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

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Message-ID: <690f2f8d-8533-4b54-adbe-206c0e06da51@intel.com>
Date: Fri, 17 Oct 2025 10:53:12 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Kai Huang <kai.huang@...el.com>, Rick Edgecombe
 <rick.p.edgecombe@...el.com>, Dan Williams <dan.j.williams@...el.com>,
 Binbin Wu <binbin.wu@...ux.intel.com>
Subject: Re: [PATCH v2 1/2] KVM: VMX: Inject #UD if guest tries to execute
 SEAMCALL or TDCALL

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@...r.kernel.org
> Cc: Kai Huang <kai.huang@...el.com>
> Cc: Xiaoyao Li <xiaoyao.li@...el.com>
> Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Binbin Wu <binbin.wu@...ux.intel.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.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,
>   };