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

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Date:   Thu, 14 Jan 2021 22:59:39 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>, <x86@...nel.org>
CC:     <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 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.

~Andrew