Re: usbdev_mmap causes type confusion in page_table_check

[David Hildenbrand <david@redhat.com>]

On 08.05.23 23:55, Pasha Tatashin wrote:
> On Mon, May 8, 2023 at 2:52 PM Matthew Wilcox <willy@infradead.org> wrote:
>>
>> On Mon, May 08, 2023 at 02:48:59PM -0700, Pasha Tatashin wrote:
>>> On Mon, May 8, 2023 at 2:36 PM Matthew Wilcox <willy@infradead.org> wrote:
>>>>
>>>> On Mon, May 08, 2023 at 05:27:10PM -0400, Pasha Tatashin wrote:
>>>>>> static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
>>>>>>                                   unsigned long pfn, unsigned long pgcnt,
>>>>>>                                   bool rw)
>>>>>> {
>>>>>>          // ...
>>>>>>          anon = PageAnon(page);
>>>>>>          for (i = 0; i < pgcnt; i++) {
>>>>>>                  // ...
>>>>>>                  if (anon) {
>>>>>>                          BUG_ON(atomic_read(&ptc->file_map_count));
>>>>>>                          BUG_ON(atomic_inc_return(&ptc->anon_map_count) > 1 && rw);
>>>>>>                  } else {
>>>>>>                          BUG_ON(atomic_read(&ptc->anon_map_count));
>>>>>>                          BUG_ON(atomic_inc_return(&ptc->file_map_count) < 0);
>>>>>>                  }
>>>>>>                  // ...
>>>>>>          }
>>>>>>          // ...
>>>>>> }
>>>>>>
>>>>>> This call to PageAnon is invalid for slab pages because slab reuses the bits
>>>>>> in struct page/folio to store its internal states, and the anonymity bit only
>>>>>> exists in struct page/folio. As a result, the counters are incorrectly updated
>>>>>> and checked in page_table_check_set and page_table_check_clear, leading to the
>>>>>> bug being raised.
>>>>>
>>>>> We should change anon boolean to be:
>>>>>
>>>>> anon = !PageSlab(page) && PageAnon(page);
>>>>
>>>> No.  Slab pages are not elegible for mapping into userspace.  That's
>>>
>>> Sure, I can add BUG_ON(PageSlab(page)); to page_table_check_set.
>>>
>>>> all.  There should be a BUG() for that.  And I do mean BUG(), not
>>>> "return error to user".  Something has gone horribly wrong, and it's
>>>> time to crash.
>>>
>>>   It is just too easy to make slab available via remap_pfn_range(), but
>>> I do not think we want to add BUG() into the remap function, otherwise
>>> we will break devices such as /dev/mem.
>>
>> Slab pages can't be mmaped.  Really, no matter what interface you're
>> using.  page->_mapcount is necessarily incremented by mapping to
>> userspace, and slab uses that space for its own purposes (and has
>> for decades).  It's similar for page tables and other allocations that
>> use PageType.
> 
> Mapping random memory in /dev/mem can cause mapping slab pages in to
> userspace, the page->_mapcount is not incremented (and other fields
> are not accessed) in that case, as we are using VM_PFNMAP type VMA,
> which does not access "struct page".

We should be using vm_normal_page() to identify if we should be looking 
at the struct page or not, no?

-- 
Thanks,

David / dhildenb