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

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Message-ID: <87im4luaq7.ffs@nanos.tec.linutronix.de>
Date:   Sat, 17 Apr 2021 00:26:56 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Sami Tolvanen <samitolvanen@...gle.com>, x86@...nel.org
Cc:     Kees Cook <keescook@...omium.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,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH 06/15] x86: Avoid CFI jump tables in IDT and entry points

On Fri, Apr 16 2021 at 13:38, Sami Tolvanen wrote:
> With CONFIG_CFI_CLANG, the compiler replaces function addresses in C
> code with jump table addresses.

Fine.

> To avoid referring to jump tables in entry code with PTI,

What has this to do with PTI?

> disable CFI for IDT and paravirt code, and use function_nocfi() to
> prevent jump table addresses from being added to the IDT or system
> call entry points.

How does this changelog make sense for anyone not familiar with the
matter at hand?

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 ...

Thanks,

        tglx