> -----Original Message-----
> From: Jan Beulich <jbeul...@suse.com>
> Sent: 07 April 2020 12:08
> To: xen-devel@lists.xenproject.org
> Cc: Andrew Cooper <andrew.coop...@citrix.com>; Roger Pau Monné
> <roger....@citrix.com>; Wei Liu
> <w...@xen.org>; Paul Durrant <p...@xen.org>
> 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 <jbeul...@suse.com>
Reviewed-by: Paul Durrant <p...@xen.org>
...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]);
> + }
> }
>
> /*
