Re: [Xen-devel] [PATCH v9 3/4] mm: make pages allocated with MEMF_no_refcount safe to assign

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.02.2020 11:12, Durrant, Paul wrote:
>> From: Julien Grall <julien@xxxxxxx>
>> Sent: 06 February 2020 10:04
>>
>> On 03/02/2020 10:56, Paul Durrant wrote:
>>> @@ -2332,11 +2350,23 @@ struct page_info *alloc_domheap_pages(
>>>                                     memflags, d)) == NULL)) )
>>>            return NULL;
>>>
>>> -    if ( d && !(memflags & MEMF_no_owner) &&
>>> -         assign_pages(d, pg, order, memflags) )
>>> +    if ( d && !(memflags & MEMF_no_owner) )
>>>       {
>>> -        free_heap_pages(pg, order, memflags & MEMF_no_scrub);
>>> -        return NULL;
>>> +        if ( memflags & MEMF_no_refcount )
>>> +        {
>>> +            unsigned long i;
>>> +
>>> +            for ( i = 0; i < (1ul << order); i++ )
>>> +            {
>>> +                ASSERT(!pg[i].count_info);
>>> +                pg[i].count_info = PGC_extra;
>>
>> ... this is pursuing the wrongness of the code above and not safe
>> against offlining.
>>
>> We could argue this is an already existing bug, however I am a bit
>> unease to add more abuse in the code. Jan, what do you think?
>>
> 
> I'd consider a straightforward patch-clash. If this patch goes in
> after yours then it needs to be modified accordingly, or vice versa.

While generally I advocate for not widening existing issues, I agree
with Paul here. His patch should not be penalized by us _later_
having found an issue (which is quite a bit wider).

Jan

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