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

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Message-ID: <112c84e8-4116-448d-9c6c-89b0312cfc01@intel.com>
Date: Wed, 11 Jun 2025 23:34:04 -0700
From: Sohil Mehta <sohil.mehta@...el.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>, Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH] x86/its: Warn when microcode is old

On 6/11/2025 5:08 PM, 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");
> +

Maybe I am missing something, but isn't "old" supposed to be an evolving
thing? I am not sure how old microcode can reliably (always) translate
to not effective against ITS.

Can you please provide more context? I feel the warning could be
misleading for userspace.



>  	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
>