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

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Message-ID: <Yvn5BNXfOm3uA7WA@worktop.programming.kicks-ass.net>
Date:   Mon, 15 Aug 2022 09:43:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ashok Raj <ashok.raj@...el.com>
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>
Subject: Re: [PATCH 3/5] x86/microcode/intel: Allow a late-load only if a min
 rev is specified

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?