unix: avoid taking two endpoint locks

If we want to take two endpoint locks, we need to be sure that we always
take them in the same order.

Accept() locks the listening endpoint to work with acceptedChan and then
it calls GetLocalAddress that locks an accepted endpoint. Actually, we
can release the listening endpoint lock before calling GetLocalAddress.

Reported-by: syzbot+f52bd603f51a4ae91054@syzkaller.appspotmail.com
PiperOrigin-RevId: 394553823

pkg/sentry/socket/unix/transport/connectioned.go

@@ -406,14 +406,15 @@ func (e *connectionedEndpoint) Listen(backlog int) *syserr.Error {
 // Accept accepts a new connection.
 func (e *connectionedEndpoint) Accept(peerAddr *tcpip.FullAddress) (Endpoint, *syserr.Error) {
 	e.Lock()
-	defer e.Unlock()
 
 	if !e.Listening() {
+		e.Unlock()
 		return nil, syserr.ErrInvalidEndpointState
 	}
 
 	select {
 	case ne := <-e.acceptedChan:
+		e.Unlock()
 		if peerAddr != nil {
 			ne.Lock()
 			c := ne.connected
@@ -429,6 +430,7 @@ func (e *connectionedEndpoint) Accept(peerAddr *tcpip.FullAddress) (Endpoint, *s
 		return ne, nil
 
 	default:
+		e.Unlock()
 		// Nothing left.
 		return nil, syserr.ErrWouldBlock
 	}