Re: [PATCH v3] KVM: x86: Restrict saved GPA writes to hardware write faults

[Sean Christopherson <seanjc@google.com>]

On Tue, Sep 15, 2026, Kyle Zeng wrote:
> On Tue, Sep 15, 2026 at 10:41:03PM +0000, sashiko-bot@kernel.org wrote:
> > > @@ -6454,7 +6456,18 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> > >  
> > >  		/* With shadow page tables, cr2 contains a GVA or nGPA. */
> > >  		if (vcpu->arch.mmu->root_role.direct) {
> > > -			ctxt->gpa_available = true;
> > > +			ctxt->gpa_access = ACC_READ_MASK;
> > > +			/*
> > > +			 * Always allow writes for SEV guests, as the guest's
> > > +			 * page tables are encrypted, i.e. KVM can't walk the
> > > +			 * guest's page tables and so must always use the GPA
> > > +			 * from the initial fault.  Restricting use of the GPA
> > > +			 * to the access type that faulted would prevent KVM
> > > +			 * from emulating RMW operations for SEV guests.
> > > +			 */
> > > +			if ((emulation_type & EMULTYPE_PF_WRITE) ||
> > > +			    (cpu_feature_enabled(X86_FEATURE_SVM) && is_sev_guest(vcpu)))
> > 
> > [Severity: High]
> > Could this lead to an out-of-bounds read and type confusion?
 
...

> Is there a configuration that gets past the initialization checks and
> demonstrates an incorrect access? I'd like to establish that before
> treating this as a functional issue.

Eh, doesn't matter, my suggestion was terrible.  I forgot that is_sev_guest()
needs to check embedded data; I was thinking of TDX and SNP VMs, which have
dedicated VM types.

I think the right way to handle this is to track if a VM has protected page
tables.  That'd also help communicate/document why KVM has this weird behavior.

Untested...

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 30ffaa65f589..90c950c12900 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1166,6 +1166,7 @@ struct kvm_arch {
 	u8 mmu_valid_gen;
 	u8 vm_type;
 	bool has_private_mem;
+	bool has_protected_page_tables;
 	bool has_protected_state;
 	bool has_protected_eoi;
 	bool has_protected_pmu;
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index b56c39c53c98..7f544658b1cc 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2955,6 +2955,7 @@ void sev_vm_init(struct kvm *kvm)
 		kvm->arch.has_protected_state = true;
 		fallthrough;
 	case KVM_X86_SEV_VM:
+		kvm->arch.has_protected_page_tables = true;
 		kvm->arch.pre_fault_allowed = !kvm->arch.has_private_mem;
 		to_kvm_sev_info(kvm)->need_init = true;
 		break;
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 014545c1839b..35022f308e06 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -616,6 +616,7 @@ int tdx_vm_init(struct kvm *kvm)
 {
 	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
 
+	kvm->arch.has_protected_page_tables = true;
 	kvm->arch.has_protected_state = true;
 	/*
 	 * TDX Module doesn't allow the hypervisor to modify the EOI-bitmap,
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f36b952624c8..7e5e2e587086 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -33,7 +33,6 @@
 #include "lapic.h"
 #include "xen.h"
 #include "smm.h"
-#include "svm/svm.h"
 
 #include <linux/clocksource.h>
 #include <linux/timekeeping.h>
@@ -6412,15 +6411,14 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 		if (vcpu->arch.mmu->root_role.direct) {
 			ctxt->gpa_access = ACC_READ_MASK;
 			/*
-			 * Always allow writes for SEV guests, as the guest's
-			 * page tables are encrypted, i.e. KVM can't walk the
-			 * guest's page tables and so must always use the GPA
-			 * from the initial fault.  Restricting use of the GPA
-			 * to the access type that faulted would prevent KVM
-			 * from emulating RMW operations for SEV guests.
+			 * Always allow writes for guests with protected page
+			 * tables, as KVM can't walk the guest's page tables,
+			 * i.e. KVM can't get the RMW protections for a given
+			 * GVA to see if the write side of a RMW operation
+			 * should be allowed.
 			 */
 			if ((emulation_type & EMULTYPE_PF_WRITE) ||
-			    (cpu_feature_enabled(X86_FEATURE_SVM) && is_sev_guest(vcpu)))
+			    vcpu->kvm->arch.has_protected_page_tables)
 				ctxt->gpa_access |= ACC_WRITE_MASK;
 			ctxt->gpa_val = cr2_or_gpa;
 		}