KVM: x86/mmu: Don't put invalid SPs back on the list of active pages

Delete a shadow page from the invalidation list instead of throwing it
back on the list of active pages when it's a root shadow page with
active users.  Invalid active root pages will be explicitly freed by
mmu_free_root_page() when the root_count hits zero, i.e. they don't need
to be put on the active list to avoid leakage.

Use sp->role.invalid to detect that a shadow page has already been
zapped, i.e. is not on a list.

WARN if an invalid page is encountered when zapping pages, as it should
now be impossible.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200623193542.7554-2-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 1b2988a..14c1677 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2748,10 +2748,23 @@ static bool __kvm_mmu_prepare_zap_page(struct kvm *kvm,
        if (!sp->root_count) {
                /* Count self */
                (*nr_zapped)++;
-               list_move(&sp->link, invalid_list);
+
+               /*
+                * Already invalid pages (previously active roots) are not on
+                * the active page list.  See list_del() in the "else" case of
+                * !sp->root_count.
+                */
+               if (sp->role.invalid)
+                       list_add(&sp->link, invalid_list);
+               else
+                       list_move(&sp->link, invalid_list);
                kvm_mod_used_mmu_pages(kvm, -1);
        } else {
-               list_move(&sp->link, &kvm->arch.active_mmu_pages);
+               /*
+                * Remove the active root from the active page list, the root
+                * will be explicitly freed when the root_count hits zero.
+                */
+               list_del(&sp->link);
 
                /*
                 * Obsolete pages cannot be used on any vCPUs, see the comment
@@ -5718,12 +5731,11 @@ restart:
                        break;
 
                /*
-                * Skip invalid pages with a non-zero root count, zapping pages
-                * with a non-zero root count will never succeed, i.e. the page
-                * will get thrown back on active_mmu_pages and we'll get stuck
-                * in an infinite loop.
+                * Invalid pages should never land back on the list of active
+                * pages.  Skip the bogus page, otherwise we'll get stuck in an
+                * infinite loop if the page gets put back on the list (again).
                 */
-               if (sp->role.invalid && sp->root_count)
+               if (WARN_ON(sp->role.invalid))
                        continue;
 
                /*
@@ -6001,7 +6013,7 @@ void kvm_mmu_zap_all(struct kvm *kvm)
        spin_lock(&kvm->mmu_lock);
 restart:
        list_for_each_entry_safe(sp, node, &kvm->arch.active_mmu_pages, link) {
-               if (sp->role.invalid && sp->root_count)
+               if (WARN_ON(sp->role.invalid))
                        continue;
                if (__kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list, &ign))
                        goto restart;