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

[Pasha Tatashin]

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().

Thanks you,
Pasha