Patch "x86/hyperv: Block root partition functionality in a Confidential VM" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    x86/hyperv: Block root partition functionality in a Confidential VM

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     x86-hyperv-block-root-partition-functionality-in-a-c.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 346bc4bbfbfd24ba59e944c8cc648925b5e538fa
Author: Michael Kelley <mikelley@xxxxxxxxxxxxx>
Date:   Wed Mar 15 08:34:13 2023 -0700

    x86/hyperv: Block root partition functionality in a Confidential VM
    
    [ 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@xxxxxxxxxxxxx>
    Link: https://lore.kernel.org/r/1678894453-95392-1-git-send-email-mikelley@xxxxxxxxxxxxx
    Signed-off-by: Wei Liu <wei.liu@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index ba0efc30fac52..8d3c649a1769b 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -289,12 +289,16 @@ 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");
 	}