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

[wangyanan (Y)]

Hi Will,

On 2020/11/30 21:21, Will Deacon wrote:
> On Mon, Nov 30, 2020 at 08:18:45PM +0800, Yanan Wang wrote:
>> When installing a new leaf pte onto an invalid ptep, we need to get_page(ptep).
>> When just updating a valid leaf ptep, we shouldn't get_page(ptep).
>> Incorrect page_count of translation tables might lead to memory leak,
>> when unmapping a stage 2 memory range.
> Did you find this by inspection, or did you hit this in practice? I'd be
> interested to see the backtrace for mapping over an existing mapping.

Actually this is found by inspection.

In the current code, get_page() will uniformly called at "out_get_page" 
in function stage2_map_walk_leaf(),

no matter the old ptep is valid or not.

When using stage2_unmap_walker() API to unmap a memory range, some 
page-table pages might not be

freed if page_count of the pages is not right.

>> Signed-off-by: Yanan Wang <wangyanan55 at huawei.com>
>> ---
>>   arch/arm64/kvm/hyp/pgtable.c | 7 ++++---
>>   1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> 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.


Yanan