linux-kernel - Re: [PATCH 17/21] x86/acpi: Convert indirect jump to retpoline

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Date:   Thu, 14 Jan 2021 17:47:37 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andrew Cooper <andrew.cooper3@...rix.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>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH 17/21] x86/acpi: Convert indirect jump to retpoline

On Thu, Jan 14, 2021 at 10:59:39PM +0000, Andrew Cooper wrote:
> On 14/01/2021 19:40, 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: "Rafael J. Wysocki" <rjw@...ysocki.net>
> > Cc: Len Brown <len.brown@...el.com>
> > Cc: Pavel Machek <pavel@....cz>
> > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > ---
> >  arch/x86/kernel/acpi/wakeup_64.S | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kernel/acpi/wakeup_64.S b/arch/x86/kernel/acpi/wakeup_64.S
> > index 5d3a0b8fd379..0b371580e620 100644
> > --- a/arch/x86/kernel/acpi/wakeup_64.S
> > +++ b/arch/x86/kernel/acpi/wakeup_64.S
> > @@ -7,6 +7,7 @@
> >  #include <asm/msr.h>
> >  #include <asm/asm-offsets.h>
> >  #include <asm/frame.h>
> > +#include <asm/nospec-branch.h>
> >  
> >  # Copyright 2003 Pavel Machek <pavel@...e.cz
> >  
> > @@ -39,7 +40,7 @@ SYM_FUNC_START(wakeup_long64)
> >  	movq	saved_rbp, %rbp
> >  
> >  	movq	saved_rip, %rax
> > -	jmp	*%rax
> > +	JMP_NOSPEC rax
> >  SYM_FUNC_END(wakeup_long64)
> 
> I suspect this won't work as you intend.
> 
> wakeup_long64() still executes on the low mappings, not the high
> mappings, so the `jmp __x86_indirect_thunk_rax` under this JMP_NOSPEC
> will wander off into the weeds.
> 
> This is why none of the startup "jmps from weird contexts onto the high
> mappings" have been retpolined-up.

D'oh.  Of course it wouldn't be that easy.  I suppose the other two
retpoline patches (15 and 21) are bogus as well.

-- 
Josh