Re: [PATCH v2 4/4] mm: page_table_check: Ensure user pages are not slab pages

[Ruihan Li]

On Mon, May 15, 2023 at 12:28:54PM -0400, Pasha Tatashin wrote:
> 
> On Mon, May 15, 2023 at 9:10 AM Ruihan Li <lrh2000@xxxxxxxxxx> wrote:
> >
> > The current uses of PageAnon in page table check functions can lead to
> > type confusion bugs between struct page and slab [1], if slab pages are
> > accidentally mapped into the user space. This is because slab reuses the
> > bits in struct page to store its internal states, which renders PageAnon
> > ineffective on slab pages.
> >
> > Since slab pages are not expected to be mapped into the user space, this
> > patch adds BUG_ON(PageSlab(page)) checks to make sure that slab pages
> > are not inadvertently mapped. Otherwise, there must be some bugs in the
> > kernel.
> >
> > Reported-by: syzbot+fcf1a817ceb50935ce99@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://lore.kernel.org/lkml/000000000000258e5e05fae79fc1@xxxxxxxxxx/ [1]
> > Fixes: df4e817b7108 ("mm: page table check")
> > Cc: <stable@xxxxxxxxxxxxxxx> # 5.17
> > Signed-off-by: Ruihan Li <lrh2000@xxxxxxxxxx>
> 
> Acked-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> 
> I would also update order in mm/memory.c
> static int validate_page_before_insert(struct page *page)
> {
> if (PageAnon(page) || PageSlab(page) || page_has_type(page))
> 
> It is not strictly a bug there, as it works by accident, but
> PageSlab() should go before PageAnon(), because without checking if
> this is PageSlab() we should not be testing for PageAnon().

Right. Perhaps it would be better to send another patch for this
separately.

> 
> Thanks you,
> Pasha

Thanks,
Ruihan Li