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

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Message-ID: <20251014231042.1399849-1-seanjc@google.com>
Date: Tue, 14 Oct 2025 16:10:42 -0700
From: Sean Christopherson <seanjc@...gle.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>, Xiaoyao Li <xiaoyao.li@...el.com>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>
Subject: [PATCH] KVM: VMX: Inject #UD if guest tries to execute SEAMCALL or TDCALL

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.

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@...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>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 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.
+		 */
+		return true;
 	default:
 		return true;
 	}
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index 097304bf1e1d..7326c68f9909 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -2127,6 +2127,9 @@ int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
 		return tdx_emulate_mmio(vcpu);
 	case EXIT_REASON_EPT_VIOLATION:
 		return tdx_handle_ept_violation(vcpu);
+	case EXIT_REASON_SEAMCALL:
+		kvm_queue_exception(vcpu, UD_VECTOR);
+		return 1;
 	case EXIT_REASON_OTHER_SMI:
 		/*
 		 * Unlike VMX, SMI in SEAM non-root mode (i.e. when
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,
 };

base-commit: 6b36119b94d0b2bb8cea9d512017efafd461d6ac
-- 
2.51.0.788.g6d19910ace-goog