Re: usbdev_mmap causes type confusion in page_table_check

[Pasha Tatashin]

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.

Pasha