linux-kernel - Re: [PATCH v3 1/5] x86/microcode/intel: Check against CPU signature before saving microcode

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Message-ID: <Yv9k6fqRANu4ojK6@zn.tnic>
Date:   Fri, 19 Aug 2022 12:24:41 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ashok Raj <ashok.raj@...el.com>
Cc:     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>,
        Jacon Jun Pan <jacob.jun.pan@...el.com>
Subject: Re: [PATCH v3 1/5] x86/microcode/intel: Check against CPU signature
 before saving microcode

On Wed, Aug 17, 2022 at 05:11:23AM +0000, Ashok Raj wrote:
> When save_microcode_patch() is looking to replace an existing microcode in
> the cache, current code is *only* checks the CPU sig/pf in the main

Write those "sig/pf" things out once so that it is clear what that is.

> header. Microcode can carry additional sig/pf combinations in the extended
> signature table, which is completely missed today.
> 
> For e.g. Current patch is a multi-stepping patch and new incoming patch is
> a specific patch just for this CPUs stepping.
> 
> patch1:
> fms3 <--- header FMS
> ...
> ext_sig:
> fms1
> fms2
> 
> patch2: new
> fms2 <--- header FMS
> 
> Current code takes only fms3 and checks with patch2 fms2.

So, find_matching_signature() does all the signatures matching and
scanning already. If anything, that function should tell its callers
whether the patch it is looking at - the fms2 one - should replace the
current one or not.

I.e., all the logic to say how strong a patch match is, should be
concentrated there. And then the caller will do the according action.

> saved_patch.header.fms3 != new_patch.header.fms2, so save_microcode_patch
> saves it to the end of list instead of replacing patch1 with patch2.
> 
> There is no functional user observable issue since find_patch() skips
> patch versions that are <= current_patch and will land on patch2 properly.
> 
> Nevertheless this will just end up storing every patch that isn't required.
> Kernel just needs to store the latest patch. Otherwise its a memory leak
> that sits in kernel and never used.

Oh well.

> Cc: stable@...r.kernel.org

Why?

This looks like a small correction to me which doesn't need to go to
stable...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette