linux-hardening - Re: [PATCH 05/15] x86: Implement function

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Date:   Sat, 17 Apr 2021 01:02:49 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        X86 ML <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,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 05/15] x86: Implement function_nocfi

On Fri, Apr 16 2021 at 15:37, Kees Cook wrote:

> On Fri, Apr 16, 2021 at 03:20:17PM -0700, Andy Lutomirski wrote:
>> But obviously there is code that needs real function pointers.  How
>> about making this a first-class feature, or at least hacking around it
>> more cleanly.  For example, what does this do:
>> 
>> char entry_whatever[];
>> wrmsrl(..., (unsigned long)entry_whatever);
>
> This is just casting. It'll still resolve to the jump table entry.
>
>> or, alternatively,
>> 
>> extern void func() __attribute__((nocfi));
>
> __nocfi says func() should not perform checking of correct jump table
> membership for indirect calls.
>
> But we don't want a global marking for a function to be ignored by CFI;
> we don't want functions to escape CFI -- we want specific _users_ to
> either not check CFI for indirect calls (__nocfi) or we want specific
> passed addresses to avoid going through the jump table
> (function_nocfi()).

And that's why you mark entire files to be exempt without any rationale
why it makes sense.

Thanks,

        tglx