Subject: Re: bug#45198: 28.0.50; Sandbox mode

From: Philipp Stephani <[email-masked]>
Date: Sun, 18 Apr 2021 11:11:28 +0200

Am So., 18. Apr. 2021 um 08:21 Uhr schrieb Eli Zaretskii <[email-masked]>:
>
> > From: Philipp <[email-masked]>
> > Date: Sat, 17 Apr 2021 21:52:59 +0200
> > Cc: Mattias Engdegård <[email-masked]>,
> >  João Távora <[email-masked]>,
> >  [email-masked],
> >  [email-masked],
> >  [email-masked],
> >  [email-masked]
> >
> > > So you are going to suggest that we rely on some auditing of the
> > > syscalls Emacs uses now to decide which ones to filter and which not?
> >
> > I don't mean that we should wade through all potential syscalls that Emacs could make.  Typically you can come up with such a Seccomp policy iteratively: run Seccomp in advisory mode (i.e. only log syscalls), then allow the syscalls that are both necessary and harmless in the policy.
>
> Emacs can be invoked to do many different things, and will
> correspondingly present very different profiles of syscalls.  Is the
> procedure you envision practical, let alone seamless, given that it
> will have to become part of the maintenance and the release process?

Yes. There aren't that many syscalls to begin with, and Emacs uses
only a small subset of them. New Emacs or libc versions occasionally
introduce new syscalls, but finding and allowing them tends to be not
that big of a deal.

> > >  And what about users who make local changes
> > > in their Emacs?
> >
> > They can provide their own Seccomp policies or modify the ones included in Emacs.
>
> What does providing a policy entail? can you describe the procedure of
> tailoring a policy to changes in the Emacs code?

1. Run the Emacs sandbox with the code you want to run.
2. Emacs will crash with SIGSYS if it hits a forbidden/unknown
syscall. Ensure that this generates a coredump.
3. Check the backtrace for the coredump (e.g. coredumpctl debug)
and/or the Seccomp audit logs (ausearch) for the syscall that
triggered the signal.
4. Add a rule for the syscall and its arguments to the BPF generation
program, e.g. lib-src/seccom-filter.c.
5. Regenerate the BPF rule file.