KVM: x86: MMU: Consolidate BUG_ON checks for reverse-mapped sptes

At some call sites of rmap_get_first() and rmap_get_next(), BUG_ON is
placed right after the call to detect unrelated sptes which must not be
found in the reverse-mapping list.

Move this check in rmap_get_first/next() so that all call sites, not
just the users of the for_each_rmap_spte() macro, will be checked the
same way.

One thing to keep in mind is that kvm_mmu_unlink_parents() also uses
rmap_get_first() to handle parent sptes.  The change will not break it
because parent sptes are present, at least until drop_parent_pte()
actually unlinks them, and not mmio-sptes.

Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 19 insertions, 11 deletions

diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt
index 3a4d681c3e98..daf9c0f742d2 100644
--- a/Documentation/virtual/kvm/mmu.txt
+++ b/Documentation/virtual/kvm/mmu.txt
@@ -203,10 +203,10 @@ Shadow pages contain the following information:
     page cannot be destroyed.  See role.invalid.
   parent_ptes:
     The reverse mapping for the pte/ptes pointing at this page's spt. If
-    parent_ptes bit 0 is zero, only one spte points at this pages and
+    parent_ptes bit 0 is zero, only one spte points at this page and
     parent_ptes points at this single spte, otherwise, there exists multiple
     sptes pointing at this page and (parent_ptes & ~0x1) points at a data
-    structure with a list of parent_ptes.
+    structure with a list of parent sptes.
   unsync:
     If true, then the translations in this page may not match the guest's
     translation.  This is equivalent to the state of the tlb when a pte is
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 3104748eeb48..5b249d4f4da1 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1098,17 +1098,23 @@ struct rmap_iterator {
 static u64 *rmap_get_first(struct kvm_rmap_head *rmap_head,
                            struct rmap_iterator *iter)
 {
+        u64 *sptep;
+
         if (!rmap_head->val)
                 return NULL;
 
         if (!(rmap_head->val & 1)) {
                 iter->desc = NULL;
-                return (u64 *)rmap_head->val;
+                sptep = (u64 *)rmap_head->val;
+                goto out;
         }
 
         iter->desc = (struct pte_list_desc *)(rmap_head->val & ~1ul);
         iter->pos = 0;
-        return iter->desc->sptes[iter->pos];
+        sptep = iter->desc->sptes[iter->pos];
+out:
+        BUG_ON(!is_shadow_present_pte(*sptep));
+        return sptep;
 }
 
 /*
@@ -1118,14 +1124,14 @@ static u64 *rmap_get_first(struct kvm_rmap_head *rmap_head,
  */
 static u64 *rmap_get_next(struct rmap_iterator *iter)
 {
+        u64 *sptep;
+
         if (iter->desc) {
                 if (iter->pos < PTE_LIST_EXT - 1) {
-                        u64 *sptep;
-
                         ++iter->pos;
                         sptep = iter->desc->sptes[iter->pos];
                         if (sptep)
-                                return sptep;
+                                goto out;
                 }
 
                 iter->desc = iter->desc->more;
@@ -1133,17 +1139,20 @@ static u64 *rmap_get_next(struct rmap_iterator *iter)
                 if (iter->desc) {
                         iter->pos = 0;
                         /* desc->sptes[0] cannot be NULL */
-                        return iter->desc->sptes[iter->pos];
+                        sptep = iter->desc->sptes[iter->pos];
+                        goto out;
                 }
         }
 
         return NULL;
+out:
+        BUG_ON(!is_shadow_present_pte(*sptep));
+        return sptep;
 }
 
 #define for_each_rmap_spte(_rmap_head_, _iter_, _spte_)                 \
         for (_spte_ = rmap_get_first(_rmap_head_, _iter_);              \
-             _spte_ && ({BUG_ON(!is_shadow_present_pte(*_spte_)); 1;}); \
-             _spte_ = rmap_get_next(_iter_))
+             _spte_; _spte_ = rmap_get_next(_iter_))
 
 static void drop_spte(struct kvm *kvm, u64 *sptep)
 {
@@ -1358,7 +1367,6 @@ static bool kvm_zap_rmapp(struct kvm *kvm, struct kvm_rmap_head *rmap_head)
         bool flush = false;
 
         while ((sptep = rmap_get_first(rmap_head, &iter))) {
-                BUG_ON(!(*sptep & PT_PRESENT_MASK));
                 rmap_printk("%s: spte %p %llx.\n", __func__, sptep, *sptep);
 
                 drop_spte(kvm, sptep);