done():
+					t.Fatal("session canceled before auth completed")
+				default:
+				}
+
+				// Mark auth as completed. Now OnPolicyChange should
+				// evaluate the policy and act accordingly.
+				c.authCompleted.Store(true)
+
+				srv.OnPolicyChange()
+				synctest.Wait()
+				select {
+				case <-ctx.Done():
+					if !tt.wantCancel {
+						t.Fatal("valid session should not have been canceled")
+					}
+				default:
+					if tt.wantCancel {
+						t.Fatal("invalid session should have been canceled")
+					}
+				}
+			})
+		})
+	}
+}
+
 func mockRecordingServer(t *testing.T, handleRecord http.HandlerFunc) *httptest.Server {
 	t.Helper()
 	mux := http.NewServeMux()