[PATCH v4 26/26] KVM: nSVM: Only copy NP_ENABLE from VMCB01's misc_ctl

[Yosry Ahmed <yosry.ahmed@linux.dev>]

The 'misc_ctl' field in VMCB02 is taken as-is from VMCB01. However, the
only bit that needs to copied is NP_ENABLE, as all other known bits in
misc_ctl are related to SEV guests, and KVM doesn't support nested
virtualization for SEV guests.

Only copy NP_ENABLE to harden against future bugs if/when other bits are
set for L1 but should not be set for L2.

Opportunistically add a comment explaining why NP_ENABLE is taken from
VMCB01 and not VMCB02.

Suggested-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
---
 arch/x86/kvm/svm/nested.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index e62fd6524feb..b3a90ff262d5 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -856,8 +856,16 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
 						V_NMI_BLOCKING_MASK);
 	}
 
-	/* Copied from vmcb01.  msrpm_base can be overwritten later.  */
-	vmcb02->control.misc_ctl = vmcb01->control.misc_ctl;
+	/*
+	 * Copied from vmcb01.  msrpm_base can be overwritten later.
+	 *
+	 * NP_ENABLE in vmcb12 is only used for consistency checks.  If L1
+	 * enables NPTs, KVM shadows L1's NPTs and uses those to run L2. If L1
+	 * disables NPT, KVM runs L2 with the same NPTs used to run L1. For the
+	 * latter, L1 runs L2 with shadow page tables that translate L2 GVAs to
+	 * L1 GPAs, so the same NPTs can be used for L1 and L2.
+	 */
+	vmcb02->control.misc_ctl = vmcb01->control.misc_ctl & SVM_MISC_CTL_NP_ENABLE;
 	vmcb02->control.iopm_base_pa = vmcb01->control.iopm_base_pa;
 	vmcb02->control.msrpm_base_pa = vmcb01->control.msrpm_base_pa;
 	vmcb_mark_dirty(vmcb02, VMCB_PERM_MAP);
-- 
2.52.0.457.g6b5491de43-goog