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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.04.2020 14:39, Paul Durrant wrote:
>> -----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>

Thanks.

> ...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?

I'd view this as useful only if ...

>> +
>> +            if ( !is_special_page(pg) &&

... this could then also be made make use of it.

Jan