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Date:   Thu, 28 Jul 2022 17:16:40 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        Dimitri John Ledkov <dimitri.ledkov@canonical.com>,
        Borislav Petkov <bp@suse.de>, stable@vger.kernel.org
Subject: Re: [PATCH] x86/bugs: Do not enable IBPB at firmware entry when IBPB
 is not available
Message-ID: <YuKoWPRt56T+FE+s@worktop.programming.kicks-ass.net>
References: <20220728122602.2500509-1-cascardo@canonical.com>
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On Thu, Jul 28, 2022 at 09:26:02AM -0300, Thadeu Lima de Souza Cascardo wrote:
> Some cloud hypervisors do not provide IBPB on very recent CPU processors,
> including AMD processors affected by Retbleed.

That's a bug in the hypervisor.

> Fixes: 28a99e95f55c ("x86/amd: Use IBPB for firmware calls")

Fixes^WCreates-a-speculation-hole-in:

> Reported-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: <stable@vger.kernel.org>
> ---
>  arch/x86/kernel/cpu/bugs.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 6454bc767f0f..6761668100b9 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1520,6 +1520,7 @@ static void __init spectre_v2_select_mitigation(void)
>  	 * enable IBRS around firmware calls.
>  	 */
>  	if (boot_cpu_has_bug(X86_BUG_RETBLEED) &&
> +	    boot_cpu_has(X86_FEATURE_IBPB) &&
>  	    (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
>  	     boot_cpu_data.x86_vendor == X86_VENDOR_HYGON)) {

At the very least we need a pr_warn() and something nasty in
retbleed_show_state() to warn the user their firmware calls are
vulnerable.