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.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index f36dc2f..f119736 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -358,12 +358,16 @@
* 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");
}
