linux-kernel - Re: [PATCH] x86/mm: Do not allow non-MAP_FIXED mapping across DEFAULT_MAP

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Date:   Mon, 13 Nov 2017 17:57:03 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...capital.net>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Nicholas Piggin <npiggin@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/mm: Do not allow non-MAP_FIXED mapping across
 DEFAULT_MAP_WINDOW border

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