Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by VM instructions

Sean Christopherson
2021-01-12T19:42:31Z

On Tue, Jan 12, 2021, Wei Huang wrote:
> +/* Emulate SVM VM execution instructions */
> +static int svm_emulate_vm_instr(struct kvm_vcpu *vcpu, u8 modrm)
> +{
> + struct vcpu_svm *svm = to_svm(vcpu);
> +
> + switch (modrm) {
> + case 0xd8: /* VMRUN */
> + return vmrun_interception(svm);
> + case 0xda: /* VMLOAD */
> + return vmload_interception(svm);
> + case 0xdb: /* VMSAVE */
> + return vmsave_interception(svm);
> + default:
> + /* inject a #GP for all other cases */
> + kvm_queue_exception_e(vcpu, GP_VECTOR, 0);
> + return 1;
> + }
> +}
v> +
> static int gp_interception(struct vcpu_svm *svm)
> {
> struct kvm_vcpu *vcpu = &svm->vcpu;
> u32 error_code = svm->vmcb->control.exit_info_1;
> -
> - WARN_ON_ONCE(!enable_vmware_backdoor);
> + int rc;
>
> /*
> - * VMware backdoor emulation on #GP interception only handles IN{S},
> - * OUT{S}, and RDPMC, none of which generate a non-zero error code.
> + * Only VMware backdoor and SVM VME errata are handled. Neither of
> + * them has non-zero error codes.
> */
> if (error_code) {
> kvm_queue_exception_e(vcpu, GP_VECTOR, error_code);
> return 1;
> }
> - return kvm_emulate_instruction(vcpu, EMULTYPE_VMWARE_GP);
> +
> + rc = kvm_emulate_instruction(vcpu, EMULTYPE_PARAVIRT_GP);
> + if (rc > 1)
> + rc = svm_emulate_vm_instr(vcpu, rc);
> + return rc;
> }

...

> +static int is_vm_instr_opcode(struct x86_emulate_ctxt *ctxt)
> +{
> + unsigned long rax;
> +
> + if (ctxt->b != 0x1)
> + return 0;
> +
> + switch (ctxt->modrm) {
> + case 0xd8: /* VMRUN */
> + case 0xda: /* VMLOAD */
> + case 0xdb: /* VMSAVE */
> + rax = kvm_register_read(emul_to_vcpu(ctxt), VCPU_REGS_RAX);
> + if (!kvm_is_host_reserved_region(rax))
> + return 0;
> + break;
> + default:
> + return 0;
> + }
> +
> + return ctxt->modrm;
> +}
> +
> static bool is_vmware_backdoor_opcode(struct x86_emulate_ctxt *ctxt)
> {
> switch (ctxt->opcode_len) {
> @@ -7305,6 +7327,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> struct x86_emulate_ctxt *ctxt = vcpu->arch.emulate_ctxt;
> bool writeback = true;
> bool write_fault_to_spt;
> + int vminstr;
>
> if (unlikely(!kvm_x86_ops.can_emulate_instruction(vcpu, insn, insn_len)))
> return 1;
> @@ -7367,10 +7390,14 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> }
> }
>
> - if ((emulation_type & EMULTYPE_VMWARE_GP) &&
> - !is_vmware_backdoor_opcode(ctxt)) {
> - kvm_queue_exception_e(vcpu, GP_VECTOR, 0);
> - return 1;
> + if (emulation_type & EMULTYPE_PARAVIRT_GP) {
> + vminstr = is_vm_instr_opcode(ctxt);
> + if (!vminstr && !is_vmware_backdoor_opcode(ctxt)) {
> + kvm_queue_exception_e(vcpu, GP_VECTOR, 0);
> + return 1;
> + }
> + if (vminstr)
> + return vminstr;

I'm pretty sure this doesn't correctly handle a VM-instr in L2 that hits a bad
L0 GPA and that L1 wants to intercept. The intercept bitmap isn't checked until
x86_emulate_insn(), and the vm*_interception() helpers expect nested VM-Exits to
be handled further up the stack.

> }
>
> /*
> --
> 2.27.0
>