From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1033398AbdAEUPg (ORCPT ); Thu, 5 Jan 2017 15:15:36 -0500 Received: from mail-ua0-f170.google.com ([209.85.217.170]:35226 "EHLO mail-ua0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1033332AbdAEUP1 (ORCPT ); Thu, 5 Jan 2017 15:15:27 -0500 MIME-Version: 1.0 In-Reply-To: <161ece66-fbf4-cb89-3da6-91b4851af69f@intel.com> References: <20161227015413.187403-1-kirill.shutemov@linux.intel.com> <20161227015413.187403-30-kirill.shutemov@linux.intel.com> <5a3dcc25-b264-37c7-c090-09981b23940d@intel.com> <20170105192910.q26ozg4ci4i3j2ai@black.fi.intel.com> <161ece66-fbf4-cb89-3da6-91b4851af69f@intel.com> From: Andy Lutomirski Date: Thu, 5 Jan 2017 12:14:47 -0800 Message-ID: Subject: Re: [RFC, PATCHv2 29/29] mm, x86: introduce RLIMIT_VADDR To: Dave Hansen Cc: "Kirill A. Shutemov" , Linus Torvalds , Andrew Morton , X86 ML , Thomas Gleixner , Ingo Molnar , Arnd Bergmann , "H. Peter Anvin" , Andi Kleen , linux-arch , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , Linux API Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 5, 2017 at 11:39 AM, Dave Hansen wrote: > On 01/05/2017 11:29 AM, Kirill A. Shutemov wrote: >> On Thu, Jan 05, 2017 at 11:13:57AM -0800, Dave Hansen wrote: >>> On 12/26/2016 05:54 PM, Kirill A. Shutemov wrote: >>>> MM would use min(RLIMIT_VADDR, TASK_SIZE) as upper limit of virtual >>>> address available to map by userspace. >>> >>> What happens to existing mappings above the limit when this upper limit >>> is dropped? >> >> Nothing: we only prevent creating new mappings. All existing are not >> affected. >> >> The semantics here the same as with other resource limits. >> >>> Similarly, why do we do with an application running with something >>> incompatible with the larger address space that tries to raise the >>> limit? Say, legacy MPX. >> >> It has to know what it does. Yes, it can change limit to the point where >> application is unusable. But you can to the same with other limits. > > I'm not sure I'm comfortable with this. Do other rlimit changes cause > silent data corruption? I'm pretty sure doing this to MPX would. > What actually goes wrong in this case? That is, what combination of MPX setup of subsequent allocations will cause a problem, and is the problem worse than just a segfault? IMO it would be really nice to keep the messy case confined to MPX. FWIW, this problem is kind of generic. If you run code in a process, MPX or otherwise, that assumes something about pointer values and then create a pointer that violates its assumptions, you will cause problems. For example, some VMs use high bits to store metadata. If you feed a pointer that's too big to such code, boom. This is exactly why high addresses need to be opt-in.