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

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Message-ID: <871rb7sh31.ffs@nanos.tec.linutronix.de>
Date:   Sun, 18 Apr 2021 18:17:06 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        David Laight <David.Laight@...lab.com>,
        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 Sat, Apr 17 2021 at 17:11, Andy Lutomirski wrote:
> On Sat, Apr 17, 2021 at 4:53 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>> which works for
>>
>>       foo = function_nocfi(bar);
>
> I agree in general.  But right now, we have, in asm/proto.h:
>
> void entry_SYSCALL_64(void);
>
> and that's pure nonsense.  Depending on your point of view,
> entry_SYSCALL_64 is a symbol that resolves to an integer or it's an
> array of bytes containing instructions, but it is most definitely not
> a function void (void).  So, regardless of any CFI stuff, I propose
> that we standardize our handling of prototypes of symbols that are
> opaque to the C compiler.  Here are a couple of choices:
>
> Easy one:
>
> extern u8 entry_SYSCALL_64[];
>
> Slightly more complicated:
>
> struct opaque_symbol;
> extern struct opaque_symbol entry_SYSCALL_64;
>
> The opaque_symbol variant avoids any possible confusion over the weird
> status of arrays in C, and it's hard to misuse, since struct
> opaque_symbol is an incomplete type.

Makes sense.