From: Michael Kelley mikelley@microsoft.com
[ Upstream commit f8acb24aaf89fc46cd953229462ea8abe31b395f ]
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@microso... Signed-off-by: Wei Liu wei.liu@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- 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 46668e2554210..1ce228dc267ae 100644 --- a/arch/x86/kernel/cpu/mshyperv.c +++ b/arch/x86/kernel/cpu/mshyperv.c @@ -291,12 +291,16 @@ 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"); }