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

[Xiaoyao Li]

On 10/15/2025 7:10 AM, Sean Christopherson wrote:
> Add VMX exit handlers for SEAMCALL and TDCALL, and a SEAMCALL handler for
> TDX, to inject a #UD if a non-TD guest attempts to execute SEAMCALL or
> TDCALL, or if a TD guest attempst to execute SEAMCALL.  

> 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.

It's true only on the hardware with SEAM support.

On older hardware without SEAM support, SEAMCALL/TDCALL gets native #UD.

> Note!  No unprivilege 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 pseudocde
> 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".
>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Kai Huang <kai.huang@xxxxxxxxx>
> Cc: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> Cc: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   arch/x86/include/uapi/asm/vmx.h | 1 +
>   arch/x86/kvm/vmx/nested.c       | 8 ++++++++
>   arch/x86/kvm/vmx/tdx.c          | 3 +++
>   arch/x86/kvm/vmx/vmx.c          | 8 ++++++++
>   4 files changed, 20 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..f64a1eb241b6 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.
> +		 */

The i.e. part is confusing? It is exactly forwarding the EXITs to L1, 
while it says L1 should never want or expect such an exit.

> +		return true;
>   	default:
>   		return true;
>   	}