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Date:   Fri, 16 Apr 2021 15:37:47 -0700
From:   Kees Cook <keescook@chromium.org>
To:     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
Message-ID: <202104161529.D9F98DA994@keescook>
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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