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

[Will Deacon]

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.

> 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.

>  	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