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Date:   Fri, 16 Apr 2021 15:16:40 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Borislav Petkov <bp@alien8.de>
Cc:     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>,
        Mark Rutland <mark.rutland@arm.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: <202104161510.246509CE@keescook>
References: <20210416203844.3803177-1-samitolvanen@google.com>
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 <20210416211855.GD22348@zn.tnic>
 <CABCJKud8TvzhcjHCpsrtCJ4B50ZUfaL48F42EhZ2zWKLteAc0Q@mail.gmail.com>
 <20210416220251.GE22348@zn.tnic>
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On Sat, Apr 17, 2021 at 12:02:51AM +0200, Borislav Petkov wrote:
> On Fri, Apr 16, 2021 at 02:49:23PM -0700, Sami Tolvanen wrote:
> > __nocfi only disables CFI checking in a function, the compiler still
> > changes function addresses to point to the CFI jump table, which is
> > why we need function_nocfi().
> 
> So call it __func_addr() or get_function_addr() or so, so that at least
> it is clear what this does.

FWIW, it's been renamed already. I'll CC Mark back into the thread.
https://lore.kernel.org/lkml/20210325101655.GB36570@C02TD0UTHF1T.local/

> Also, am I going to get a steady stream of patches adding that wrapper
> to function names or is this it? IOW, have you built an allyesconfig to
> see how many locations need touching?

Nooo. Much like __nocfi, this should be extremely rare and is only used in
places that must not be doing CFI nor working on the jump tables (e.g. the
syscall MSR). There list for arm64 in -next, for example, is short:


429d9a552e81 arm64: ftrace: use function_nocfi for ftrace_call
fbcdf27674bc arm64: add __nocfi to __apply_alternatives
f2324191e959 arm64: add __nocfi to functions that jump to a physical address
c4a384170f17 arm64: use function_nocfi with __pa_symbol
5198a15901d2 psci: use function_nocfi for cpu_resume
8e284f3ebed2 bpf: disable CFI in dispatcher functions


-- 
Kees Cook