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 <[email protected]>
---
arch/x86/kernel/cpu/mshyperv.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index ff348eb..ac630ec 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -356,12 +356,16 @@ 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");
}
--
1.8.3.1
On Wed, Mar 15, 2023 at 08:34:13AM -0700, Michael Kelley wrote:
> 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 <[email protected]>
Applied to hyperv-fixes. Thanks.
> ---
> arch/x86/kernel/cpu/mshyperv.c | 12 ++++++++----
> 1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index ff348eb..ac630ec 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -356,12 +356,16 @@ 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");
> }
> --
> 1.8.3.1
>