Today I Learned: Binfmt_misc(dfir.ch)
58 pointsby malmoeb7 days ago7 comments
  • Brian_K_White37 minutes ago
    I used to run sco xenix and unix binaries on linux via ibcs. That worked by registering a binfmt-something-else not -misc, because it didn't load an interpreter like qemu or wine, the kernel ran the binary directly, so the binfmt was something like -sysv or -ibcs2 or something. Not for real / production, just for fun. I got it go but no situation ever arised that wasn't better solved some other way. And good thing because I don't think that has worked for many years.
  • throwawayqqq115 hours ago
    Why can you register interpreters as non-root and why do these custom interpreters take precedence?

    EDIT: Checked on may dated ubuntu laptop, /proc/sys/fs/binfmt_misc/register is root:root owned with --w-------. An important detail that the article omits and that changes this "vulnerability".

    • dathinab4 hours ago
      > provides a nifty way (once the attacker has gained root rights on the machine) to create a little backdoor to regain root access when the original access no longer works

      so it does imply it needs root rights

      but it's an example of why it's a bad idea to "cleanup" a system from a virus without a full reinstall

      it also matters for other reasons, as some ways to gain root are unreliable and don't persist reboot and you don't want to hide that you have root access

    • duskwuff4 hours ago
      You can't. This is a classic example of an "other side of this airtight hatchway"[1] problem.

      [1]: https://devblogs.microsoft.com/oldnewthing/20060508-22/?p=31...

      • jchw4 hours ago
        To be fair, the article is not claiming that binfmt_misc is a security vulnerability, or at least I didn't come away with that impression (and the word "vulnerability" doesn't appear in the page either.) It's just being pointed out that you can use it as a pretty sneaky way to leave yourself a backdoor, which I think it is, among many.
    • littlestymaar3 hours ago
      It's not omitted by the article, the threat model is stated explicitly:

      > TL;DR: binfmt_misc provides a nifty way (once the attacker has gained root rights on the machine) to create a little backdoor to regain root access when the original access no longer works.

  • dandersch8 hours ago
    Before knowing about binfmt, I always wondered how wine is able is able to execute .exe files directly, i.e. ./prog.exe instead of wine ./prog.exe. Turns out the wine package (at least on Arch) comes with a handler for them and the Arch wiki mentions that you may want to remove it for security reasons.
    • thayne7 hours ago
      It can also be used to automatically execute jar files with "java -jar". I don't think arch is set up to do that automatically, but it is fairly easy to do[1].

      [1]: https://wiki.archlinux.org/title/Binfmt_misc_for_Java

    • WhyNotHugo3 hours ago
      binfmt can also be used to register qemu for binaries for foreign architectures. This allows running programs compiled for another architecture, and makes it really simply to run podman/docker containers with images for other architectures.
      • nicman2323 minutes ago
        also chroot into ie raspi sd cards.
  • DominoTree9 hours ago
    Traditionally I've seen these adapters primarily used to pass binaries for other architectures to QEMU and similar.

    Years ago on FreeBSD I created a "Volkswagen mode" by using the similar `imgact_binmisc` kernel module to register a handler for binaries with the system's native ELF headers. It took a bit of hacking to make it all work with the native architecture, but when it was done, the handler would simply execute the binary, drop its return code, and return 0 instead - effectively making the system think that every command was "successful"

    The system failed to boot when I finally got it all working (which was expected) but it was a fun adventure to do something so pointless and silly.

    It would be a similarly clever place to maintain persistence and transparently inject bytecode or do other rude things on FreeBSD as well

    • jeroenhdan hour ago
      > effectively making the system think that every command was "successful"

      I can only imagine the havoc this would wreak on shell scripts that call out to the test/[/[[ binaries on a system.

    • Twirrim3 hours ago
      Yup, using this approach it's possible to build/use aarch64 containers on an x86 machine. This technique means that a much smaller set of operations are being emulated (doesn't have to emulate the entire kernel etc)

      For something I was building, it enabled me to get a full aarch64 compilation done, with a native toolkit, without having to run a full emulation layer. The time savings of doing it this way vs full emulation were huge. Off the top of my head, emulated it was taking over an hour to do the full build, whereas within a container it was only about 10-15 minutes.

  • Valodim3 hours ago
    https://search.nixos.org/options?show=boot.binfmt.emulatedSy...

    Set this one line setting on a nixos system, and it can run foreign binaries. Magic.

  • dathinab2 hours ago
    binfmt_misc helped me out a lot some years ago

    I had a build system which was able to cross compile.

    And a test system which wasn't able to handle cross compiled/emulated/remote code but needed to run test on cross compiled code.

    In the end with binfmt the test system never knew it was running the code with qemu instead of native and "just worked".

    • adastra222 hours ago
      Sounds like a useful trick for getting a coding agent to run/test/debug cross compiling rules.
  • muppetman3 hours ago
    Another reason I compile my own kernels and disable features like this. I also disable loadable kernel modules. Of course this makes standard support channels... Difficult.