linux-kernel - Re: [PATCH] x86/cpu: Fix migration safety with X86_BUG_NULL

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Message-ID: <YV2zFlaLvZzNPkjh@zn.tnic>
Date:   Wed, 6 Oct 2021 16:30:46 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Jane Malalane <jane.malalane@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Pu Wen <puwen@...on.cn>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Yazen Ghannam <Yazen.Ghannam@....com>,
        Brijesh Singh <brijesh.singh@....com>,
        Huang Rui <ray.huang@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Kim Phillips <kim.phillips@....com>, stable@...r.kernel.org
Subject: Re: [PATCH] x86/cpu: Fix migration safety with X86_BUG_NULL_SEL

On Wed, Oct 06, 2021 at 03:15:51PM +0100, Andrew Cooper wrote:
> The case which goes wrong is this:
> 
> 1. Zen1 (or earlier) and Zen2 (or later) in a migration pool
> 2. Linux boots on Zen2, probes and finds the absence of X86_BUG_NULL_SEL
> 3. Linux is then migrated to Zen1
> 
> Linux is now running on a X86_BUG_NULL_SEL-impacted CPU while believing
> that the bug is fixed.
> 
> The only way to address the problem is to fully trust the "no longer
> affected" CPUID bit when virtualised, because in the above case it would
> be clear deliberately to indicate the fact "you might migrate to
> somewhere which really is affected".

Yap, makes sense.

Thanks for taking the time - that's what I was looking for.

Please add to the commit message of the next version.

-- 
Regards/Gruss,
    Boris.

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