linux-hardening - Re: [PATCH v3 11/16] x86/purgatory: Disable CFI

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Date:   Tue, 14 Sep 2021 15:31:14 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     X86 ML <x86@...nel.org>, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v3 11/16] x86/purgatory: Disable CFI

On Tue, Sep 14, 2021 at 1:30 PM Sami Tolvanen <samitolvanen@...gle.com> wrote:
>
> On Tue, Sep 14, 2021 at 1:02 PM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> >
> > On Tue, Sep 14, 2021 at 12:11 PM Sami Tolvanen <samitolvanen@...gle.com> wrote:
> > >
> > > Disable CONFIG_CFI_CLANG for the stand-alone purgatory.ro.
> > >
> > > Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> >
> > I kind of prefer the existing convention that has explicit guards on
> > specific configs (ie. CONFIG_FUNCTION_TRACER, CONFIG_STACKPROTECTOR,
> > CONFIG_STACKPROTECTOR_STRONG, CONFIG_RETPOLINE); it's more obvious
> > which configs may introduce which flags that are problematic. This
> > patch is ok as is, but it kind of makes this Makefile more
> > inconsistent.  I would prefer we had the explicit checks.
>
> The Makefile does already use DISABLE_STACKLEAK_PLUGIN in a similar
> way, but I don't have a strong preference here.

mmm...DISABLE_STACKLEAK_PLUGIN adds to PURGATORY_CFLAGS. This patch
adds to PURGATORY_CFLAGS_REMOVE.

> I can move this into
> an ifdef if it makes things cleaner.
>
> > Does CFI actually do any instrumentation in these object files? I
> > guess issues in purgatory cause silent/hard to debug kexec failures?
>
> The compiler shouldn't add any actual CFI instrumentation here right
> now, but I would prefer to avoid issues in future.

Ok, good to know.
-- 
Thanks,
~Nick Desaulniers