Re: [Xen-devel] [PATCH v7 1/3] AMD/IOMMU: allocate one device table per PCI segment

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 26/09/2019 15:28, Jan Beulich wrote:
> @@ -1068,8 +1067,29 @@ static void * __init allocate_ppr_log(st
>                                  IOMMU_PPR_LOG_DEFAULT_ENTRIES, "PPR Log");
>  }
>  
> +/*
> + * Within ivrs_mappings[] we allocate an extra array element to store
> + * - segment number,
> + * - device table.
> + */
> +#define IVRS_MAPPINGS_SEG(m) (m)[ivrs_bdf_entries].dte_requestor_id
> +#define IVRS_MAPPINGS_DEVTAB(m) (m)[ivrs_bdf_entries].intremap_table
> +
> +static void __init free_ivrs_mapping(void *ptr)
> +{
> +    const struct ivrs_mappings *ivrs_mappings = ptr;

How absolutely certain are we that ptr will never be NULL?

It might be better to rename this to radix_tree_free_ivrs_mappings() to
make it clear who calls it, and also provide a hint as to why the
parameter is void.

> +
> +    if ( IVRS_MAPPINGS_DEVTAB(ivrs_mappings) )
> +        deallocate_buffer(IVRS_MAPPINGS_DEVTAB(ivrs_mappings),
> +                          dt_alloc_size());
> +
> +    xfree(ptr);
> +}
> +
>  static int __init amd_iommu_init_one(struct amd_iommu *iommu, bool intr)
>  {
> +    const struct ivrs_mappings *ivrs_mappings;
> +
>      if ( allocate_cmd_buffer(iommu) == NULL )
>          goto error_out;
>  
> @@ -1082,13 +1102,15 @@ static int __init amd_iommu_init_one(str
>      if ( intr && !set_iommu_interrupt_handler(iommu) )
>          goto error_out;
>  
> -    /* To make sure that device_table.buffer has been successfully allocated */
> -    if ( device_table.buffer == NULL )
> +    /* Make sure that the device table has been successfully allocated. */
> +    ivrs_mappings = get_ivrs_mappings(iommu->seg);
> +    if ( !IVRS_MAPPINGS_DEVTAB(ivrs_mappings) )

This is still going to crash with a NULL pointer deference in the case
described by the comment.  (Then again, it may not crash, and hit
userspace at the 64M mark.)

You absolutely need to check ivrs_mappings being non NULL before using
IVRS_MAPPINGS_DEVTAB(), or perhaps roll the check into the macro.

~Andrew

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