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Date:   Tue, 16 Aug 2022 16:51:49 +0000
From:   Ashok Raj <ashok.raj@intel.com>
To:     Borislav Petkov <bp@alien8.de>
CC:     Ashok Raj <ashok_raj@linux.intel.com>, X86 ML <x86@kernel.org>,
        "Andrew Cooper" <amc96@srcf.net>,
        LKML <linux-kernel@vger.kernel.org>,
        =?utf-8?Q?=C8=98tefan?= Talpalaru <stefantalpalaru@yahoo.com>,
        Ashok Raj <ashok.raj@intel.com>
Subject: Re: [PATCH] x86/microcode/AMD: Attempt applying on every logical
 thread
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On Tue, Aug 16, 2022 at 02:41:39PM +0200, Borislav Petkov wrote:
> On Tue, Aug 16, 2022 at 09:00:14AM +0000, Ashok Raj wrote:
> > A re-application means, you want to apply even if the cpu_rev <= patch.rev
> 
> Yes.

I see, so we probably shouldn't remove the rev check completely but just
permit to reload if the rev is equal?

> 
> > if cpu_rev is > patch_rev, clearly its ahead?. say BIOS has a newer
> > version than in the initrd image, do we want to replace the BIOS
> > version since we do no revid checks here.
> 
> Can you even downgrade the microcode through the MSR?

I'm not sure what is true for AMD.

For Intel's there is a Security Version Number in the signed encrypted part
of the microcode. As long as the new image has SVN >= what the CPU's SVN is
you can update the microcode. 

So if a rev2 is loaded, and rev1 is the new MCU, and rev2 and rev1 have the
same SVN, you can go down rev from rev2->rev1 where rev2 > rev1.

Microcode enforcement for rollback protection has been the SVN. Version
number is a SW feel good thing to identifying which image you are running,
not intended for preventing rollback or any security enforcement.

Cheers,
Ashok