linux-kernel - Re: [RFC PATCH 3/3] x86/microcode: Taint and warn on late loading

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Message-ID: <Yo5LAenZIsYmM9Ie@zn.tnic>
Date:   Wed, 25 May 2022 17:28:01 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 3/3] x86/microcode: Taint and warn on late loading

On Wed, May 25, 2022 at 02:50:59PM +0000, Luck, Tony wrote:
> Are taint flags in such short supply that you couldn't create a new
> one?

Yes, they can be as many as there are letters in the english alphabet,
it seems:

struct taint_flag {
        char c_true;    /* character printed when tainted */
	^^^^^^^^^^^^

and there are already

#define TAINT_FLAGS_COUNT               18

in use.

> The OUT_OF_SPEC one already seems to be used in some dubious
> ways:
> 1) Command line argument to clear a X86_FEATURES bit
> 2) Forcing PAE
> 3) Writing to an MSR not on the "approved" list
> 
> As you add more ways to set this taint bit, it becomes less useful
> for debugging ...

Look at the other taint flags - they're set in a bunch of different
places so it is hard to unambiguously decide where the taint was set. If
we wanna use it for debugging, then the taint_flag struct above should
probably save the caller address which set the taint... or something to
that effect.

> since now you have to dig into which of the possible cases set the bit
> to decide whether it might have contributed to the OOPS.

So I'm still not convinced this should have a special taint flag.

-- 
Regards/Gruss,
    Boris.

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