hyperv-fixes for 6.3-rc6

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Merge tag 'hyperv-fixes-signed-20230402' of git://git.kernel.org/pub/scm/linux/kernel/git/hyperv/linux

Pull hyperv fixes from Wei Liu:

 - Fix a bug in channel allocation for VMbus (Mohammed Gamal)

 - Do not allow root partition functionality in CVM (Michael Kelley)

* tag 'hyperv-fixes-signed-20230402' of git://git.kernel.org/pub/scm/linux/kernel/git/hyperv/linux:
  x86/hyperv: Block root partition functionality in a Confidential VM
  Drivers: vmbus: Check for channel allocation before looking up relids

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
+	 * Hyper-V should never specify running as root and as a Confidential
-	 * root.
+	 * 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");
 	}

drivers/hv/connection.c

@@ -409,6 +409,10 @@ void vmbus_disconnect(void)
  */
 struct vmbus_channel *relid2channel(u32 relid)
 {
+	if (vmbus_connection.channels == NULL) {
+		pr_warn_once("relid2channel: relid=%d: No channels mapped!\n", relid);
+		return NULL;
+	}
 	if (WARN_ON(relid >= MAX_CHANNEL_RELIDS))
 		return NULL;
 	return READ_ONCE(vmbus_connection.channels[relid]);