Re: [PATCH] mm: Check writable zero page in page table check

[Pasha Tatashin]

Sorry: s/Rock/Rick

:-)

On Mon, Sep 5, 2022 at 8:38 PM Pasha Tatashin <pasha.tatashin@xxxxxxxxxx> wrote:
>
> Hi Rock,
>
> Good idea to add a check for write access to zero page. Can you please
> also update Documentation/mm/page_table_check.rst ?
>
> Thank you,
> Pasha
>
> On Fri, Sep 2, 2022 at 7:31 PM Rick Edgecombe
> <rick.p.edgecombe@xxxxxxxxx> wrote:
> >
> > The zero page should remain all zero, so that it can be mapped as
> > read-only for read faults of memory that should be zeroed. If it is ever
> > mapped writable to userspace, it could become non-zero and so other apps
> > would unexpectedly get non-zero data. So the zero page should never be
> > mapped writable to userspace. Check for this condition in
> > page_table_check_set().
> >
> > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> >
> > ---
> >
> > Hi,
> >
> > CONFIG_PAGE_TABLE_CHECK is pretty explicit about what it checks (and
> > doesn't mention the zero page), but this condition seems to fit with the
> > general category of "pages mapped wrongly to userspace". I added it
> > locally to help me debug something. Maybe it's more widely useful.
> >
> >  mm/page_table_check.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/mm/page_table_check.c b/mm/page_table_check.c
> > index e2062748791a..665ece0d55d4 100644
> > --- a/mm/page_table_check.c
> > +++ b/mm/page_table_check.c
> > @@ -102,6 +102,8 @@ static void page_table_check_set(struct mm_struct *mm, unsigned long addr,
> >         if (!pfn_valid(pfn))
> >                 return;
> >
> > +       BUG_ON(is_zero_pfn(pfn) && rw);
> > +
> >         page = pfn_to_page(pfn);
> >         page_ext = lookup_page_ext(page);
> >         anon = PageAnon(page);
> >
> > base-commit: b90cb1053190353cc30f0fef0ef1f378ccc063c5
> > --
> > 2.17.1
> >