How the xz backdoor highlights a major flaw in Nix | Shade's Blog
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Background On Friday, March 29th, 2024, a historical and sophisticated security vulnerability (CVE-2024-3094) was discovered in the XZ Utils package and liblzma api in version 5.6.0 and 5.6.1. While this vulnerability mostly affects Debian and RedHat distributions, there was some interesting discussion regarding xz and Nix. How did this affect Nix and NixOS? The truth is not a whole lot in reality. I saw conflicting reports, but supposedly, the tarballs of xz that Nix downloads were not infected.

The main issue is the handling of security updates within the Nixpkgs ecosystem, which relies on Nix’s CI system, Hydra, to test and build packages. Due to the extensive number of packages in the Nixpkgs repository, the process can be slow, causing delays in the release of updates. As an example, the updated xz 5.4.6 package took nearly 5 days to become available in the unstable branch!

Fundamentally, there needs to be a change in how security fixes are handled in Hydra. As stated in the article, Nix was lucky to be unaffected, but multiple days to push out a security patch of this severity is concerning, even if there was no reason for concern.

  • Atemu@lemmy.ml
    23·
    3 months ago

    This blog post misses entirely that this has nothing to do with the unstable channel. It just happened to only affect unstable this time because it gets updates first. If we had found out about the xz backdoor two months later (totally possible; we were really lucky this time), this would have affected a stable channel in exactly the same way. (It’d be slightly worse actually because that’d be a potentially breaking change too but I digress.)

    I see two way to “fix” this:

    • Throw a shitton of money at builders. I could see this getting staging-next rebuild times down to just 1-2 days which I’d say is almost acceptable. This could even be a temporary thing to reduce cost; quickly renting an extremely large on-demand fleet from some cloud provider for a day whenever a critical world rebuild needs to be done which shouldn’t be too often.
    • Implement pure grafting for important security patches through a second overlay-like mechanism.