linux-kernel - [PATCH 4/9] KVM: x86/mmu: Add sanity checks that KVM doesn't create EPT #VE SPTEs

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Date: Fri, 17 May 2024 17:04:25 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 4/9] 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@...gle.com>
---
 arch/x86/kvm/mmu/mmu.c      | 3 +++
 arch/x86/kvm/mmu/spte.h     | 9 +++++++++
 arch/x86/kvm/mmu/tdp_iter.h | 2 ++
 3 files changed, 14 insertions(+)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 5095fb46713e..d2af077d8b34 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/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);
 }
 
diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index 5dd5405fa07a..52fa004a1fbc 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/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
diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
index fae559559a80..2880fd392e0c 100644
--- a/arch/x86/kvm/mmu/tdp_iter.h
+++ b/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);
 }
 
-- 
2.45.0.215.g3402c0e53f-goog