linux-kernel - Re: [PATCH 3/5] x86/microcode/intel: Allow a late-load only if a min rev is specified

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Date:   Mon, 15 Aug 2022 12:29:11 +0000
From:   Ashok Raj <ashok.raj@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Tony Luck" <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "LKML Mailing List" <linux-kernel@...r.kernel.org>,
        X86-kernel <x86@...nel.org>,
        "Andy Lutomirski" <luto@...capital.net>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ashok Raj <ashok.raj@...el.com>
Subject: Re: [PATCH 3/5] x86/microcode/intel: Allow a late-load only if a min
 rev is specified

On Mon, Aug 15, 2022 at 09:43:00AM +0200, Peter Zijlstra wrote:
> On Sat, Aug 13, 2022 at 10:38:23PM +0000, Ashok Raj wrote:
> 
> > The proposal here is an even simpler option. The criteria for a microcode to
> > be a viable late-load candidate is that no CPUID or OS visible MSR features
> > are removed with respect to an earlier version of the microcode.
> > 
> > Pseudocode for late-load is as follows:
> > 
> > if header.min_required_id == 0
> > 	This is old format microcode, block late-load
> > else if current_ucode_version < header.min_required_id
> > 	Current version is too old, block late-load of this microcode.
> > else
> > 	OK to proceed with late-load.
> 
> What about ucode that adds CPUID bits? Since the kernel will not re-init
> it will not pick up on those. But userspace might.
> 
> Should we at all time enable CPUID intercept to ensure user visible
> CPUID doesn't change?

The protection is to make sure any existing users won't experience a
feature pulled under their feet.

Using new features aren't dangerous though, and should be permitted.