x86/hyperv: Block root partition functionality in a Confidential VM

Hyper-V should never specify a VM that is a Confidential VM and also
running in the root partition.  Nonetheless, explicitly block such a
combination to guard against a compromised Hyper-V maliciously trying to
exploit root partition functionality in a Confidential VM to expose
Confidential VM secrets. No known bug is being fixed, but the attack
surface for Confidential VMs on Hyper-V is reduced.
Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/1678894453-95392-1-git-send-email-mikelley@microsoft.comSigned-off-by: Wei Liu <wei.liu@kernel.org>

arch/x86/kernel/cpu/mshyperv.c

@@ -358,12 +358,16 @@ static void __init ms_hyperv_init_platform(void)
 	 * To mirror what Windows does we should extract CPU management
 	 * features and use the ReservedIdentityBit to detect if Linux is the
 	 * root partition. But that requires negotiating CPU management
-	 * interface (a process to be finalized).
+	 * interface (a process to be finalized). For now, use the privilege
+	 * flag as the indicator for running as root.
 	 *
-	 * For now, use the privilege flag as the indicator for running as
-	 * root.
+	 * Hyper-V should never specify running as root and as a Confidential
+	 * VM. But to protect against a compromised/malicious Hyper-V trying
+	 * to exploit root behavior to expose Confidential VM memory, ignore
+	 * the root partition setting if also a Confidential VM.
 	 */
-	if (cpuid_ebx(HYPERV_CPUID_FEATURES) & HV_CPU_MANAGEMENT) {
+	if ((ms_hyperv.priv_high & HV_CPU_MANAGEMENT) &&
+	    !(ms_hyperv.priv_high & HV_ISOLATION)) {
 		hv_root_partition = true;
 		pr_info("Hyper-V: running as root partition\n");
 	}