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 f36dc2f796c5c0eca284a9d58c8139d2a108472f..f1197366a97dc44278d6fa829df7c15eec16bd65 100644 (file)
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/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");
        }