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 <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20250919223258.1604852-22-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 5865c9b..e8d74e9 100644 (file)
--- 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 b4b6860..f630740 100644 (file)
--- 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;