[PATCH 5.12 072/700] KVM: x86/mmu: Treat NX as used (not reserved) for all !TDP shadow MMUs

[Greg Kroah-Hartman]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

commit 112022bdb5bc372e00e6e43cb88ee38ea67b97bd upstream.

Mark NX as being used for all non-nested shadow MMUs, as KVM will set the
NX bit for huge SPTEs if the iTLB mutli-hit mitigation is enabled.
Checking the mitigation itself is not sufficient as it can be toggled on
at any time and KVM doesn't reset MMU contexts when that happens.  KVM
could reset the contexts, but that would require purging all SPTEs in all
MMUs, for no real benefit.  And, KVM already forces EFER.NX=1 when TDP is
disabled (for WP=0, SMEP=1, NX=0), so technically NX is never reserved
for shadow MMUs.

Fixes: b8e8c8303ff2 ("kvm: mmu: ITLB_MULTIHIT mitigation")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Message-Id: <20210622175739.3610207-3-seanjc@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/x86/kvm/mmu/mmu.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4155,7 +4155,15 @@ static inline u64 reserved_hpa_bits(void
 void
 reset_shadow_zero_bits_mask(struct kvm_vcpu *vcpu, struct kvm_mmu *context)
 {
-	bool uses_nx = context->nx ||
+	/*
+	 * KVM uses NX when TDP is disabled to handle a variety of scenarios,
+	 * notably for huge SPTEs if iTLB multi-hit mitigation is enabled and
+	 * to generate correct permissions for CR0.WP=0/CR4.SMEP=1/EFER.NX=0.
+	 * The iTLB multi-hit workaround can be toggled at any time, so assume
+	 * NX can be used by any non-nested shadow MMU to avoid having to reset
+	 * MMU contexts.  Note, KVM forces EFER.NX=1 when TDP is disabled.
+	 */
+	bool uses_nx = context->nx || !tdp_enabled ||
 		context->mmu_role.base.smep_andnot_wp;
 	struct rsvd_bits_validate *shadow_zero_check;
 	int i;