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

[Josh Poimboeuf]

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@xxxxxxxxxx>
> > Cc: Juergen Gross <jgross@xxxxxxxx>
> > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > ---
> >   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