[PATCH 5/5] KVM: x86: MMU: Consolidate WARN_ON/BUG_ON checks for reverse-mapped sptes

[Takuya Yoshikawa]

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 should 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.  In addition, change the BUG_ON to WARN_ON since killing the
whole host is the last thing that KVM should try.

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@xxxxxxxxxxxxx>
---
 Documentation/virtual/kvm/mmu.txt |  4 ++--
 arch/x86/kvm/mmu.c                | 31 ++++++++++++++++++++++---------
 2 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt
index 3a4d681..daf9c0f 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 c5e2363..353d752 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1099,17 +1099,28 @@ struct rmap_iterator {
  */
 static u64 *rmap_get_first(unsigned long rmap, struct rmap_iterator *iter)
 {
+	u64 *sptep;
+
 	if (!rmap)
 		return NULL;
 
 	if (!(rmap & 1)) {
 		iter->desc = NULL;
-		return (u64 *)rmap;
+		sptep = (u64 *)rmap;
+		goto out;
 	}
 
 	iter->desc = (struct pte_list_desc *)(rmap & ~1ul);
 	iter->pos = 0;
-	return iter->desc->sptes[iter->pos];
+	sptep = iter->desc->sptes[iter->pos];
+out:
+	/*
+	 * Parent sptes found in sp->parent_ptes lists are also checked here
+	 * since kvm_mmu_unlink_parents() uses this function.  If the condition
+	 * needs to be changed for them, make another wrapper function.
+	 */
+	WARN_ON(!is_shadow_present_pte(*sptep));
+	return sptep;
 }
 
 /*
@@ -1119,14 +1130,14 @@ static u64 *rmap_get_first(unsigned long rmap, struct rmap_iterator *iter)
  */
 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;
@@ -1134,17 +1145,20 @@ 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:
+	WARN_ON(!is_shadow_present_pte(*sptep));
+	return sptep;
 }
 
 #define for_each_rmap_spte(_rmap_, _iter_, _spte_)			    \
 	   for (_spte_ = rmap_get_first(*_rmap_, _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 +1372,6 @@ static bool kvm_zap_rmapp(struct kvm *kvm, unsigned long *rmapp)
 	bool flush = false;
 
 	while ((sptep = rmap_get_first(*rmapp, &iter))) {
-		BUG_ON(!(*sptep & PT_PRESENT_MASK));
 		rmap_printk("%s: spte %p %llx.\n", __func__, sptep, *sptep);
 
 		drop_spte(kvm, sptep);
-- 
2.1.0

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