Re: [PATCH] x86/mm: Do not allow non-MAP_FIXED mapping across DEFAULT_MAP_WINDOW border

[Kirill A. Shutemov]

On Mon, Nov 13, 2017 at 05:57:03PM +0100, Thomas Gleixner wrote:
> On Mon, 13 Nov 2017, Kirill A. Shutemov wrote:
> 
> > On Mon, Nov 13, 2017 at 04:43:26PM +0100, Thomas Gleixner wrote:
> > > On Tue, 7 Nov 2017, Kirill A. Shutemov wrote:
> > > 
> > > > In case of 5-level paging, we don't put any mapping above 47-bit, unless
> > > > userspace explicitly asked for it.
> > > > 
> > > > Userspace can ask for allocation from full address space by specifying
> > > > hint address above 47-bit.
> > > > 
> > > > Nicholas noticed that current implementation violates this interface:
> > > > we can get vma partly in high addresses if we ask for a mapping at very
> > > > end of 47-bit address space.
> > > > 
> > > > Let's make sure that, when consider hint address for non-MAP_FIXED
> > > > mapping, start and end of resulting vma are on the same side of 47-bit
> > > > border.
> > > 
> > > What happens for mappings with MAP_FIXED which cross the border?
> > 
> > It will succeed with 5-level paging.
> 
> And why is this allowed?
> 
> > It should be safe as with 4-level paging such request would fail and it's
> > reasonable to expect that userspace is not relying on the failure to
> > function properly.
> 
> Huch?
> 
> The first rule when looking at user space is that is broken or
> hostile. Reasonable and user space are mutually exclusive.

That's basically the same assumption we made to implement current
interface of allocation memory above 47-bits.

The premise is that nobody in right mind would try mmap(addr, MAP_FIXED)
where addr >= (1UL << 47) as it will always fail. So we can allow this to
succeed on 5-level paging machine as a way to allocate from larger address
space.

By the same logic we can allow allocation for cases where addr is below
(1UL << 47), but addr+size is above the limit.

-- 
 Kirill A. Shutemov