linux-kernel - Re: FSGSBASE causing panic on 5.9-rc1

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Message-ID: <CALCETrXoquUOZEtYUHA8ZgbihbUHK=ShaHndy5Tk4WUFJrjqqw@mail.gmail.com>
Date:   Thu, 20 Aug 2020 10:51:10 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Joerg Roedel <joro@...tes.org>,
        "Christopherson, Sean J" <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sasha Levin <sashal@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: FSGSBASE causing panic on 5.9-rc1

On Thu, Aug 20, 2020 at 6:43 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 19/08/20 23:25, Andy Lutomirski wrote:
> >         wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gsbase);
> >         load_gs_index(svm->host.gs);
> >
> > Surely that should do load_gs_index() *before* wrmsrl().  But that's
> > not the problem at hand.
>
> The wrmsrl is writing the inactive GS base so the ordering between
> load_gs_index and wrmsrl(MSR_KERNEL_GS_BASE) should be irrelevant?

load_gs_index() sets the index between a pair of swapgs's -- it writes
the inactive base, too.

--Andy