Re: [PATCH v2 11/17] selftests/x86/ldt_gdt: Prepare for access bit forced

[Peter Zijlstra]

On Thu, Dec 14, 2017 at 02:14:00PM -0800, Linus Torvalds wrote:
> On Thu, Dec 14, 2017 at 2:02 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > _Should_ being the operative word, because I cannot currently see it
> > DTRT. But maybe I'm missing the obvious -- I tend to do that at times.
> 
> At least the old get_user_pages_fast() code used to check the USER bit:
> 
>         unsigned long need_pte_bits = _PAGE_PRESENT|_PAGE_USER;
> 
>         if (write)
>                 need_pte_bits |= _PAGE_RW;
> 
> but that may have been lost when we converted over to the generic code.

The generic gup_pte_range() has pte_access_permitted() (which has the
above test) in the right place.

> It shouldn't actually _matter_, since we'd need to change access_ok()
> anyway (and gup had better check that!)

get_user_pages_fast() (both of them) do indeed test access_ok(), but the
regular get_user_pages() does not, I suspect because it can operate on a
foreign mm.

And its the regular old get_user_pages() that's all sorts of broken wrt
!PAGE_USER too.