kernel: check that a task has a network namespace

task.GetNetworkNamespace has to be used when we try to access a remote task.

PiperOrigin-RevId: 552593738

pkg/sentry/fsimpl/proc/task_net.go

@@ -43,7 +43,12 @@ func (fs *filesystem) newTaskNetDir(ctx context.Context, task *kernel.Task) kern
 	root := auth.NewRootCredentials(pidns.UserNamespace())
 
 	var contents map[string]kernfs.Inode
-	if stack := task.NetworkNamespace().Stack(); stack != nil {
+	var stack inet.Stack
+	if netns := task.GetNetworkNamespace(); netns != nil {
+		netns.DecRef(ctx)
+		stack = netns.Stack()
+	}
+	if stack != nil {
 		const (
 			arp       = "IP address       HW type     Flags       HW address            Mask     Device\n"
 			netlink   = "sk       Eth Pid    Groups   Rmem     Wmem     Dump     Locks     Drops     Inode\n"