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Date: Mon, 13 Nov 2017 19:41:54 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@amacapital.net>,
        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
Message-ID: <20171113164154.fp5fd2seozbmxcbs@node.shutemov.name>
References: <20171107130539.52676-1-kirill.shutemov@linux.intel.com>
 <alpine.DEB.2.20.1711131642370.1851@nanos>
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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.

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.

-- 
 Kirill A. Shutemov