Hold baseEndpoint.mu when calling baseEndpoint.Connected()

Connected must be called with baseEndpoint.mu locked. Updated method comment
to include this precondition.

Reported-by: syzbot+3ee57917033a3a23c8e0@syzkaller.appspotmail.com
PiperOrigin-RevId: 432506434

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

@@ -519,6 +519,8 @@ func (e *connectionedEndpoint) State() uint32 {
 
 // OnSetSendBufferSize implements tcpip.SocketOptionsHandler.OnSetSendBufferSize.
 func (e *connectionedEndpoint) OnSetSendBufferSize(v int64) (newSz int64) {
+	e.Lock()
+	defer e.Unlock()
 	if e.Connected() {
 		return e.baseEndpoint.connected.SetSendBufferSize(v)
 	}

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

@@ -222,6 +222,8 @@ func (e *connectionlessEndpoint) State() uint32 {
 
 // OnSetSendBufferSize implements tcpip.SocketOptionsHandler.OnSetSendBufferSize.
 func (e *connectionlessEndpoint) OnSetSendBufferSize(v int64) (newSz int64) {
+	e.Lock()
+	defer e.Unlock()
 	if e.Connected() {
 		return e.baseEndpoint.connected.SetSendBufferSize(v)
 	}

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

@@ -813,6 +813,8 @@ func (e *baseEndpoint) ConnectedPasscred() bool {
 }
 
 // Connected implements ConnectingEndpoint.Connected.
+//
+// Preconditions: e.mu must be held.
 func (e *baseEndpoint) Connected() bool {
 	return e.receiver != nil && e.connected != nil
 }