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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Kees Cook <keescook@chromium.org>,
        Andy Lutomirski <luto@kernel.org>
Cc:     Borislav Petkov <bp@alien8.de>,
        Sami Tolvanen <samitolvanen@google.com>,
        X86 ML <x86@kernel.org>, Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        linux-hardening@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH 05/15] x86: Implement function_nocfi
In-Reply-To: <202104161529.D9F98DA994@keescook>
References: <20210416203844.3803177-1-samitolvanen@google.com> <20210416203844.3803177-6-samitolvanen@google.com> <20210416211855.GD22348@zn.tnic> <CABCJKud8TvzhcjHCpsrtCJ4B50ZUfaL48F42EhZ2zWKLteAc0Q@mail.gmail.com> <20210416220251.GE22348@zn.tnic> <CALCETrVTtKqD6fonUmT_qr0HJ0X9TWzLGq-wpm+A7XKyjn3W5g@mail.gmail.com> <20210416221414.GF22348@zn.tnic> <CALCETrUo+tR+YmfoBPWV9z_7QhU74=7tmCBD_zsfa24ZxNvfxg@mail.gmail.com> <202104161529.D9F98DA994@keescook>
Date:   Sat, 17 Apr 2021 01:02:49 +0200
Message-ID: <87fszpu92e.ffs@nanos.tec.linutronix.de>
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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