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
From: Paolo Bonzini
Date: Thu Mar 10 2016 - 05:03:01 EST
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@xxxxxxxxxxxxxxx
>> Cc: Xiao Guangrong <guangrong.xiao@xxxxxxxxxx>
> As a redhat guy i am so proud. :)
>> Fixes: c258b62b264fdc469b6d3610a907708068145e3b
> Thanks for you fixing it, Paolo!
> Reviewed-by: Xiao Guangrong <guangrong.xiao@xxxxxxxxxxxxxxx>
>> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> 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,
>> reset_shadow_zero_bits_mask(struct kvm_vcpu *vcpu, struct kvm_mmu
>> + int uses_nx = context->nx || context->base_role.smep_andnot_wp;
> It would be better if it is 'bool'
Ok, will do.