linux-kernel - Re: [RFC PATCH v19 1/5] exec: Add a new AT

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Message-ID: <20240709.gae4cu4Aiv6s@digikod.net>
Date: Tue, 9 Jul 2024 11:18:00 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: Jeff Xu <jeffxu@...gle.com>
Cc: Florian Weimer <fweimer@...hat.com>, Al Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Kees Cook <keescook@...omium.org>, 
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	linux-security-module@...r.kernel.org
Subject: Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2)

On Mon, Jul 08, 2024 at 10:52:36AM -0700, Jeff Xu wrote:
> On Mon, Jul 8, 2024 at 10:33 AM Florian Weimer <fweimer@...hat.com> wrote:
> >
> > * Jeff Xu:
> >
> > > On Mon, Jul 8, 2024 at 9:26 AM Florian Weimer <fweimer@...hat.com> wrote:
> > >>
> > >> * Jeff Xu:
> > >>
> > >> > Will dynamic linkers use the execveat(AT_CHECK) to check shared
> > >> > libraries too ?  or just the main executable itself.
> > >>
> > >> I expect that dynamic linkers will have to do this for everything they
> > >> map.
> > > Then all the objects (.so, .sh, etc.) will go through  the check from
> > > execveat's main  to security_bprm_creds_for_exec(), some of them might
> > > be specific for the main executable ?

Yes, we should check every executable code (including seccomp filters)
to get a consistent policy.

What do you mean by "specific for the main executable"?

> >
> > If we want to avoid that, we could have an agreed-upon error code which
> > the LSM can signal that it'll never fail AT_CHECK checks, so we only
> > have to perform the extra system call once.

I'm not sure to follow.  Either we check executable code or we don't,
but it doesn't make sense to only check some parts (except for migration
of user space code in a system, which is one purpose of the securebits
added with the next patch).

The idea with AT_CHECK is to unconditionnaly check executable right the
same way it is checked when a file is executed.  User space can decide
to check that or not according to its policy (i.e. securebits).

> >
> Right, something like that.
> I would prefer not having AT_CHECK specific code in LSM code as an
> initial goal, if that works, great.

LSMs should not need to change anything, but they are free to implement
new access right according to AT_CHECK.

> 
> -Jeff
> 
> > Thanks,
> > Florian
> >