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Date:   Mon, 8 May 2023 22:36:44 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Pasha Tatashin <pasha.tatashin@soleen.com>
Cc:     Ruihan Li <lrh2000@pku.edu.cn>,
        syzbot+fcf1a817ceb50935ce99@syzkaller.appspotmail.com,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, gregkh@linuxfoundation.org,
        linux-usb@vger.kernel.org, syzkaller-bugs@googlegroups.com
Subject: Re: usbdev_mmap causes type confusion in page_table_check
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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
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.