[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

[Paolo Bonzini]

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>
Fixes: c258b62b264fdc469b6d3610a907708068145e3b
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,
 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;
+
 	/*
 	 * Passing "true" to the last argument is okay; it adds a check
 	 * on bit 8 of the SPTEs which KVM doesn't use anyway.
 	 */
 	__reset_rsvds_bits_mask(vcpu, &context->shadow_zero_check,
 				boot_cpu_data.x86_phys_bits,
-				context->shadow_root_level, context->nx,
+				context->shadow_root_level, uses_nx,
 				guest_cpuid_has_gbpages(vcpu), is_pse(vcpu),
 				true);
 }
-- 
1.8.3.1