linux-kernel - Re: [PATCH V2 6/9] x86/speculation: Change __FILL_RETURN

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Date:   Tue, 7 Apr 2020 08:27:45 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, peterz@...radead.org,
        jthierry@...hat.com, tglx@...utronix.de
Subject: Re: [PATCH V2 6/9] x86/speculation: Change __FILL_RETURN_BUFFER to
 work with objtool

On Tue, Apr 07, 2020 at 09:31:39AM +0200, Alexandre Chartre wrote:
> Change __FILL_RETURN_BUFFER so that the stack state is deterministically
> defined for each iteration and that objtool can have an accurate view
> of the stack.
> 
> Signed-off-by: Alexandre Chartre <alexandre.chartre@...cle.com>
> ---
>  arch/x86/include/asm/nospec-branch.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 5c24a7b35166..9a946fd5e824 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -60,8 +60,8 @@
>  	jmp	775b;				\
>  774:						\
>  	dec	reg;				\
> -	jnz	771b;				\
> -	add	$(BITS_PER_LONG/8) * nr, sp;
> +	add	$(BITS_PER_LONG/8) * 2, sp;	\
> +	jnz	771b;
>  
>  #ifdef __ASSEMBLY__

This still isn't a complete fix because the macro is used in an
alternative.  So in the !X86_FEATURE_RSB_CTXSW case, this code isn't
patched in and the ORC data which refers to it is wrong.

As I said before I think the easiest fix would be to convert RSB and
retpolines to use static branches instead of alternatives.

-- 
Josh