Re: [PATCH] KVM: SVM: Fix CPL export via SS.DPL

From: Jan Kiszka <jan.kiszka@web.de>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Gleb Natapov" <gleb@kernel.org>,
	"Borislav Petkov" <bp@alien8.de>,
	lkml <linux-kernel@vger.kernel.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Steven Rostedt" <rostedt@goodmis.org>, x86-ml <x86@kernel.org>,
	kvm@vger.kernel.org, "Jörg Rödel" <joro@8bytes.org>
Subject: Re: [PATCH] KVM: SVM: Fix CPL export via SS.DPL
Date: Mon, 30 Jun 2014 17:03:57 +0200	[thread overview]
Message-ID: <53B17C5D.5050407@web.de> (raw)
In-Reply-To: <53B17BDD.5030301@redhat.com>

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On 2014-06-30 17:01, Paolo Bonzini wrote:
> Il 29/06/2014 17:12, Jan Kiszka ha scritto:
>> From: Jan Kiszka <jan.kiszka@siemens.com>
>>
>> We import the CPL via SS.DPL since ae9fedc793. However, we fail to
>> export it this way so far. This caused spurious guest crashes, e.g. of
>> Linux when accessing the vmport from guest user space which triggered
>> register saving/restoring to/from host user space.
>>
>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>> ---
>>
>> Just in time for the next match :D
>>
>>  arch/x86/kvm/svm.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>> index ec8366c..b5e994a 100644
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -1462,6 +1462,7 @@ static void svm_get_segment(struct kvm_vcpu *vcpu,
>>           */
>>          if (var->unusable)
>>              var->db = 0;
>> +        var->dpl = to_svm(vcpu)->vmcb->save.cpl;
>>          break;
>>      }
>>  }
>>
> 
> Thanks.  In theory this is not necessary, the SS.DPL should be the same
> as the CPL according to the manuals (the manual say that the SS.DPL
> "should match" the CPL, and that's the only reason why I included the
> import in ae9fedc793).  But apparently this is not the case.

15.5.1:

"When examining segment attributes after a #VMEXIT:
[...]
• Retrieve the CPL from the CPL field in the VMCB, not from any segment
DPL."

Jan


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