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

[Andrew Cooper]

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@xxxxxxxxxxxxx>
> Cc: Len Brown <len.brown@xxxxxxxxx>
> Cc: Pavel Machek <pavel@xxxxxx>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> ---
>  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@xxxxxxx
>  
> @@ -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