Re: [PATCH 02/14] KVM: arm64: Tear down unlinked stage-2 subtree after break-before-make

[Quentin Perret <qperret@google.com>]

Hi Oliver,

On Tuesday 30 Aug 2022 at 19:41:20 (+0000), Oliver Upton wrote:
>  static int stage2_map_walk_table_pre(u64 addr, u64 end, u32 level,
>  				     kvm_pte_t *ptep,
>  				     struct stage2_map_data *data)
>  {
> -	if (data->anchor)
> -		return 0;
> +	struct kvm_pgtable_mm_ops *mm_ops = data->mm_ops;
> +	kvm_pte_t *childp = kvm_pte_follow(*ptep, mm_ops);
> +	struct kvm_pgtable *pgt = data->mmu->pgt;
> +	int ret;
>  
>  	if (!stage2_leaf_mapping_allowed(addr, end, level, data))
>  		return 0;
>  
> -	data->childp = kvm_pte_follow(*ptep, data->mm_ops);
>  	kvm_clear_pte(ptep);
>  
>  	/*
> @@ -782,8 +786,13 @@ static int stage2_map_walk_table_pre(u64 addr, u64 end, u32 level,
>  	 * individually.
>  	 */
>  	kvm_call_hyp(__kvm_tlb_flush_vmid, data->mmu);
> -	data->anchor = ptep;
> -	return 0;
> +
> +	ret = stage2_map_walk_leaf(addr, end, level, ptep, data);
> +
> +	mm_ops->put_page(ptep);
> +	mm_ops->free_removed_table(childp, level + 1, pgt);

By the look of it, __kvm_pgtable_visit() has saved the table PTE on the
stack prior to calling the TABLE_PRE callback, and it then uses the PTE
from its stack and does kvm_pte_follow() to find the childp, and walks
from there. Would that be a UAF now?

> +	return ret;
>  }