From 296599346c671f31854d674db2891ff2e1654b6a Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Fri, 19 Sep 2025 15:32:28 -0700 Subject: [PATCH] KVM: x86/mmu: WARN on attempt to check permissions for Shadow Stack #PF 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=1 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. Reviewed-by: Binbin Wu Reviewed-by: Xiaoyao Li Link: https://lore.kernel.org/r/20250919223258.1604852-22-seanjc@google.com Signed-off-by: Sean Christopherson --- 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 5865c9b77b6d2..e8d74e949f911 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 b4b6860ab971c..f63074048ec61 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; -- 2.47.3