RE: [PATCH] x86/PoD: correct ordering of checks in p2m_pod_zero_check()

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 07 April 2020 12:08
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; Roger Pau Monné 
> <roger.pau@xxxxxxxxxx>; Wei Liu
> <wl@xxxxxxx>; Paul Durrant <paul@xxxxxxx>
> Subject: [PATCH] x86/PoD: correct ordering of checks in p2m_pod_zero_check()
> 
> Commit 0537d246f8db ("mm: add 'is_special_page' inline function...")
> moved the is_special_page() checks first in its respective changes to
> PoD code. While this is fine for p2m_pod_zero_check_superpage(), the
> validity of the MFN is inferred in both cases from the p2m_is_ram()
> check, which therefore also needs to come first in this 2nd instance.
> 
> Take the opportunity and address latent UB here as well - transform
> the MFN into struct page_info * only after having established that
> this is a valid page.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Paul Durrant <paul@xxxxxxx>

...with a suggestion below

> ---
> I will admit that this was build tested only. I did observe the crash
> late yesterday while in the office, but got around to analyzing it only
> today, where I'm again restricted in what I can reasonably test.
> 
> --- a/xen/arch/x86/mm/p2m-pod.c
> +++ b/xen/arch/x86/mm/p2m-pod.c
> @@ -877,23 +877,25 @@ p2m_pod_zero_check(struct p2m_domain *p2
>      for ( i = 0; i < count; i++ )
>      {
>          p2m_access_t a;
> -        struct page_info *pg;
> 
>          mfns[i] = p2m->get_entry(p2m, gfns[i], types + i, &a,
>                                   0, NULL, NULL);
> -        pg = mfn_to_page(mfns[i]);
> 
>          /*
>           * If this is ram, and not a pagetable or a special page, and
>           * probably not mapped elsewhere, map it; otherwise, skip.
>           */
> -        if ( !is_special_page(pg) && p2m_is_ram(types[i]) &&
> -             (pg->count_info & PGC_allocated) &&
> -             !(pg->count_info & PGC_page_table) &&
> -             ((pg->count_info & PGC_count_mask) <= max_ref) )
> -            map[i] = map_domain_page(mfns[i]);
> -        else
> -            map[i] = NULL;
> +        map[i] = NULL;
> +        if ( p2m_is_ram(types[i]) )
> +        {
> +            const struct page_info *pg = mfn_to_page(mfns[i]);

Perhaps have local scope stack variable for count_info too?

> +
> +            if ( !is_special_page(pg) &&
> +                 (pg->count_info & PGC_allocated) &&
> +                 !(pg->count_info & PGC_page_table) &&
> +                 ((pg->count_info & PGC_count_mask) <= max_ref) )
> +                map[i] = map_domain_page(mfns[i]);
> +        }
>      }
> 
>      /*