linux-kernel - Re: [PATCH] x86/its: Warn when microcode is old

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Message-ID: <f9e2dcaa-6eff-4eda-8c7c-0d19c2d11e73@suse.com>
Date: Thu, 12 Jun 2025 09:58:35 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, x86@...nel.org
Cc: linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
 Josh Poimboeuf <jpoimboe@...nel.org>
Subject: Re: [PATCH] x86/its: Warn when microcode is old



On 6/12/25 03:08, Pawan Gupta wrote:
> A microcode update is required for IBPB to be effective against ITS. On
> Intel, X86_BUG_OLD_MICROCODE is set already when old microcode is detected.
> In such a case system is tainted and a warning is issued.
> 
> Also warn that userspace could be vulnerable to ITS.
> 
> Suggested-by: Borislav Petkov <bp@...en8.de>
> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> ---
>   arch/x86/kernel/cpu/bugs.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 7f94e6a5497d9a2d312a76095e48d6b364565777..7aa3ae00e6b4daa5e42217b66c50cd46d6bcc115 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1427,6 +1427,10 @@ static void __init its_select_mitigation(void)
>   		return;
>   	}
>   
> +	/* For IBPB to be effective against ITS */
> +	if (boot_cpu_has_bug(X86_BUG_OLD_MICROCODE))
> +		pr_warn("Old microcode, userspace may be vulnerable to ITS\n");
> +

That check is too coarse and probably has a high chance of false 
positive, i.e

Latest firmware for your CPU is version Z as defined in 
intel-ucode-defs.h, current running version is X which is older than Z , 
but it already contains IBPB fixups for ITS. Then it will trigger a 
false positive.

>   	if (its_mitigation == ITS_MITIGATION_RETPOLINE_STUFF &&
>   	    !IS_ENABLED(CONFIG_MITIGATION_CALL_DEPTH_TRACKING)) {
>   		pr_err("RSB stuff mitigation not supported, using default\n");
> 
> ---
> base-commit: 19272b37aa4f83ca52bdf9c16d5d81bdd1354494
> change-id: 20250611-its-warn-b292b10896e3
>