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Date:   Fri, 16 Apr 2021 15:37:47 -0700
From:   Kees Cook <keescook@...omium.org>
To:     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 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()).

So, instead of a cast, a wrapper is used to bypass instrumentation in
the very few cases its needed.

(Note that such a wrapper is no-op without CFI enabled.)

-- 
Kees Cook

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