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 > >
