linux-kernel - Re: [PATCH 2/2] KVM: MMU: fix reserved bit check for pte.u=0/pte.w=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0

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Message-ID: <56E14647.8040803@redhat.com>
Date:	Thu, 10 Mar 2016 11:02:47 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:	stable@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: MMU: fix reserved bit check for
 pte.u=0/pte.w=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0



On 10/03/2016 09:36, Xiao Guangrong wrote:
> 
> 
> On 03/08/2016 07:44 PM, Paolo Bonzini wrote:
>> KVM handles supervisor writes of a pte.u=0/pte.w=0/CR0.WP=0 page by
>> setting U=0 and W=1 in the shadow PTE.  This will cause a user write
>> to fault and a supervisor write to succeed (which is correct because
>> CR0.WP=0).  A user read instead will flip U=0 to 1 and W=1 back to 0.
>> This enables user reads; it also disables supervisor writes, the next
>> of which will then flip the bits again.
>>
>> When SMEP is in effect, however, pte.u=0 will enable kernel execution
>> of this page.  To avoid this, KVM also sets pte.nx=1.  The reserved bit
>> catches this because it only looks at the guest's EFER.NX bit.  Teach it
>> that smep_andnot_wp will also use the NX bit of SPTEs.
>>
>> Cc: stable@...r.kernel.org
>> Cc: Xiao Guangrong <guangrong.xiao@...hat.com>
> 
> As a redhat guy i am so proud. :)
> 
>> Fixes: c258b62b264fdc469b6d3610a907708068145e3b
> 
> Thanks for you fixing it, Paolo!
> 
> Reviewed-by: Xiao Guangrong <guangrong.xiao@...ux.intel.com>
> 
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>> ---
>>   arch/x86/kvm/mmu.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index 95a955de5964..0cd4ee01de94 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -3721,13 +3721,15 @@ static void reset_rsvds_bits_mask_ept(struct
>> kvm_vcpu *vcpu,
>>   void
>>   reset_shadow_zero_bits_mask(struct kvm_vcpu *vcpu, struct kvm_mmu
>> *context)
>>   {
>> +    int uses_nx = context->nx || context->base_role.smep_andnot_wp;
> 
> It would be better if it is 'bool'

Ok, will do.

Paolo