linux-kernel - Re: [PATCH 15/21] x86/xen/pvh: Convert indirect jump to retpoline

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Date:   Fri, 15 Jan 2021 09:08:11 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jürgen Groß <jgross@...e.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        Miroslav Benes <mbenes@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH 15/21] x86/xen/pvh: Convert indirect jump to retpoline

On Fri, Jan 15, 2021 at 06:24:10AM +0100, Jürgen Groß wrote:
> On 14.01.21 20:40, Josh Poimboeuf wrote:
> > It's kernel policy to not have (unannotated) indirect jumps because of
> > Spectre v2.  This one's probably harmless, but better safe than sorry.
> > Convert it to a retpoline.
> > 
> > Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> > Cc: Juergen Gross <jgross@...e.com>
> > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > ---
> >   arch/x86/platform/pvh/head.S | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
> > index 43b4d864817e..d87cebd08d32 100644
> > --- a/arch/x86/platform/pvh/head.S
> > +++ b/arch/x86/platform/pvh/head.S
> > @@ -16,6 +16,7 @@
> >   #include <asm/boot.h>
> >   #include <asm/processor-flags.h>
> >   #include <asm/msr.h>
> > +#include <asm/nospec-branch.h>
> >   #include <xen/interface/elfnote.h>
> >   	__HEAD
> > @@ -105,7 +106,7 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
> >   	/* startup_64 expects boot_params in %rsi. */
> >   	mov $_pa(pvh_bootparams), %rsi
> >   	mov $_pa(startup_64), %rax
> > -	jmp *%rax
> > +	JMP_NOSPEC rax
> 
> I'd rather have it annotated only.
> 
> Using ALTERNATIVE in very early boot code is just adding needless
> clutter, as the retpoline variant won't ever be active.

Yeah, Andy pointed out something similar.  I'll be changing this to an
annotation.

-- 
Josh