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Date: Mon, 13 Nov 2017 17:57:03 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>,
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        Cyrill Gorcunov <gorcunov@openvz.org>,
        Nicholas Piggin <npiggin@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86/mm: Do not allow non-MAP_FIXED mapping across
 DEFAULT_MAP_WINDOW border
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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.

Thanks,

	tglx