From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754600AbdKNMGp (ORCPT ); Tue, 14 Nov 2017 07:06:45 -0500 Received: from mail-wm0-f67.google.com ([74.125.82.67]:40194 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752747AbdKNMGj (ORCPT ); Tue, 14 Nov 2017 07:06:39 -0500 X-Google-Smtp-Source: AGs4zMaCT95As2XcKt+i9GqvCqOOjnsUydYcDhQjTP+3mUs/59U4IxXfmU3sYODLp7DomnftZYZeww== Date: Tue, 14 Nov 2017 15:06:36 +0300 From: "Kirill A. Shutemov" To: Thomas Gleixner Cc: "Kirill A. Shutemov" , Ingo Molnar , Linus Torvalds , x86@kernel.org, "H. Peter Anvin" , Andy Lutomirski , Cyrill Gorcunov , Nicholas Piggin , 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: <20171114120636.fsrerghzdctivtdl@node.shutemov.name> References: <20171107130539.52676-1-kirill.shutemov@linux.intel.com> <20171113164154.fp5fd2seozbmxcbs@node.shutemov.name> <20171113200005.xzbhenukotzicwd2@node.shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 13, 2017 at 10:17:30PM +0100, Thomas Gleixner wrote: > 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. Okay, I'll update the patch. -- Kirill A. Shutemov