ipn/ipnserver: move more connection acceptance logic to LocalBackend

Follow-up to #6467 and #6506.

LocalBackend knows the server-mode state, so move more auth checking
there, removing some bookkeeping from ipnserver.Server.

Updates #6417
Updates tailscale/corp#8051

Change-Id: Ic5d14a077bf0dccc92a3621bd2646bab2cc5b837
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -200,7 +200,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/health/healthmsg                               from tailscale.com/ipn/ipnlocal
         tailscale.com/hostinfo                                       from tailscale.com/control/controlclient+
         tailscale.com/ipn                                            from tailscale.com/ipn/ipnlocal+
-        tailscale.com/ipn/ipnauth                                    from tailscale.com/ipn/ipnserver
+        tailscale.com/ipn/ipnauth                                    from tailscale.com/ipn/ipnserver+
         tailscale.com/ipn/ipnlocal                                   from tailscale.com/ssh/tailssh+
         tailscale.com/ipn/ipnserver                                  from tailscale.com/cmd/tailscaled
         tailscale.com/ipn/ipnstate                                   from tailscale.com/control/controlclient+

ipn/ipnauth/ipnauth.go

@@ -44,7 +44,16 @@ type ConnIdentity struct {
 	user   *user.User
 }
 
-func (ci *ConnIdentity) UserID() string         { return ci.userID }
+// UserID returns the local machine's userid of the connection.
+//
+// It's suitable for passing to LookupUserFromID (os/user.LookupId) on any
+// operating system.
+//
+// TODO(bradfitz): it currently returns an empty string on everything
+// but Windows. We should make it return the actual uid also on all supported
+// peercred platforms from the creds if non-nil.
+func (ci *ConnIdentity) UserID() string { return ci.userID }
+
 func (ci *ConnIdentity) User() *user.User       { return ci.user }
 func (ci *ConnIdentity) Pid() int               { return ci.pid }
 func (ci *ConnIdentity) IsUnixSock() bool       { return ci.isUnixSock }

ipn/ipnlocal/local.go

@@ -38,6 +38,7 @@ import (
 	"tailscale.com/health/healthmsg"
 	"tailscale.com/hostinfo"
 	"tailscale.com/ipn"
+	"tailscale.com/ipn/ipnauth"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/ipn/policy"
 	"tailscale.com/net/dns"
@@ -2029,6 +2030,34 @@ func (b *LocalBackend) InServerMode() bool {
 	return b.inServerMode
 }
 
+// CheckIPNConnectionAllowed returns an error if the identity in ci should not
+// be allowed to connect or make requests to the LocalAPI currently.
+//
+// Currently (as of 2022-11-23), this is only used on Windows to check if
+// we started in server mode and ci is from an identity other than the one
+// that started the server.
+func (b *LocalBackend) CheckIPNConnectionAllowed(ci *ipnauth.ConnIdentity) error {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	serverModeUid := b.pm.CurrentUser()
+	if serverModeUid == "" {
+		// Either this platform isn't a "multi-user" platform or we're not yet
+		// running as one.
+		return nil
+	}
+	if !b.inServerMode {
+		return nil
+	}
+	uid := ci.UserID()
+	if uid == "" {
+		return errors.New("empty user uid in connection identity")
+	}
+	if uid != serverModeUid {
+		return fmt.Errorf("Tailscale running in server mode (uid=%v); connection from %q not allowed", serverModeUid, uid)
+	}
+	return nil
+}
+
 // Login implements Backend.
 // As of 2022-11-15, this is only exists for Android.
 func (b *LocalBackend) Login(token *tailcfg.Oauth2Token) {
@@ -4383,11 +4412,3 @@ func (b *LocalBackend) ListProfiles() []ipn.LoginProfile {
 	defer b.mu.Unlock()
 	return b.pm.Profiles()
 }
-
-// CurrentUser returns the current server mode user ID. It is only non-empty on
-// Windows where we have a multi-user system.
-func (b *LocalBackend) CurrentUser() string {
-	b.mu.Lock()
-	defer b.mu.Unlock()
-	return b.pm.CurrentUser()
-}

ipn/ipnserver/server.go

@@ -304,8 +304,8 @@ func (s *Server) checkConnIdentityLocked(ci *ipnauth.ConnIdentity) error {
 			return inUseOtherUserError{fmt.Errorf("Tailscale already in use by %s, pid %d", active.User().Username, active.Pid())}
 		}
 	}
-	if cu := s.b.CurrentUser(); cu != "" && cu != ci.UserID() {
-		return inUseOtherUserError{fmt.Errorf("Tailscale already in use by %s", s.b.CurrentUser())}
+	if err := s.b.CheckIPNConnectionAllowed(ci); err != nil {
+		return inUseOtherUserError{err}
 	}
 	return nil
 }