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.go

@@ -192,9 +192,9 @@ func (srv *server) OnPolicyChange() {
 	srv.mu.Lock()
 	defer srv.mu.Unlock()
 	for c := range srv.activeConns {
-		if c.info == nil {
-			// c.info is nil when the connection hasn't been authenticated yet.
-			// In that case, the connection will be terminated when it is.
+		if !c.authCompleted.Load() {
+			// The connection hasn't completed authentication yet.
+			// In that case, the connection will be terminated when it does.
 			continue
 		}
 		go c.checkStillValid()
@@ -236,14 +236,26 @@ type conn struct {
 	// Banners cannot be sent after auth completes.
 	spac gossh.ServerPreAuthConn
 
+	// The following fields are set during clientAuth and are used for policy
+	// evaluation and session management. They are immutable after clientAuth
+	// completes. They must not be read from other goroutines until
+	// authCompleted is set to true.
+
 	action0     *tailcfg.SSHAction // set by clientAuth
 	finalAction *tailcfg.SSHAction // set by clientAuth
 
-	info         *sshConnInfo // set by setInfo
+	info         *sshConnInfo // set by setInfo (during clientAuth)
 	localUser    *userMeta    // set by clientAuth
 	userGroupIDs []string     // set by clientAuth
 	acceptEnv    []string
 
+	// authCompleted is set to true after clientAuth has finished writing
+	// all authentication state fields (info, localUser, action0,
+	// finalAction, userGroupIDs, acceptEnv). It provides a memory
+	// barrier so that concurrent readers (e.g. OnPolicyChange) see
+	// fully-initialized values.
+	authCompleted atomic.Bool
+
 	// mu protects the following fields.
 	//
 	// srv.mu should be acquired prior to mu.
@@ -369,6 +381,7 @@ func (c *conn) clientAuth(cm gossh.ConnMetadata) (perms *gossh.Permissions, retE
 				}
 			}
 			c.finalAction = action
+			c.authCompleted.Store(true)
 			return &gossh.Permissions{}, nil
 		case action.Reject:
 			metricTerminalReject.Add(1)

ssh/tailssh/tailssh_test.go

@@ -31,6 +31,7 @@
 	"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()