KVM: x86/mmu: Add sanity checks that KVM doesn't create EPT #VE SPTEs

Assert that KVM doesn't set a SPTE to a value that could trigger an EPT
Violation #VE on a non-MMIO SPTE, e.g. to help detect bugs even without
KVM_INTEL_PROVE_VE enabled, and to help debug actual #VE failures.

Note, this will run afoul of TDX support, which needs to reflect emulated
MMIO accesses into the guest as #VEs (which was the whole point of adding
EPT Violation #VE support in KVM).  The obvious fix for that is to exempt
MMIO SPTEs, but that's annoyingly difficult now that is_mmio_spte() relies
on a per-VM value.  However, resolving that conundrum is a future problem,
whereas getting KVM_INTEL_PROVE_VE healthy is a current problem.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20240518000430.1118488-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -336,16 +336,19 @@ static int is_cpuid_PSE36(void)
 #ifdef CONFIG_X86_64
 static void __set_spte(u64 *sptep, u64 spte)
 {
+	KVM_MMU_WARN_ON(is_ept_ve_possible(spte));
 	WRITE_ONCE(*sptep, spte);
 }
 
 static void __update_clear_spte_fast(u64 *sptep, u64 spte)
 {
+	KVM_MMU_WARN_ON(is_ept_ve_possible(spte));
 	WRITE_ONCE(*sptep, spte);
 }
 
 static u64 __update_clear_spte_slow(u64 *sptep, u64 spte)
 {
+	KVM_MMU_WARN_ON(is_ept_ve_possible(spte));
 	return xchg(sptep, spte);
 }
 

arch/x86/kvm/mmu/spte.h

@@ -3,6 +3,8 @@
 #ifndef KVM_X86_MMU_SPTE_H
 #define KVM_X86_MMU_SPTE_H
 
+#include <asm/vmx.h>
+
 #include "mmu.h"
 #include "mmu_internal.h"
 
@@ -276,6 +278,13 @@ static inline bool is_shadow_present_pte(u64 pte)
 	return !!(pte & SPTE_MMU_PRESENT_MASK);
 }
 
+static inline bool is_ept_ve_possible(u64 spte)
+{
+	return (shadow_present_mask & VMX_EPT_SUPPRESS_VE_BIT) &&
+	       !(spte & VMX_EPT_SUPPRESS_VE_BIT) &&
+	       (spte & VMX_EPT_RWX_MASK) != VMX_EPT_MISCONFIG_WX_VALUE;
+}
+
 /*
  * Returns true if A/D bits are supported in hardware and are enabled by KVM.
  * When enabled, KVM uses A/D bits for all non-nested MMUs.  Because L1 can

arch/x86/kvm/mmu/tdp_iter.h

@@ -21,11 +21,13 @@ static inline u64 kvm_tdp_mmu_read_spte(tdp_ptep_t sptep)
 
 static inline u64 kvm_tdp_mmu_write_spte_atomic(tdp_ptep_t sptep, u64 new_spte)
 {
+	KVM_MMU_WARN_ON(is_ept_ve_possible(new_spte));
 	return xchg(rcu_dereference(sptep), new_spte);
 }
 
 static inline void __kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 new_spte)
 {
+	KVM_MMU_WARN_ON(is_ept_ve_possible(new_spte));
 	WRITE_ONCE(*rcu_dereference(sptep), new_spte);
 }