[PATCH v16 21/51] KVM: x86/mmu: WARN on attempt to check permissions for Shadow Stack #PF

Sean Christopherson
2025-09-19T22:33:43Z

Add PFERR_SS_MASK, a.k.a. Shadow Stack access, and WARN if KVM attempts to
check permissions for a Shadow Stack access as KVM hasn't been taught to
understand the magic Writable=0,Dirty=0 combination that is required for
Shadow Stack accesses, and likely will never learn. There are no plans to
support Shadow Stacks with the Shadow MMU, and the emulator rejects all
instructions that affect Shadow Stacks, i.e. it should be impossible for
KVM to observe a #PF due to a shadow stack access.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/include/asm/kvm_host.h | 1 +
arch/x86/kvm/mmu.h | 2 +-
2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 7a7e6356a8dd..554d83ff6135 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -267,6 +267,7 @@ enum x86_intercept_stage;
#define PFERR_RSVD_MASK BIT(3)
#define PFERR_FETCH_MASK BIT(4)
#define PFERR_PK_MASK BIT(5)
+#define PFERR_SS_MASK BIT(6)
#define PFERR_SGX_MASK BIT(15)
#define PFERR_GUEST_RMP_MASK BIT_ULL(31)
#define PFERR_GUEST_FINAL_MASK BIT_ULL(32)
diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index b4b6860ab971..f63074048ec6 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -212,7 +212,7 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,

fault = (mmu->permissions[index] >> pte_access) & 1;

- WARN_ON(pfec & (PFERR_PK_MASK | PFERR_RSVD_MASK));
+ WARN_ON_ONCE(pfec & (PFERR_PK_MASK | PFERR_SS_MASK | PFERR_RSVD_MASK));
if (unlikely(mmu->pkru_mask)) {
u32 pkru_bits, offset;

--
2.51.0.470.ga7dc726c21-goog