Re: [PATCH] binfmt_misc: add two-steps registration (opt-in)

[Domenico Andreoli <domenico.andreoli@linux.com>]

On Thu, Mar 10, 2022 at 08:13:25AM -0800, Kees Cook wrote:
> On Tue, Mar 01, 2022 at 02:28:22PM +0100, Domenico Andreoli wrote:
> > From: Domenico Andreoli <domenico.andreoli@linux.com>
> > 
> > Experimenting with new interpreter configurations can lead to annoying
> > failures, when the system is left unable to load ELF binaries power
> > cycling is the only way to get it back operational.
> > 
> > This patch tries to mitigate such conditions by adding an opt-in
> > two-steps registration.
> > 
> > A new optional field is added to the configuration string, it's an
> > expiration interval for the newly added interpreter. If the user is
> > not able to confirm in time, possibly because the system is broken,
> > the new interpreter is automatically disabled.
> 
> Hi!

Hi!

> 
> As this both changes the userspace API and adds timers, I'd like the

Right but 1. it's backward compatible, 2. it fails on unsupporting
kernels.

Curiosity, I understand why API changes require care but what's so
special about the timers?

> change to be really well justified. Can you explain the conditions you
> get into that can't be escaped by just disabling the bad binfmt_misc
> entry?

It happened when I somehow messed up with the ELF loader of my system,
it was the very first time I tried to manually configure qemu-user-static
for a foreign architecture.

Suddenly I could not do anything, no ls, no cat. Did not realize that
my shell has built-in echo and that I could cut-and-paste the path for
disabling the bad interpreter.  I did not investigate what I did wrong
or what I could do better, I simply didn't do it again.

I just got a deeper understanding of the note in Debian's update-binfmts
manpage:

  If you're not careful, you can break your system with update-binfmts.
  An easy way to do this is to register an ELF binary as a handler for
  ELF, which will almost certainly cause your system to hang immediately;
  even if it doesn't, you won't be able to run update-binfmts to fix it.

I shot on my foot and I thought the API could be made a bit more friendly.

Thanks,
Dom

> 
> -Kees
> 
> -- 
> Kees Cook

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