linux-kernel - Re: [PATCH 2/2] x86/paravirt: add extra clobbers with ZERO_CALL_USED

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Message-ID: <202209071609.BE5BF18D6@keescook>
Date:   Wed, 7 Sep 2022 16:10:43 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Bill Wendling <morbo@...gle.com>,
        Juergen Gross <jgross@...e.com>,
        "Srivatsa S. Bhat (VMware)" <srivatsa@...il.mit.edu>,
        Alexey Makhalov <amakhalov@...are.com>,
        VMware PV-Drivers Reviewers <pv-drivers@...are.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        virtualization@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        clang-built-linux <llvm@...ts.linux.dev>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 2/2] x86/paravirt: add extra clobbers with
 ZERO_CALL_USED_REGS enabled

On Wed, Sep 07, 2022 at 10:50:03AM +0200, Peter Zijlstra wrote:
> On Tue, Sep 06, 2022 at 11:00:07PM -0700, Nick Desaulniers wrote:
> > On Sun, Sep 4, 2022 at 11:02 PM Bill Wendling <morbo@...gle.com> wrote:
> > >
> > > On Sat, Sep 3, 2022 at 12:18 AM Kees Cook <keescook@...omium.org> wrote:
> > > >
> > > > On Fri, Sep 02, 2022 at 09:37:50PM +0000, Bill Wendling wrote:
> > > > > [...]
> > > > >         callq   *pv_ops+536(%rip)
> > > >
> > > > Do you know which pv_ops function is this? I can't figure out where
> > > > pte_offset_kernel() gets converted into a pv_ops call....
> > > >
> > > This one is _paravirt_ident_64, I believe. I think that the original
> > > issue Nathan was seeing was with another seemingly innocuous function.
> > 
> > _paravirt_ident_64 is marked noinstr, which makes me suspect that it
> > really needs to not be touched at all by the compiler for
> > these...special features.
> 
> My source tree sayeth:
> 
>   u64 notrace _paravirt_ident_64(u64 x)

I don't see noinstr either. But it seems a reasonable thing to do?

Bill, does fixing up the noinstr macro and adding it here fix the
problem?

-- 
Kees Cook