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 09:46:04 +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.
> 
> Any microcode that removes a feature will set the min_version to itself.
> This will enforce this microcode is not suitable for late-loading.
> 
> The enforcement is not in hardware and limited to kernel loader enforcing
> the requirement. It is not required for early loading of microcode to
> enforce this requirement, since the new features are only
> evaluated after early loading in the boot process.
> 
> 
> Test cases covered:
> 
> 1. With new kernel, attempting to load an older format microcode with the
>    min_rev=0 should be blocked by kernel.
> 
>    [  210.541802] microcode: Header MUST specify min version for late-load
> 
> 2. New microcode with a non-zero min_rev in the header, but the specified
>    min_rev is greater than what is currently loaded in the CPU should be
>    blocked by kernel.
> 
>    245.139828] microcode: Current revision 0x8f685300 is too old to update,
> must be at 0xaa000050 version or higher
> 
> 3. New microcode with a min_rev < currently loaded should allow loading the
>    microcode
> 
> 4. Build initrd with microcode that has min_rev=0, or min_rev > currently
>    loaded should permit early loading microcode from initrd.

What if any validation do you have to ensure min_rev does as promised?
That is, ucode can very easily lie about the number and still remove an
MSR or CPUID enumerated feature.