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

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Message-ID: <20240708.ujeeDahkee4e@digikod.net>
Date: Mon, 8 Jul 2024 19:05:39 +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 09:40:45AM -0700, Jeff Xu wrote:
> 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.

Correct, that would enable to safely handle LD_PRELOAD for instance.

> 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 ?
> e.g. ChromeOS uses security_bprm_creds_for_exec to block executable
> memfd [1], applying this means automatically extending the block to
> the .so object.

That's a good example of how this AT_CHECK check makes sense.

Landlock will probably get a similar (optional) restriction too:
https://github.com/landlock-lsm/linux/issues/37

> 
> I'm not sure if other LSMs need to be updated ?  e.g.  will  SELINUX
> check for .so with its process transaction policy ?

LSM should not need to be updated with this patch series.  However,
systems/components/containers enabling this new check should make sure
it works with their current policy.

> 
> [1] https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3834992
> 
> -Jeff
> 
> 
> > Usually, that does not include the maim program, but this can
> > happen with explicit loader invocations (“ld.so /bin/true”).
> >
> > Thanks,
> > Florian
> >