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

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Date:   Wed, 6 Oct 2021 15:15:51 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Borislav Petkov <bp@...en8.de>,
        Jane Malalane <jane.malalane@...rix.com>
CC:     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 01/10/2021 15:19, Borislav Petkov wrote:
> On Fri, Oct 01, 2021 at 02:33:49PM +0100, Jane Malalane wrote:
>> Subject: Re: [PATCH] x86/cpu: Fix migration safety with X86_BUG_NULL_SEL
>> ...
>> Currently, Linux probes for X86_BUG_NULL_SEL unconditionally which
>> makes it unsafe to migrate in a virtualised environment as the
>> properties across the migration pool might differ.
> Sorry but you need to explain "migration safety" in greater detail -
> we're not all virtualizers.

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".

~Andrew