Re: Potential security weakness in Guix services

[Maxime Devos <maximedevos@telenet.be>]

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On Fri, 2021-02-05 at 10:57 +0100, Ludovic Courtès wrote:
> Hi Maxime,

> 
> > I don't know how I should implement this properly in Guile, though.
> > In C, I would use loop using openat with O_NOFOLLOW, in combination
> > with stat, but Guile doesn't have openat or O_NOFOLLOW.
> 
> In this case we need a solution without openat for now.  Perhaps simply
> changing ‘mkdir-p/perms’ to ‘lstat’ components as it goes?

A compromised service could create a component as a regular file or
directory, and quickly replace it with a symlink after the activation
gexp checks the component wasn't a symlink but before the chown or
chmod.

It's a tiny window though.  I was going to say this tiny ‘exploitation
window’ could be ignored for now until an API to openat & other *at
C functions makes it into guile, but then I checked the inotify(7)
API.  Apparently, inotify events are generated for directory accesses.

> > [...]
> > I'll look into writing a concrete proposal for *at in guile.
> > I'll post a link to the guile mailing list message when it has
> > been composed and sent.

Link: https://lists.gnu.org/archive/html/bug-guile/2021-02/msg00002.html

> The difficulty in designing such an interface is that the Scheme API is
> more about ports than it’s about file names and file descriptors.

This doesn't seem a large issue to me.
Ports for directories can be made with 'open': (open "/home" O_RDONLY).
Example use of for changing permission bits of "/home/USER" with
proposed API:

(let* ((directory (open "/home" O_RDONLY))
       ;; in the C API, this translates to:
       ;; openat(directory-fd, "USER", O_IDK)
       ;; (replace O_IDK with appropriate open flags)
       (file-in-dir (open  (make-path-at directory "USER") O_IDK)))
  (chmod file-in-dir #o077)
  (chown file-in-dir 0 0))

I'll try to implement this API in Scheme (using the FFI), and post
it at https://notabug.org/mdevos/things.  I'll post a follow-up
messsage once I've implemented the basics (openat, chmodat,
chownat).

Greetings,
Maxime.

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