Re: [RFC PATCH 1/3] KVM: arm64: Fix possible memory leak in kvm stage2

Will Deacon
2020-12-01T14:18:04Z

On Tue, Dec 01, 2020 at 03:21:23PM +0800, wangyanan (Y) wrote:
> On 2020/11/30 21:21, Will Deacon wrote:
> > On Mon, Nov 30, 2020 at 08:18:45PM +0800, Yanan Wang wrote:
> > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > > index 0271b4a3b9fe..696b6aa83faf 100644
> > > --- a/arch/arm64/kvm/hyp/pgtable.c
> > > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > > @@ -186,6 +186,7 @@ static bool kvm_set_valid_leaf_pte(kvm_pte_t *ptep, u64 pa, kvm_pte_t attr,
> > > return old == pte;
> > > smp_store_release(ptep, pte);
> > > + get_page(virt_to_page(ptep));
> > This is also used for the hypervisor stage-1 page-table, so I'd prefer to
> > leave this function as-is.
> I agree at this point.
> > > return true;
> > > }
> > > @@ -476,6 +477,7 @@ static bool stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> > > /* There's an existing valid leaf entry, so perform break-before-make */
> > > kvm_set_invalid_pte(ptep);
> > > kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, level);
> > > + put_page(virt_to_page(ptep));
> > > kvm_set_valid_leaf_pte(ptep, phys, data->attr, level);
> > > out:
> > > data->phys += granule;
> > Isn't this hunk alone sufficient to solve the problem?
> >
> > Will
> > .
>
> Not sufficient enough. When the old ptep is valid and old pte equlas new
> pte, in this case, "True" is also returned by kvm_set_valid_leaf_pte()
>
> and get_page() will still be called.

I had a go at fixing this without introducing refcounting to the hyp stage-1
case, and ended up with the diff below. What do you think?

Will

--->8

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 0271b4a3b9fe..78e2c0dc47ae 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -170,23 +170,16 @@ static void kvm_set_table_pte(kvm_pte_t *ptep, kvm_pte_t *childp)
smp_store_release(ptep, pte);
}

-static bool kvm_set_valid_leaf_pte(kvm_pte_t *ptep, u64 pa, kvm_pte_t attr,
- u32 level)
+static kvm_pte_t kvm_init_valid_leaf_pte(u64 pa, kvm_pte_t attr, u32 level)
{
- kvm_pte_t old = *ptep, pte = kvm_phys_to_pte(pa);
+ kvm_pte_t pte = kvm_phys_to_pte(pa);
u64 type = (level == KVM_PGTABLE_MAX_LEVELS - 1) ? KVM_PTE_TYPE_PAGE :
KVM_PTE_TYPE_BLOCK;

pte |= attr & (KVM_PTE_LEAF_ATTR_LO | KVM_PTE_LEAF_ATTR_HI);
pte |= FIELD_PREP(KVM_PTE_TYPE, type);
pte |= KVM_PTE_VALID;
-
- /* Tolerate KVM recreating the exact same mapping. */
- if (kvm_pte_valid(old))
- return old == pte;
-
- smp_store_release(ptep, pte);
- return true;
+ return pte;
}

static int kvm_pgtable_visitor_cb(struct kvm_pgtable_walk_data *data, u64 addr,
@@ -341,12 +334,17 @@ static int hyp_map_set_prot_attr(enum kvm_pgtable_prot prot,
static bool hyp_map_walker_try_leaf(u64 addr, u64 end, u32 level,
kvm_pte_t *ptep, struct hyp_map_data *data)
{
+ kvm_pte_t new, old = *ptep;
u64 granule = kvm_granule_size(level), phys = data->phys;

if (!kvm_block_mapping_supported(addr, end, phys, level))
return false;

- WARN_ON(!kvm_set_valid_leaf_pte(ptep, phys, data->attr, level));
+ /* Tolerate KVM recreating the exact same mapping. */
+ new = kvm_init_valid_leaf_pte(phys, data->attr, level);
+ if (old != new && !WARN_ON(kvm_pte_valid(old)))
+ smp_store_release(ptep, new);
+
data->phys += granule;
return true;
}
@@ -465,19 +463,24 @@ static bool stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
kvm_pte_t *ptep,
struct stage2_map_data *data)
{
+ kvm_pte_t new, old = *ptep;
u64 granule = kvm_granule_size(level), phys = data->phys;

if (!kvm_block_mapping_supported(addr, end, phys, level))
return false;

- if (kvm_set_valid_leaf_pte(ptep, phys, data->attr, level))
- goto out;
+ new = kvm_init_valid_leaf_pte(phys, data->attr, level);
+ if (kvm_pte_valid(old)) {
+ /*
+ * There's an existing valid leaf entry, so perform
+ * break-before-make.
+ */
+ kvm_set_invalid_pte(ptep);
+ kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, level);
+ put_page(virt_to_page(ptep));
+ }

- /* There's an existing valid leaf entry, so perform break-before-make */
- kvm_set_invalid_pte(ptep);
- kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, level);
- kvm_set_valid_leaf_pte(ptep, phys, data->attr, level);
-out:
+ smp_store_release(ptep, new);
data->phys += granule;
return true;
}