Re: [PATCH v2 1/2] KVM: VMX: Inject #UD if guest tries to execute SEAMCALL or TDCALL

Xiaoyao Li
2025-10-17T02:53:17Z

On 10/17/2025 2:21 AM, Sean Christopherson wrote:
> Add VMX exit handlers for SEAMCALL and TDCALL to inject a #UD if a non-TD
> guest attempts to execute SEAMCALL or TDCALL. Neither SEAMCALL nor TDCALL
> is gated by any software enablement other than VMXON, and so will generate
> a VM-Exit instead of e.g. a native #UD when executed from the guest kernel.
>
> Note! No unprivileged DoS of the L1 kernel is possible as TDCALL and
> SEAMCALL #GP at CPL > 0, and the CPL check is performed prior to the VMX
> non-root (VM-Exit) check, i.e. userspace can't crash the VM. And for a
> nested guest, KVM forwards unknown exits to L1, i.e. an L2 kernel can
> crash itself, but not L1.
>
> Note #2! The Intel® Trust Domain CPU Architectural Extensions spec's
> pseudocode shows the CPL > 0 check for SEAMCALL coming _after_ the VM-Exit,
> but that appears to be a documentation bug (likely because the CPL > 0
> check was incorrectly bundled with other lower-priority #GP checks).
> Testing on SPR and EMR shows that the CPL > 0 check is performed before
> the VMX non-root check, i.e. SEAMCALL #GPs when executed in usermode.
>
> Note #3! The aforementioned Trust Domain spec uses confusing pseudocode
> that says that SEAMCALL will #UD if executed "inSEAM", but "inSEAM"
> specifically means in SEAM Root Mode, i.e. in the TDX-Module. The long-
> form description explicitly states that SEAMCALL generates an exit when
> executed in "SEAM VMX non-root operation". But that's a moot point as the
> TDX-Module injects #UD if the guest attempts to execute SEAMCALL, as
> documented in the "Unconditionally Blocked Instructions" section of the
> TDX-Module base specification.
>
> Cc: stable@vger.kernel.org
> Cc: Kai Huang <kai.huang@intel.com>
> Cc: Xiaoyao Li <xiaoyao.li@intel.com>
> Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Binbin Wu <binbin.wu@linux.intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>

> ---
> arch/x86/include/uapi/asm/vmx.h | 1 +
> arch/x86/kvm/vmx/nested.c | 8 ++++++++
> arch/x86/kvm/vmx/vmx.c | 8 ++++++++
> 3 files changed, 17 insertions(+)
>
> diff --git a/arch/x86/include/uapi/asm/vmx.h b/arch/x86/include/uapi/asm/vmx.h
> index 9792e329343e..1baa86dfe029 100644
> --- a/arch/x86/include/uapi/asm/vmx.h
> +++ b/arch/x86/include/uapi/asm/vmx.h
> @@ -93,6 +93,7 @@
> #define EXIT_REASON_TPAUSE 68
> #define EXIT_REASON_BUS_LOCK 74
> #define EXIT_REASON_NOTIFY 75
> +#define EXIT_REASON_SEAMCALL 76
> #define EXIT_REASON_TDCALL 77
> #define EXIT_REASON_MSR_READ_IMM 84
> #define EXIT_REASON_MSR_WRITE_IMM 85
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 76271962cb70..bcea087b642f 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -6728,6 +6728,14 @@ static bool nested_vmx_l1_wants_exit(struct kvm_vcpu *vcpu,
> case EXIT_REASON_NOTIFY:
> /* Notify VM exit is not exposed to L1 */
> return false;
> + case EXIT_REASON_SEAMCALL:
> + case EXIT_REASON_TDCALL:
> + /*
> + * SEAMCALL and TDCALL unconditionally VM-Exit, but aren't
> + * virtualized by KVM for L1 hypervisors, i.e. L1 should
> + * never want or expect such an exit.
> + */
> + return false;
> default:
> return true;
> }
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 546272a5d34d..d1b34b7ca4a3 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6033,6 +6033,12 @@ static int handle_vmx_instruction(struct kvm_vcpu *vcpu)
> return 1;
> }
>
> +static int handle_tdx_instruction(struct kvm_vcpu *vcpu)
> +{
> + kvm_queue_exception(vcpu, UD_VECTOR);
> + return 1;
> +}
> +
> #ifndef CONFIG_X86_SGX_KVM
> static int handle_encls(struct kvm_vcpu *vcpu)
> {
> @@ -6158,6 +6164,8 @@ static int (*kvm_vmx_exit_handlers[])(struct kvm_vcpu *vcpu) = {
> [EXIT_REASON_ENCLS] = handle_encls,
> [EXIT_REASON_BUS_LOCK] = handle_bus_lock_vmexit,
> [EXIT_REASON_NOTIFY] = handle_notify,
> + [EXIT_REASON_SEAMCALL] = handle_tdx_instruction,
> + [EXIT_REASON_TDCALL] = handle_tdx_instruction,
> [EXIT_REASON_MSR_READ_IMM] = handle_rdmsr_imm,
> [EXIT_REASON_MSR_WRITE_IMM] = handle_wrmsr_imm,
> };