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About Meltdown and Spectre

Oh, no! Not yet another blog article! This article expresses my view on the Meltdown and Spectre vulnerabilities, with a focus on FreeBSD and HardenedBSD. Most other articles focus on Windows, Linux, and macOS. Since there are plenty of other articles diving into the gory details, I’ll skip those in this article.

While we see a lot of speculation and rumors about Meltdown, it’s important to keep a level head. Meltdown is primarily a local attack. Building a ROP chain for remote exploitation of Meltdown would likely be extremely difficult. We are seeing Spectre proofs of concept written in javascript.

Meltdown is a read-only attack. It is not possible to gain write access in the kernel with Meltdown only. However, I would think it is possible to gain write access by combining Meltdown with Row Hammer on systems without ECC RAM.

The Meltdown vulnerability was disclosed to FreeBSD late in December. HardenedBSD and OpenBSD received no advanced notification. Some hardware and software vendors were notified months ago. The embargo is technically still applied and the official PoC hasn’t been released.

FreeBSD is working on a mitigation. We don’t know how they plan to approach the mitigation. We don’t know if they’ll issue a Call For Testing (CFT) first, or if they will commit to HEAD. In either case, HardenedBSD will thoroughly test and integrate their patch. It’s likely that the patch will cause merge conflicts, so we’ll need to work through that.

llvm is also working on at least one compiler-based mitigation approach, called retpoline. Their plan is to introduce retpoline as an optional feature in 6.0.0 with backports to older releases. The llvm project recommends compiling retpoline-enabled applications to use immediate binding (-z now) in order to make the retpoline PLT code smaller. HardenedBSD has had full RELRO enabled (RELRO + BIND_NOW) for base and ports, with only a few applications opting out of it, for almost two years. retpoline also requires use of lld, which is the default linker in HardenedBSD on both amd64 and arm64.

Once llvm’s patch is committed, I will create a feature branch in HardenedBSD’s playground repo to test it with world and kernel. If I succeed in enabling it in base, I will look into enabling it in ports. At least one experimental package build (also known as an “exp-run”) will be performed. Doing an exp-run will also give us real-world metrics on any potential overhead retpoline will give us.

If retpoline works out, then we in HardenedBSD will have two mitigations against Spectre and/or Meltdown: the mitigation from FreeBSD and the mitigation from llvm. retpoline will move from the playground repo to our main repo.