This blog post is a continuation of my previous posts about Confidential Computing.

tl;dr

Complexity frequently leands to security issues. Adding support for a bunch of confidential computing technologies to KVM increases its complexity and thus softens its security stance.

Longer Version

While scrolling through KVM security vulnerabilities, it’s hard not to notice an uptick of vulnerabilities related to confidential computing, specifically AMD SEV. Here are some examples. These vulnerabilities typically don’t break the security promises of the confidential VM, but open up issues on the host.

I have been wondering whether the enabling of confidential computing features in KVM inadvertently lowers the security of KVM as a whole. The confidential guest may enjoy the benefits of some protection against malicious hypervisors, but the hypervisor has a harder time enforcing isolation on the whole system.

KVM on x86 is already a beast through no fault of its maintainers. x86 is notoriously hard to virtualize because it is an architecture with lots of legacy. The complexity of KVM reflects that. Also, KVM has often been the first public implementation of many virtualization features and thus can’t enjoy the benefit of hindsight. It also has many users, so rectifying any unfortunate API design or implementation choice is extremely hard because someone’s problem is another person’s feature.

Given the complexities, our open-source virtualization stack would benefit from some big corporation money and brains to simplify, harden its security, and improve its trustworthiness. But as the incentive structures are, CPU vendors instead have started pouring money into developing mutually incompatible confidential computing solutions.

AMD, Intel, and ARM designed their confidential computing projects so they can be bolted onto the existing software stack. As such, each of these technologies adds not only thousands of lines of code to KVM but also further increases the complexity of the code base. Due to the increased complexity, we now unsurprisingly see security issues in the modified code.

So the technology that is supposed to help to increase trust in virtualization has ultimately weakened the security of virtualization for many users. Isn’t this ironic?