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

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Message-ID: <20250612132250.k2loomkqhkfvksvk@desk>
Date: Thu, 12 Jun 2025 06:22:50 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Nikolay Borisov <nik.borisov@...e.com>
Cc: x86@...nel.org, 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 Thu, Jun 12, 2025 at 09:58:35AM +0300, Nikolay Borisov wrote:
> 
> 
> 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.

Agree, as I mentioned in the other email, I am okay dropping this patch.
This creates more confusion than it helps.