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: Xiao Guangrong
Date: Thu Mar 10 2016 - 03:37:35 EST

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 *context)
+ int uses_nx = context->nx || context->base_role.smep_andnot_wp;

It would be better if it is 'bool'