Re: [Xen-devel] [PATCH v2 6/7] x86/mm: handle foreign mappings in p2m_entry_modify

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.02.19 at 18:46, <roger.pau@xxxxxxxxxx> wrote:
> @@ -948,6 +951,11 @@ static inline void p2m_entry_modify(struct p2m_domain 
> *p2m, p2m_type_t nt,
>          p2m->ioreq.entry_count++;
>          break;
>  
> +    case p2m_map_foreign:
> +        BUG_ON(!mfn_valid(nfn) ||
> +               !page_get_owner_and_reference(mfn_to_page(nfn)));
> +        break;

Asserting that the passed in MFN is valid is fine. Asserting that a
reference can be got is not, as this sets us up for a DoS in case
of a refcount overflow, or the page having got ballooned out by
its owner. That is, the issue of you folding the two original calls
into one is wider than just the two distinct error codes getting lost
that were previously produced - you can't (currently) report up
any error from this low layer. (And I'm sorry, I should have noticed
this on v1 already.)

Jan



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