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

[Thomas Gleixner]

On Mon, 13 Nov 2017, Kirill A. Shutemov wrote:
> 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.

Makes some sense, but it would be nice to have this documented exactly in
arch_get_unmapped_area_topdown(), i.e. the function where you are adding
the border check to. Otherwise 3 month from now somebody will look at that
and ask exactly the same question again.

Thanks,

	tglx