Fix SCM Rights reference leaks.

Control messages should be released on Read (which ignores the control message)
or zero-byte Send. Otherwise, open fds sent through the control messages will
be leaked.

PiperOrigin-RevId: 337110774

pkg/sentry/socket/unix/unix.go

@@ -573,13 +573,17 @@ func (s *SocketOperations) Read(ctx context.Context, _ *fs.File, dst usermem.IOS
 	if dst.NumBytes() == 0 {
 		return 0, nil
 	}
-	return dst.CopyOutFrom(ctx, &EndpointReader{
+	r := &EndpointReader{
 		Ctx:       ctx,
 		Endpoint:  s.ep,
 		NumRights: 0,
 		Peek:      false,
 		From:      nil,
-	})
+	}
+	n, err := dst.CopyOutFrom(ctx, r)
+	// Drop control messages.
+	r.Control.Release(ctx)
+	return n, err
 }
 
 // RecvMsg implements the linux syscall recvmsg(2) for sockets backed by

pkg/sentry/socket/unix/unix_vfs2.go

@@ -267,13 +267,17 @@ func (s *SocketVFS2) Read(ctx context.Context, dst usermem.IOSequence, opts vfs.
 	if dst.NumBytes() == 0 {
 		return 0, nil
 	}
-	return dst.CopyOutFrom(ctx, &EndpointReader{
+	r := &EndpointReader{
 		Ctx:       ctx,
 		Endpoint:  s.ep,
 		NumRights: 0,
 		Peek:      false,
 		From:      nil,
-	})
+	}
+	n, err := dst.CopyOutFrom(ctx, r)
+	// Drop control messages.
+	r.Control.Release(ctx)
+	return n, err
 }
 
 // PWrite implements vfs.FileDescriptionImpl.

pkg/sentry/syscalls/linux/sys_socket.go

@@ -1052,7 +1052,9 @@ func sendSingleMsg(t *kernel.Task, s socket.Socket, file *fs.File, msgPtr userme
 	// Call the syscall implementation.
 	n, e := s.SendMsg(t, src, to, int(flags), haveDeadline, deadline, controlMessages)
 	err = handleIOError(t, n != 0, e.ToError(), syserror.ERESTARTSYS, "sendmsg", file)
-	if err != nil {
+	// Control messages should be released on error as well as for zero-length
+	// messages, which are discarded by the receiver.
+	if n == 0 || err != nil {
 		controlMessages.Release(t)
 	}
 	return uintptr(n), err

pkg/sentry/syscalls/linux/vfs2/socket.go

@@ -1055,7 +1055,9 @@ func sendSingleMsg(t *kernel.Task, s socket.SocketVFS2, file *vfs.FileDescriptio
 	// Call the syscall implementation.
 	n, e := s.SendMsg(t, src, to, int(flags), haveDeadline, deadline, controlMessages)
 	err = slinux.HandleIOErrorVFS2(t, n != 0, e.ToError(), syserror.ERESTARTSYS, "sendmsg", file)
-	if err != nil {
+	// Control messages should be released on error as well as for zero-length
+	// messages, which are discarded by the receiver.
+	if n == 0 || err != nil {
 		controlMessages.Release(t)
 	}
 	return uintptr(n), err