Re: index-out-of-range ubsan warnings

Paolo Bonzini
2016-02-23T13:21:12Z

>
> +#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