Re: [PATCH v2 for-4.19 1/3] x86/EPT: correct special page checking in epte_get_entry_emt()

["Oleksii K." <oleksii.kurochko@xxxxxxxxx>]

On Wed, 2024-06-12 at 15:16 +0200, Jan Beulich wrote:
> mfn_valid() granularity is (currently) 256Mb. Therefore the start of
> a
> 1Gb page passing the test doesn't necessarily mean all parts of such
> a
> range would also pass. Yet using the result of mfn_to_page() on an
> MFN
> which doesn't pass mfn_valid() checking is liable to result in a
> crash
> (the invocation of mfn_to_page() alone is presumably "just" UB in
> such a
> case).
> 
> Fixes: ca24b2ffdbd9 ("x86/hvm: set 'ipat' in EPT for special pages")
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

~ Oleksii
> ---
> Of course we could leverage mfn_valid() granularity here to do an
> increment by more than 1 if mfn_valid() returned false. Yet doing so
> likely would want a suitable helper to be introduced first, rather
> than
> open-coding such logic here.
> ---
> v2: New.
> 
> --- a/xen/arch/x86/mm/p2m-ept.c
> +++ b/xen/arch/x86/mm/p2m-ept.c
> @@ -519,8 +519,12 @@ int epte_get_entry_emt(struct domain *d,
>      }
>  
>      for ( special_pgs = i = 0; i < (1ul << order); i++ )
> -        if ( is_special_page(mfn_to_page(mfn_add(mfn, i))) )
> +    {
> +        mfn_t cur = mfn_add(mfn, i);
> +
> +        if ( mfn_valid(cur) && is_special_page(mfn_to_page(cur)) )
>              special_pgs++;
> +    }
>  
>      if ( special_pgs )
>      {
>