Re: usbdev_mmap causes type confusion in page_table_check

[Matthew Wilcox <willy@infradead.org>]

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.