Re: index-out-of-range ubsan warnings

[Paolo Bonzini <pbonzini@redhat.com>]

> 
> +#define INVALID_INDEX        (-1)
> +
>   static int mmu_unsync_walk(struct kvm_mmu_page *sp,
>                              struct kvm_mmu_pages *pvec)
>   {
>           if (!sp->unsync_children)
>                   return 0;
> 
> -        mmu_pages_add(pvec, sp, 0);
> +        /*
> +         * do not count the index in the parent of the sp we're
> +         * walking start from.
> +         */
> +        mmu_pages_add(pvec, sp, INVALID_INDEX);
>           return __mmu_unsync_walk(sp, pvec);
>   }
> 
> @@ -1980,8 +1986,11 @@ static int mmu_pages_next(struct kvm_mmu_pages *pvec,
>                           return n;
>                   }
> 
> -                parents->parent[sp->role.level-2] = sp;
> -                parents->idx[sp->role.level-1] = pvec->page[n].idx;
> +                parents->parent[sp->role.level - 2] = sp;
> +
> +                /* skip setting idex of the sp we start from. */
> +                if (pvec->page[n].idx != INVALID_INDEX)
> +                        parents->idx[sp->role.level - 1] = pvec->page[n].idx;

So far so good.

>           }
> 
>           return n;
> @@ -1999,6 +2008,7 @@ static void mmu_pages_clear_parents(struct mmu_page_path *parents)
>                   if (!sp)
>                           return;
> 
> +                WARN_ON(idx != INVALID_INDEX);

Shouldn't this warn if idx is *equal* to INVALID_INDEX?

>                   clear_unsync_child_bit(sp, idx);
>                   level++;
>           } while (level < PT64_ROOT_LEVEL-1 && !sp->unsync_children);
> @@ -2008,7 +2018,7 @@ static void kvm_mmu_pages_init(struct kvm_mmu_page *parent,
>                                  struct mmu_page_path *parents,
>                                  struct kvm_mmu_pages *pvec)
>   {
> -        parents->parent[parent->role.level-1] = NULL;
> +        parents->parent[parent->role.level - 2] = NULL;

This is meant to stop mmu_pages_clear_parents _after_ it has
processed sp, so the "-1" is correct.  The right fix would be:

        if (parent->role.level < PT64_ROOT_LEVEL-1)
                parents->parent[parent->role.level - 1] = NULL;

and no one has noticed it so far because if the bug hits you just write 
over the beginning of ->idx (which so far is uninitialized) and nothing
bad happens.

I'm thinking of the following (untested) patch instead:

-------- 8< --------------
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: [PATCH] KVM: MMU: Fix ubsan warnings

kvm_mmu_pages_init is doing some really yucky stuff.  It is setting
up a sentinel for mmu_page_clear_parents; however, because of a) the
way levels are numbered starting from 1 and b) the way mmu_page_path
sizes its arrays with PT64_ROOT_LEVEL-1 elements, the access can be
out of bounds.  This is harmless because the code overwrites up to the
first two elements of parents->idx and these are initialized, and
because the sentinel is not needed in this case---mmu_page_clear_parents
exits anyway when it gets to the end of the array.  However ubsan
complains, and everyone else should too.

This fix does three things.  First it makes the mmu_page_path arrays
PT64_ROOT_LEVEL elements in size, so that we can write to them without
checking the level in advance.  Second it disintegrates kvm_mmu_pages_init
between mmu_unsync_walk (which resets struct kvm_mmu_pages) and
mmu_pages_next (which unconditionally places a NULL sentinel at the
end of the current path).  This is okay because the mmu_page_path is
only used in mmu_pages_clear_parents; mmu_pages_clear_parents itself
is called within a for_each_sp iterator, and hence always after a
call to mmu_pages_next.  Third it changes mmu_pages_clear_parents to
just use the sentinel to stop iteration, without checking the bounds
on level.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Reported-by: Mike Krinkin <krinkin.m.u@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 33cc9f3f5b02..f4f5e7ca14dd 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1843,6 +1843,7 @@ static int __mmu_unsync_walk(struct kvm_mmu_page *sp,
 static int mmu_unsync_walk(struct kvm_mmu_page *sp,
 			   struct kvm_mmu_pages *pvec)
 {
+	pvec->nr = 0;
 	if (!sp->unsync_children)
 		return 0;
 
@@ -1956,8 +1957,8 @@ static void kvm_sync_pages(struct kvm_vcpu *vcpu,  gfn_t gfn)
 }
 
 struct mmu_page_path {
-	struct kvm_mmu_page *parent[PT64_ROOT_LEVEL-1];
-	unsigned int idx[PT64_ROOT_LEVEL-1];
+	struct kvm_mmu_page *parent[PT64_ROOT_LEVEL];
+	unsigned int idx[PT64_ROOT_LEVEL];
 };
 
 #define for_each_sp(pvec, sp, parents, i)			\
@@ -1971,19 +1972,21 @@ static int mmu_pages_next(struct kvm_mmu_pages *pvec,
 			  int i)
 {
 	int n;
+	int level = PT_PAGE_TABLE_LEVEL;
 
 	for (n = i+1; n < pvec->nr; n++) {
 		struct kvm_mmu_page *sp = pvec->page[n].sp;
 
-		if (sp->role.level == PT_PAGE_TABLE_LEVEL) {
-			parents->idx[0] = pvec->page[n].idx;
-			return n;
-		}
+		level = sp->role.level;
+		parents->idx[level-1] = pvec->page[n].idx;
 
-		parents->parent[sp->role.level-2] = sp;
-		parents->idx[sp->role.level-1] = pvec->page[n].idx;
+		if (level == PT_PAGE_TABLE_LEVEL)
+			break;
+
+		parents->parent[level-2] = sp;
 	}
 
+	parents->parent[level-1] = NULL;
 	return n;
 }
 
@@ -1994,22 +1996,13 @@ static void mmu_pages_clear_parents(struct mmu_page_path *parents)
 
 	do {
 		unsigned int idx = parents->idx[level];
-
 		sp = parents->parent[level];
 		if (!sp)
 			return;
 
 		clear_unsync_child_bit(sp, idx);
 		level++;
-	} while (level < PT64_ROOT_LEVEL-1 && !sp->unsync_children);
-}
-
-static void kvm_mmu_pages_init(struct kvm_mmu_page *parent,
-			       struct mmu_page_path *parents,
-			       struct kvm_mmu_pages *pvec)
-{
-	parents->parent[parent->role.level-1] = NULL;
-	pvec->nr = 0;
+	} while (!sp->unsync_children);
 }
 
 static void mmu_sync_children(struct kvm_vcpu *vcpu,
@@ -2021,7 +2014,6 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu,
 	struct kvm_mmu_pages pages;
 	LIST_HEAD(invalid_list);
 
-	kvm_mmu_pages_init(parent, &parents, &pages);
 	while (mmu_unsync_walk(parent, &pages)) {
 		bool protected = false;
 
@@ -2037,7 +2029,6 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu,
 		}
 		kvm_mmu_commit_zap_page(vcpu->kvm, &invalid_list);
 		cond_resched_lock(&vcpu->kvm->mmu_lock);
-		kvm_mmu_pages_init(parent, &parents, &pages);
 	}
 }
 
@@ -2269,7 +2260,6 @@ static int mmu_zap_unsync_children(struct kvm *kvm,
 	if (parent->role.level == PT_PAGE_TABLE_LEVEL)
 		return 0;
 
-	kvm_mmu_pages_init(parent, &parents, &pages);
 	while (mmu_unsync_walk(parent, &pages)) {
 		struct kvm_mmu_page *sp;
 
@@ -2278,7 +2268,6 @@ static int mmu_zap_unsync_children(struct kvm *kvm,
 			mmu_pages_clear_parents(&parents);
 			zapped++;
 		}
-		kvm_mmu_pages_init(parent, &parents, &pages);
 	}
 
 	return zapped;


Paolo