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

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Message-ID: <20250612131909.saj4lq7dh2gqznyc@desk>
Date: Thu, 12 Jun 2025 06:19:09 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Sohil Mehta <sohil.mehta@...el.com>
Cc: x86@...nel.org, 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 Wed, Jun 11, 2025 at 11:34:04PM -0700, Sohil Mehta wrote:
> 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.

Yes, "old" does not always means vulnerable. Looks like "... *may* be
vulnerable" is too subtle here.

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

Since the ITS software mitigation does not depend on the microcode, there
is no enumeration. Hence, hitch-hiking on the existing X86_BUG_OLD_MICROCODE.

On a second thought, cpu_set_bug_bits() already issues a warning, and even
taints the kernel when old microcode is detected. So warning here seems
redundant, and also misleading as you said. I am okay dropping this patch.