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Date:   Thu, 21 Oct 2021 20:46:30 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc:     Michael Roth <michael.roth@amd.com>,
        Brijesh Singh <brijesh.singh@amd.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
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        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Jim Mattson <jmattson@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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        David Rientjes <rientjes@google.com>,
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        Andi Kleen <ak@linux.intel.com>, tony.luck@intel.com,
        marcorr@google.com, sathyanarayanan.kuppuswamy@linux.intel.com
Subject: Re: [PATCH v6 08/42] x86/sev-es: initialize sev_status/features
 within #VC handler
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On Thu, Oct 21, 2021 at 06:47:50PM +0100, Dr. David Alan Gilbert wrote:
> Hang on, I think it's perfectly fine for it to clear that bit - it just
> gets caught if it *sets* it (i.e. claims to be a chip unaffected by the
> bug).
> 
> i.e. if guestval=0 then (GustVal & whatever) == GuestVal
>   fine
> 
> ?

Bah, ofc. The name of the bit is NullSelectorClearsBase - so when it is
clear, we will note we're affected, as that patch does:

+       /*
+        * CPUID bit above wasn't set. If this kernel is still running
+        * as a HV guest, then the HV has decided not to advertize
+        * that CPUID bit for whatever reason.  For example, one
+        * member of the migration pool might be vulnerable.  Which
+        * means, the bug is present: set the BUG flag and return.
+        */
+       if (cpu_has(c, X86_FEATURE_HYPERVISOR)) {
+               set_cpu_bug(c, X86_BUG_NULL_SEG);
+               return;
+       }

I have managed to flip the meaning in my mind.

Ok, that makes more sense.

Thx.

-- 
Regards/Gruss,
    Boris.

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