[PATCH v6 03/21] x86/virt/tdx: Disable TDX if X2APIC is not enabled

[Kai Huang <kai.huang@intel.com>]

The MMIO/xAPIC interface has some problems, most notably the APIC LEAK
[1].  This bug allows an attacker to use the APIC MMIO interface to
extract data from the SGX enclave.

TDX is not immune from this either.  Early check X2APIC and disable TDX
if X2APIC is not enabled, and make INTEL_TDX_HOST depend on X86_X2APIC.

More info:

https://lore.kernel.org/lkml/d6ffb489-7024-ff74-bd2f-d1e06573bb82@intel.com/
https://lore.kernel.org/lkml/ba80b303-31bf-d44a-b05d-5c0f83038798@intel.com/

[1]: https://aepicleak.com/aepicleak.pdf

Signed-off-by: Kai Huang <kai.huang@intel.com>
---
 arch/x86/Kconfig            |  1 +
 arch/x86/virt/vmx/tdx/tdx.c | 11 +++++++++++
 2 files changed, 12 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index b9bd5d994ba7..f6f5e4f7a760 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1960,6 +1960,7 @@ config INTEL_TDX_HOST
 	depends on CPU_SUP_INTEL
 	depends on X86_64
 	depends on KVM_INTEL
+	depends on X86_X2APIC
 	help
 	  Intel Trust Domain Extensions (TDX) protects guest VMs from malicious
 	  host and certain physical attacks.  This option enables necessary TDX
diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index 982d9c453b6b..8d943bdc8335 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -12,6 +12,7 @@
 #include <linux/printk.h>
 #include <asm/msr-index.h>
 #include <asm/msr.h>
+#include <asm/apic.h>
 #include <asm/tdx.h>
 #include "tdx.h"
 
@@ -81,6 +82,16 @@ static int __init tdx_init(void)
 		goto no_tdx;
 	}
 
+	/*
+	 * TDX requires X2APIC being enabled to prevent potential data
+	 * leak via APIC MMIO registers.  Just disable TDX if not using
+	 * X2APIC.
+	 */
+	if (!x2apic_enabled()) {
+		pr_info("Disable TDX as X2APIC is not enabled.\n");
+		goto no_tdx;
+	}
+
 	return 0;
 no_tdx:
 	clear_tdx();
-- 
2.37.3