Re: usbdev_mmap causes type confusion in page_table_check

[Pasha Tatashin <pasha.tatashin@soleen.com>]

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".

Pasha