KVM: X86/MMU: Fix shadowing 5-level NPT for 4-level NPT L1 guest

When shadowing 5-level NPT for 4-level NPT L1 guest, the root_sp is
allocated with role.level = 5 and the guest pagetable's root gfn.

And root_sp->spt[0] is also allocated with the same gfn and the same
role except role.level = 4. Luckily that they are different shadow
pages, but only root_sp->spt[0] is the real translation of the guest
pagetable.

Here comes a problem:

If the guest switches from gCR4_LA57=0 to gCR4_LA57=1 (or vice verse)
and uses the same gfn as the root page for nested NPT before and after
switching gCR4_LA57. The host (hCR4_LA57=1) might use the same root_sp
for the guest even the guest switches gCR4_LA57. The guest will see
unexpected page mapped and L2 may exploit the bug and hurt L1. It is
lucky that the problem can't hurt L0.

And three special cases need to be handled:

The root_sp should be like role.direct=1 sometimes: its contents are
not backed by gptes, root_sp->gfns is meaningless. (For a normal high
level sp in shadow paging, sp->gfns is often unused and kept zero, but
it could be relevant and meaningful if sp->gfns is used because they
are backed by concrete gptes.)

For such root_sp in the case, root_sp is just a portal to contribute
root_sp->spt[0], and root_sp->gfns should not be used and
root_sp->spt[0] should not be dropped if gpte[0] of the guest root
pagetable is changed.

Such root_sp should not be accounted too.

So add role.passthrough to distinguish the shadow pages in the hash
when gCR4_LA57 is toggled and fix above special cases by using it in
kvm_mmu_page_{get|set}_gfn() and sp_has_gptes().

Signed-off-by: Lai Jiangshan <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

Documentation/virt/kvm/x86/mmu.rst

···
     Is 1 if the MMU instance cannot use A/D bits.  EPT did not have A/D
     bits before Haswell; shadow EPT page tables also cannot use A/D bits
     if the L1 hypervisor does not enable them.
+  role.passthrough:
+    The page is not backed by a guest page table, but its first entry
+    points to one.  This is set if NPT uses 5-level page tables (host
+    CR4.LA57=1) and is shadowing L1's 4-level NPT (L1 CR4.LA57=1).
   gfn:
     Either the guest page table containing the translations shadowed by this
     page, or the base page frame for linear translations.  See role.direct.

arch/x86/include/asm/kvm_host.h

···
  * minimize the size of kvm_memory_slot.arch.gfn_track, i.e. allows allocating
  * 2 bytes per gfn instead of 4 bytes per gfn.
  *
- * Indirect upper-level shadow pages are tracked for write-protection via
+ * Upper-level shadow pages having gptes are tracked for write-protection via
  * gfn_track.  As above, gfn_track is a 16 bit counter, so KVM must not create
  * more than 2^16-1 upper-level shadow pages at a single gfn, otherwise
  * gfn_track will overflow and explosions will ensure.
···
 		unsigned smap_andnot_wp:1;
 		unsigned ad_disabled:1;
 		unsigned guest_mode:1;
-		unsigned :6;
+		unsigned passthrough:1;
+		unsigned :5;
 
 		/*
 		 * This is left at the top of the word so that

arch/x86/kvm/mmu/mmu.c

···
 
 static gfn_t kvm_mmu_page_get_gfn(struct kvm_mmu_page *sp, int index)
 {
+	if (sp->role.passthrough)
+		return sp->gfn;
+
 	if (!sp->role.direct)
 		return sp->gfns[index];
 
···
 
 static void kvm_mmu_page_set_gfn(struct kvm_mmu_page *sp, int index, gfn_t gfn)
 {
+	if (sp->role.passthrough) {
+		WARN_ON_ONCE(gfn != sp->gfn);
+		return;
+	}
+
 	if (!sp->role.direct) {
 		sp->gfns[index] = gfn;
 		return;
···
 	if (sp->role.direct)
 		return false;
 
+	if (sp->role.passthrough)
+		return false;
+
 	return true;
 }
 
···
 		quadrant &= (1 << ((PT32_PT_BITS - PT64_PT_BITS) * level)) - 1;
 		role.quadrant = quadrant;
 	}
+	if (level <= vcpu->arch.mmu->cpu_role.base.level)
+		role.passthrough = 0;
 
 	sp_list = &vcpu->kvm->arch.mmu_page_hash[kvm_page_table_hashfn(gfn)];
 	for_each_valid_sp(vcpu->kvm, sp, sp_list) {
···
 
 	root_role = cpu_role.base;
 	root_role.level = kvm_mmu_get_tdp_level(vcpu);
+	if (root_role.level == PT64_ROOT_5LEVEL &&
+	    cpu_role.base.level == PT64_ROOT_4LEVEL)
+		root_role.passthrough = 1;
 
 	shadow_mmu_init_context(vcpu, context, cpu_role, root_role);
 	kvm_mmu_new_pgd(vcpu, nested_cr3);

arch/x86/kvm/mmu/paging_tmpl.h

···
 		.level = 0xf,
 		.access = 0x7,
 		.quadrant = 0x3,
+		.passthrough = 0x1,
 	};
 
 	/*