[PATCH 4.4 11/50] KVM: MMU: fix reserved bit check for ept=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0

From: Greg Kroah-Hartman
Date: Mon Mar 14 2016 - 14:02:24 EST

4.4-stable review patch. If anyone has any objections, please let me know.


From: Paolo Bonzini <pbonzini@xxxxxxxxxx>

commit 5f0b819995e172f48fdcd91335a2126ba7d9deae upstream.

KVM has special logic to handle pages with pte.u=1 and pte.w=0 when
CR0.WP=1. These pages' SPTEs flip continuously between two states:
U=1/W=0 (user and supervisor reads allowed, supervisor writes not allowed)
and U=0/W=1 (supervisor reads and writes allowed, user writes not allowed).

When SMEP is in effect, however, U=0 will enable kernel execution of
this page. To avoid this, KVM also sets NX=1 in the shadow PTE together
with U=0, making the two states U=1/W=0/NX=gpte.NX and U=0/W=1/NX=1.
When guest EFER has the NX bit cleared, the reserved bit check thinks
that the latter state is invalid; teach it that the smep_andnot_wp case
will also use the NX bit of SPTEs.

Reviewed-by: Xiao Guangrong <guangrong.xiao@xxxxxxxxxxxxxx>
Fixes: c258b62b264fdc469b6d3610a907708068145e3b
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

arch/x86/kvm/mmu.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -3754,13 +3754,15 @@ static void reset_rsvds_bits_mask_ept(st
reset_shadow_zero_bits_mask(struct kvm_vcpu *vcpu, struct kvm_mmu *context)
+ bool 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,
- context->shadow_root_level, context->nx,
+ context->shadow_root_level, uses_nx,
guest_cpuid_has_gbpages(vcpu), is_pse(vcpu),