linux-hardening - Re: [PATCH RFC] gcc-plugins: Handle GCC version mismatch for OOT modules

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Message-ID: <202101261459.C90E9F97D@keescook>
Date:   Tue, 26 Jan 2021 14:59:57 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-hardening@...r.kernel.org, linux-kbuild@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Justin Forbes <jforbes@...hat.com>,
        Ondrej Mosnacek <omosnace@...hat.com>
Subject: Re: [PATCH RFC] gcc-plugins: Handle GCC version mismatch for OOT
 modules

On Tue, Jan 26, 2021 at 12:43:16PM -0600, Josh Poimboeuf wrote:
> On Tue, Jan 26, 2021 at 09:56:10AM -0800, Kees Cook wrote:
> > On Mon, Jan 25, 2021 at 04:19:53PM -0600, Josh Poimboeuf wrote:
> > > On Mon, Jan 25, 2021 at 02:03:07PM -0800, Kees Cook wrote:
> > > > On Mon, Jan 25, 2021 at 02:42:10PM -0600, Josh Poimboeuf wrote:
> > > > > When a GCC version mismatch is detected, print a warning and disable the
> > > > > plugin.  The only exception is the RANDSTRUCT plugin which needs all
> > > > > code to see the same struct layouts.  In that case print an error.
> > > > 
> > > > I prefer this patch as-is: only randstruct needs a hard failure. The
> > > > others likely work (in fact, randstruct likely works too).
> > > 
> > > I'm curious about this last statement, why would randstruct likely work?
> > > 
> > > Even struct module has '__randomize_layout', wouldn't basic module init
> > > go splat?
> > 
> > No; the seed is part of the generate includes -- you'll get the same
> > layout with the same seed.
> 
> Right, but don't you need the plugin enabled to make use of that seed,
> so the structs get interpreted properly by the module?  Or am I
> completely misunderstanding how this plugin works?

Having the plugin enabled or not is part of the Kconfig ... you can't
build anything if you change Kconfig. I feel like I'm missing
something...

-- 
Kees Cook