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To:     Josh Poimboeuf <jpoimboe@redhat.com>, x86@kernel.org
Cc:     linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        clang-built-linux@googlegroups.com,
        Miroslav Benes <mbenes@suse.cz>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>
References: <cover.1610652862.git.jpoimboe@redhat.com>
 <adfa2afe5ddc831017222db9f48ad0fbff17c807.1610652862.git.jpoimboe@redhat.com>
From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
Subject: Re: [PATCH 15/21] x86/xen/pvh: Convert indirect jump to retpoline
Message-ID: <12afb52c-f555-656e-d544-c2965a616bdc@suse.com>
Date:   Fri, 15 Jan 2021 06:24:10 +0100
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From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
To: Josh Poimboeuf <jpoimboe@redhat.com>, x86@kernel.org
Cc: linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
 Sami Tolvanen <samitolvanen@google.com>, Sedat Dilek
 <sedat.dilek@gmail.com>, Kees Cook <keescook@chromium.org>,
 Nick Desaulniers <ndesaulniers@google.com>,
 clang-built-linux@googlegroups.com, Miroslav Benes <mbenes@suse.cz>,
 Boris Ostrovsky <boris.ostrovsky@oracle.com>
Message-ID: <12afb52c-f555-656e-d544-c2965a616bdc@suse.com>
Subject: Re: [PATCH 15/21] x86/xen/pvh: Convert indirect jump to retpoline
References: <cover.1610652862.git.jpoimboe@redhat.com>
 <adfa2afe5ddc831017222db9f48ad0fbff17c807.1610652862.git.jpoimboe@redhat.com>
In-Reply-To: <adfa2afe5ddc831017222db9f48ad0fbff17c807.1610652862.git.jpoimboe@redhat.com>

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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.
>=20
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: Juergen Gross <jgross@suse.com>
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> ---
>   arch/x86/platform/pvh/head.S | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>=20
> 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>
>  =20
>   	__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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