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Subject: Re: [PATCH 1/1] KVM: SVM: do not zero out segment attributes if
 segment is unusable or not present
To: Roman Pen <roman.penyaev@profitbricks.com>
Cc: Mikhail Sennikovskii <mikhail.sennikovskii@profitbricks.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20170601085503.12852-1-roman.penyaev@profitbricks.com>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <16c1d0ed-2c05-d5e1-4540-b862aed660ab@redhat.com>
Date: Thu, 1 Jun 2017 11:22:00 +0200
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On 01/06/2017 10:55, Roman Pen wrote:
> This is a fix for the problem [1], where VMCB.CPL was set to 0 and interrupt
> was taken on userspace stack.  The root cause lies in the specific AMD CPU
> behaviour which manifests itself as unusable segment attributes on SYSRET.
> The corresponding work around for the kernel is the following:
> 
> 61f01dd941ba ("x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue")
> 
> In other turn virtualization side treated unusable segment incorrectly and
> restored CPL from SS attributes, which were zeroed out few lines above.
> 
> In current patch it is assured only that P bit is cleared in VMCB.save state
> and segment attributes are not zeroed out if segment is not presented or is
> unusable, therefore CPL can be safely restored from DPL field.
> 
> This is only one part of the fix, since QEMU side should be fixed accordingly
> not to zero out attributes on its side.  Corresponding patch will follow.
> 
> [1] Message id: CAJrWOzD6Xq==b-zYCDdFLgSRMPM-NkNuTSDFEtX=7MreT45i7Q@mail.gmail.com
> 
> Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
> Signed-off-by: Mikhail Sennikovskii <mikhail.sennikovskii@profitbricks.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: kvm@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  arch/x86/kvm/svm.c | 24 +++++++++++-------------
>  1 file changed, 11 insertions(+), 13 deletions(-)

Queued, thanks!

Paolo