KVM: nSVM: Save/load CET Shadow Stack state to/from vmcb12/vmcb02

Transfer the three CET Shadow Stack VMCB fields (S_CET, ISST_ADDR, and
SSP) on VMRUN, #VMEXIT, and loading nested state (saving nested state
simply copies the entire save area).  SVM doesn't provide a way to
disallow L1 from enabling Shadow Stacks for L2, i.e. KVM *must* provide
nested support before advertising SHSTK to userspace.

Link: https://lore.kernel.org/r/20250919223258.1604852-37-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/nested.c

@@ -636,6 +636,14 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm *svm, struct vmcb *vmcb12
 		vmcb_mark_dirty(vmcb02, VMCB_DT);
 	}
 
+	if (guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) &&
+	    (unlikely(new_vmcb12 || vmcb_is_dirty(vmcb12, VMCB_CET)))) {
+		vmcb02->save.s_cet  = vmcb12->save.s_cet;
+		vmcb02->save.isst_addr = vmcb12->save.isst_addr;
+		vmcb02->save.ssp = vmcb12->save.ssp;
+		vmcb_mark_dirty(vmcb02, VMCB_CET);
+	}
+
 	kvm_set_rflags(vcpu, vmcb12->save.rflags | X86_EFLAGS_FIXED);
 
 	svm_set_efer(vcpu, svm->nested.save.efer);
@@ -1044,6 +1052,12 @@ void svm_copy_vmrun_state(struct vmcb_save_area *to_save,
 	to_save->rsp = from_save->rsp;
 	to_save->rip = from_save->rip;
 	to_save->cpl = 0;
+
+	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
+		to_save->s_cet  = from_save->s_cet;
+		to_save->isst_addr = from_save->isst_addr;
+		to_save->ssp = from_save->ssp;
+	}
 }
 
 void svm_copy_vmloadsave_state(struct vmcb *to_vmcb, struct vmcb *from_vmcb)
@@ -1111,6 +1125,12 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
 	vmcb12->save.dr6    = svm->vcpu.arch.dr6;
 	vmcb12->save.cpl    = vmcb02->save.cpl;
 
+	if (guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK)) {
+		vmcb12->save.s_cet	= vmcb02->save.s_cet;
+		vmcb12->save.isst_addr	= vmcb02->save.isst_addr;
+		vmcb12->save.ssp	= vmcb02->save.ssp;
+	}
+
 	vmcb12->control.int_state         = vmcb02->control.int_state;
 	vmcb12->control.exit_code         = vmcb02->control.exit_code;
 	vmcb12->control.exit_code_hi      = vmcb02->control.exit_code_hi;