KVM: nSVM: Only copy SVM_MISC_ENABLE_NP from VMCB01's misc_ctl

The 'misc_ctl' field in VMCB02 is taken as-is from VMCB01. However, the
only bit that needs to copied is SVM_MISC_ENABLE_NP, 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 SVM_MISC_ENABLE_NP 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 SVM_MISC_ENABLE_NP is
taken from VMCB01 and not VMCB02.

Suggested-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260303003421.2185681-26-yosry@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index bf1e1ca22d9cabf2bce6587da8ebefc7c24e1f7e..b191c6cab57dbcaeb3ffcd878d09390481418e41 100644 (file)
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -837,8 +837,16 @@ 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.
+        *
+        * SVM_MISC_ENABLE_NP 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_ENABLE_NP;
        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);