Re: [PATCH v2 02/11] mm: Hardened usercopy

Balbir Singh
2016-07-15T12:55:04Z

On Thu, Jul 14, 2016 at 09:53:31PM -0700, Kees Cook wrote:
> On Thu, Jul 14, 2016 at 9:05 PM, Kees Cook <keescook@chromium.org> wrote:
> > On Thu, Jul 14, 2016 at 6:41 PM, Balbir Singh <bsingharora@gmail.com> wrote:
> >> On Thu, Jul 14, 2016 at 09:04:18PM -0400, Rik van Riel wrote:
> >>> On Fri, 2016-07-15 at 09:20 +1000, Balbir Singh wrote:
> >>>
> >>> > > ==
> >>> > > + ((unsigned long)end & (unsigned
> >>> > > long)PAGE_MASK)))
> >>> > > + return NULL;
> >>> > > +
> >>> > > + /* Allow if start and end are inside the same compound
> >>> > > page. */
> >>> > > + endpage = virt_to_head_page(end);
> >>> > > + if (likely(endpage == page))
> >>> > > + return NULL;
> >>> > > +
> >>> > > + /* Allow special areas, device memory, and sometimes
> >>> > > kernel data. */
> >>> > > + if (PageReserved(page) && PageReserved(endpage))
> >>> > > + return NULL;
> >>> >
> >>> > If we came here, it's likely that endpage > page, do we need to check
> >>> > that only the first and last pages are reserved? What about the ones
> >>> > in
> >>> > the middle?
> >>>
> >>> I think this will be so rare, we can get away with just
> >>> checking the beginning and the end.
> >>>
> >>
> >> But do we want to leave a hole where an aware user space
> >> can try a longer copy_* to avoid this check? If it is unlikely
> >> should we just bite the bullet and do the check for the entire
> >> range?
> >
> > I'd be okay with expanding the test -- it should be an extremely rare
> > situation already since the common Reserved areas (kernel data) will
> > have already been explicitly tested.
> >
> > What's the best way to do "next page"? Should it just be:
> >
> > for ( ; page <= endpage ; ptr += PAGE_SIZE, page = virt_to_head_page(ptr) ) {
> > if (!PageReserved(page))
> > return "<spans multiple pages>";
> > }
> >
> > return NULL;
> >
> > ?
>
> Er, I was testing the wrong thing. How about:
>
> /*
> * Reject if range is not Reserved (i.e. special or device memory),
> * since then the object spans several independently allocated pages.
> */
> for (; ptr <= end ; ptr += PAGE_SIZE, page = virt_to_head_page(ptr)) {
> if (!PageReserved(page))
> return "<spans multiple pages>";
> }
>
> return NULL;

That looks reasonable to me

Balbir