linux-kernel - Re: [PATCH 15/21] x86/xen/pvh: Convert indirect jump to retpoline

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Message-ID: <12afb52c-f555-656e-d544-c2965a616bdc@suse.com>
Date:   Fri, 15 Jan 2021 06:24:10 +0100
From:   Jürgen Groß <jgross@...e.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>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH 15/21] x86/xen/pvh: Convert indirect jump to retpoline

On 14.01.21 20:40, 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: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Cc: Juergen Gross <jgross@...e.com>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> ---
>   arch/x86/platform/pvh/head.S | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
> index 43b4d864817e..d87cebd08d32 100644
> --- a/arch/x86/platform/pvh/head.S
> +++ b/arch/x86/platform/pvh/head.S
> @@ -16,6 +16,7 @@
>   #include <asm/boot.h>
>   #include <asm/processor-flags.h>
>   #include <asm/msr.h>
> +#include <asm/nospec-branch.h>
>   #include <xen/interface/elfnote.h>
>   
>   	__HEAD
> @@ -105,7 +106,7 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
>   	/* startup_64 expects boot_params in %rsi. */
>   	mov $_pa(pvh_bootparams), %rsi
>   	mov $_pa(startup_64), %rax
> -	jmp *%rax
> +	JMP_NOSPEC rax

I'd rather have it annotated only.

Using ALTERNATIVE in very early boot code is just adding needless
clutter, as the retpoline variant won't ever be active.


Juergen

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