From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: [PATCH v5 07/27] mm/mmap: Create a guard area between VMAs Date: Thu, 11 Oct 2018 13:55:26 -0700 Message-ID: References: <20181011151523.27101-1-yu-cheng.yu@intel.com> <20181011151523.27101-8-yu-cheng.yu@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Jann Horn Cc: Yu-cheng Yu , X86 ML , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , LKML , linux-doc@vger.kernel.org, Linux-MM , linux-arch , Linux API , Arnd Bergmann , Balbir Singh , Cyrill Gorcunov , Dave Hansen , Eugene Syromiatnikov , Florian Weimer , "H. J. Lu" , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit , Oleg List-Id: linux-arch.vger.kernel.org On Thu, Oct 11, 2018 at 1:39 PM Jann Horn wrote: > > On Thu, Oct 11, 2018 at 5:20 PM Yu-cheng Yu wrote: > > Create a guard area between VMAs to detect memory corruption. > [...] > > +config VM_AREA_GUARD > > + bool "VM area guard" > > + default n > > + help > > + Create a guard area between VM areas so that access beyond > > + limit can be detected. > > + > > endmenu > > Sorry to bring this up so late, but Daniel Micay pointed out to me > that, given that VMA guards will raise the number of VMAs by > inhibiting vma_merge(), people are more likely to run into > /proc/sys/vm/max_map_count (which limits the number of VMAs to ~65k by > default, and can't easily be raised without risking an overflow of > page->_mapcount on systems with over ~800GiB of RAM, see > https://lore.kernel.org/lkml/20180208021112.GB14918@bombadil.infradead.org/ > and replies) with this change. > > Playing with glibc's memory allocator, it looks like glibc will use > mmap() for 128KB allocations; so at 65530*128KB=8GB of memory usage in > 128KB chunks, an application could run out of VMAs. Ugh. Do we have a free VM flag so we could do VM_GUARD to force a guard page? (And to make sure that, when a new VMA is allocated, it won't be directly adjacent to a VM_GUARD VMA.) From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr1-f65.google.com ([209.85.221.65]:40517 "EHLO mail-wr1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726762AbeJLEYi (ORCPT ); Fri, 12 Oct 2018 00:24:38 -0400 Received: by mail-wr1-f65.google.com with SMTP id d2-v6so11172554wro.7 for ; Thu, 11 Oct 2018 13:55:39 -0700 (PDT) MIME-Version: 1.0 References: <20181011151523.27101-1-yu-cheng.yu@intel.com> <20181011151523.27101-8-yu-cheng.yu@intel.com> In-Reply-To: From: Andy Lutomirski Date: Thu, 11 Oct 2018 13:55:26 -0700 Message-ID: Subject: Re: [PATCH v5 07/27] mm/mmap: Create a guard area between VMAs Content-Type: text/plain; charset="UTF-8" Sender: linux-arch-owner@vger.kernel.org List-ID: To: Jann Horn Cc: Yu-cheng Yu , X86 ML , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , LKML , linux-doc@vger.kernel.org, Linux-MM , linux-arch , Linux API , Arnd Bergmann , Balbir Singh , Cyrill Gorcunov , Dave Hansen , Eugene Syromiatnikov , Florian Weimer , "H. J. Lu" , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , Peter Zijlstra , Randy Dunlap , "Ravi V. Shankar" , "Shanbhogue, Vedvyas" , Daniel Micay Message-ID: <20181011205526.lZkKN-fbYbSfQz2BAiiTUUnu__vnia63cdFT9vqY-Yc@z> On Thu, Oct 11, 2018 at 1:39 PM Jann Horn wrote: > > On Thu, Oct 11, 2018 at 5:20 PM Yu-cheng Yu wrote: > > Create a guard area between VMAs to detect memory corruption. > [...] > > +config VM_AREA_GUARD > > + bool "VM area guard" > > + default n > > + help > > + Create a guard area between VM areas so that access beyond > > + limit can be detected. > > + > > endmenu > > Sorry to bring this up so late, but Daniel Micay pointed out to me > that, given that VMA guards will raise the number of VMAs by > inhibiting vma_merge(), people are more likely to run into > /proc/sys/vm/max_map_count (which limits the number of VMAs to ~65k by > default, and can't easily be raised without risking an overflow of > page->_mapcount on systems with over ~800GiB of RAM, see > https://lore.kernel.org/lkml/20180208021112.GB14918@bombadil.infradead.org/ > and replies) with this change. > > Playing with glibc's memory allocator, it looks like glibc will use > mmap() for 128KB allocations; so at 65530*128KB=8GB of memory usage in > 128KB chunks, an application could run out of VMAs. Ugh. Do we have a free VM flag so we could do VM_GUARD to force a guard page? (And to make sure that, when a new VMA is allocated, it won't be directly adjacent to a VM_GUARD VMA.)