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

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Message-ID: <20240709.eud4ao8Kie6n@digikod.net>
Date: Tue, 9 Jul 2024 22:41:54 +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 Tue, Jul 09, 2024 at 11:57:27AM -0700, Jeff Xu wrote:
> On Tue, Jul 9, 2024 at 2:18 AM Mickaël Salaün <mic@...ikod.net> wrote:
> >
> > 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"?
> >
> I meant:
> 
> The check is for the exe itself, not .so, etc.
> 
> For example:  /usr/bin/touch is checked.
> not the shared objects:
> ldd /usr/bin/touch
> linux-vdso.so.1 (0x00007ffdc988f000)
> libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f59b6757000)
> /lib64/ld-linux-x86-64.so.2 (0x00007f59b6986000)

ld.so should be patched to check shared-objects.

> 
> Basically, I asked if the check can be extended to shared-objects,
> seccomp filters, etc, without modifying existing LSMs.

Yes, the check should be used against any piece of code such as
shared-objects, seccomp filters...

> you pointed out "LSM should not need to be updated with this patch
> series.", which already answered my question.
> 
> Thanks.
> -Jeff
> 
> -Jeff