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

Quentin Perret
2022-09-06T15:25:08Z

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;
> }