Re: [for-4.15][PATCH v3 1/3] xen/iommu: x86: Clear the root page-table before freeing the page-tables

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.02.2021 15:24, Julien Grall wrote:
> --- a/xen/drivers/passthrough/amd/pci_amd_iommu.c
> +++ b/xen/drivers/passthrough/amd/pci_amd_iommu.c
> @@ -381,9 +381,18 @@ static int amd_iommu_assign_device(struct domain *d, u8 
> devfn,
>      return reassign_device(pdev->domain, d, devfn, pdev);
>  }
>  
> +static void iommu_clear_root_pgtable(struct domain *d)

Nit: amd_iommu_ as a prefix would be okay here considering other
(static) functions also use it. Since it is a static function,
no prefix at all would also do (my personal preference). But
iommu_ as a prefix isn't helpful and results in needless re-use
of VT-d's name.

> --- a/xen/drivers/passthrough/x86/iommu.c
> +++ b/xen/drivers/passthrough/x86/iommu.c
> @@ -267,6 +267,15 @@ int iommu_free_pgtables(struct domain *d)
>      struct page_info *pg;
>      unsigned int done = 0;
>  
> +    if ( !is_iommu_enabled(d) )
> +        return 0;
> +
> +    /*
> +     * Pages will be moved to the free list below. So we want to
> +     * clear the root page-table to avoid any potential use after-free.
> +     */
> +    hd->platform_ops->clear_root_pgtable(d);

Taking amd_iommu_alloc_root() as example, is this really correct
prior to what is now patch 2? What guarantees a new root table
won't get allocated subsequently?

Jan