Open ports mean there is a service listening. A service listening means that there is code on your box interfacing with the internet. That means there is the po

Open ports mean there is a service listening. A service listening means that there is code on your box interfacing with the internet. That means there is the possibility that an exploit in that code could lead to remote code execution. The meltdown vulnerability means that any remote code execution vulnerability can be escalated to the highest levels. Exploiting meltdown means you can read all of kernel memory, that destroys any additional layers of protection you might fantasize you had. It means that in memory passwords and encryption keys can be copied. It means that IO between other processes can be spied on. It means that all of the juicy locations to exploit for "return oriented programming" in order to gain local root will be laid bare. And so on. That's why it's called meltdown. Imagine someone who had hand crafted the equivalent of a modern OS with all of the best practices in place for an internet server. You have kernel address randomization. You have each service running as its own non-privileged user and also in a chroot jail. You have your filesystem permissions locked down tight. And so on. All of that is gone out the window with meltdown, it doesn't matter, because even code running at the lowest possible privilege level can gain access to the contents of any part of kernel memory. As for "workstation" systems. If you are behind a firewall, you never run untrusted code, and you have JS disabled then potentially you are safe.