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

Kees Cook
2016-07-15T04:53:37Z

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;

--
Kees Cook
Chrome OS & Brillo Security