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

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Message-ID: <CABCJKufQqhXsP0Nv1hu60YoH=Tetp35=yENS_9LWZvzptFvjDQ@mail.gmail.com>
Date:   Mon, 19 Apr 2021 08:20:09 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Kees Cook <keescook@...omium.org>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>, 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 Sun, Apr 18, 2021 at 3:57 PM Andy Lutomirski <luto@...nel.org> wrote:
>
> On Sun, Apr 18, 2021 at 9:17 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > 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.
>
> Sami, do you want to do this as part of your series or should I write a patch?

I can certainly include this in the next version, but that might have
to wait a bit for compiler changes, so if you want this done sooner,
please go ahead.

Sami