linux-kernel - Re: [PATCH 06/15] x86: Avoid CFI jump tables in IDT and entry points

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Date:   Sat, 17 Apr 2021 02:02:31 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>, x86@...nel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH 06/15] x86: Avoid CFI jump tables in IDT and entry points

On Fri, Apr 16 2021 at 16:56, Kees Cook wrote:
> On Sat, Apr 17, 2021 at 12:26:56AM +0200, Thomas Gleixner wrote:
>> Where is the analysis why excluding 
>> 
>> > +CFLAGS_REMOVE_idt.o		:= $(CC_FLAGS_CFI)
>> > +CFLAGS_REMOVE_paravirt.o	:= $(CC_FLAGS_CFI)
>> 
>> all of idt.c and paravirt.c is correct and how that is going to be
>> correct in the future?
>> 
>> These files are excluded from CFI, so I can add whatever I want to them
>> and circumvent the purpose of CFI, right?
>> 
>> Brilliant plan that. But I know, sekurity ...
>
> *sigh* we're on the same side. :P I will choose to understand your
> comments here as:
>
> "How will enforcement of CFI policy be correctly maintained here if
> the justification for disabling it for whole compilation units is not
> clearly understandable by other developers not familiar with the nuances
> of its application?"

Plus, if there is a justification for disabling it for a whole
compilation unit:

 Where is the tooling which makes sure that this compilation unit is not
 later on filled with code which should be subject to CFI?

Thanks,

        tglx