From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f199.google.com (mail-wr0-f199.google.com [209.85.128.199]) by kanga.kvack.org (Postfix) with ESMTP id 843EC6B025F for ; Mon, 13 Nov 2017 16:17:41 -0500 (EST) Received: by mail-wr0-f199.google.com with SMTP id v88so9800357wrb.22 for ; Mon, 13 Nov 2017 13:17:41 -0800 (PST) Received: from Galois.linutronix.de (Galois.linutronix.de. [2a01:7a0:2:106d:700::1]) by mx.google.com with ESMTPS id c5si14171049wrd.102.2017.11.13.13.17.40 for (version=TLS1_2 cipher=AES128-SHA bits=128/128); Mon, 13 Nov 2017 13:17:40 -0800 (PST) Date: Mon, 13 Nov 2017 22:17:30 +0100 (CET) From: Thomas Gleixner Subject: Re: [PATCH] x86/mm: Do not allow non-MAP_FIXED mapping across DEFAULT_MAP_WINDOW border In-Reply-To: <20171113200005.xzbhenukotzicwd2@node.shutemov.name> Message-ID: 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 Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" 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 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org