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Message-ID: <5017a6c5-55fa-0767-b1ed-2bd9e2a5efc1@citrix.com>
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
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