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

["wangyanan (Y)" <wangyanan55@huawei.com>]

On 2020/12/1 22:16, Will Deacon wrote:

> 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?

Hi Will,

Functionally this diff looks fine to me. A small comment inline, please 
see below.

I had made an alternative fix (after sending v1) and it looks much more 
concise.

If you're ok with it, I can send it as v2 (together with patch#2 and #3) 
after some tests.


Thanks,

Yanan


diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 0271b4a3b9fe..b232bdd142a6 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -470,6 +470,9 @@ static bool stage2_map_walker_try_leaf(u64 addr, u64 
end, u32 level,
         if (!kvm_block_mapping_supported(addr, end, phys, level))
                 return false;

+       if (kvm_pte_valid(*ptep))
+               put_page(virt_to_page(ptep));
+
         if (kvm_set_valid_leaf_pte(ptep, phys, data->attr, level))
                 goto out;

>
> --->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));

When old PTE is valid and equals to the new one, we will also perform 
break-before-make and the new PTE installation. But they're actually not 
necessary in this case.

> +	}
>   
> -	/* 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;
>   }
> .