Re: usbdev_mmap causes type confusion in page_table_check

[Pasha Tatashin]

On Mon, May 8, 2023 at 3:46 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 08.05.23 23:55, Pasha Tatashin wrote:
> > On Mon, May 8, 2023 at 2:52 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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?

For normal Kernel-MM operations, vm_normal_page() should be used to
get "struct page" based on vma+addr+pte combination, but
page_table_check does not use vma for its operation in order to
strengthen the verification of no invalid page sharing. But, even
vm_normal_page() can cause access to the "struct page" for VM_PFNMAP
if pfn_valid(pfn) is true. So, vm_normal_page() can return a struct
page for a user mapped slab page.

Pasha

>
> --
> Thanks,
>
> David / dhildenb
>