From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: [RFC, PATCHv2 29/29] mm, x86: introduce RLIMIT_VADDR Date: Thu, 5 Jan 2017 13:27:40 -0800 Message-ID: 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> <978d5f1a-ec4d-f747-93fd-27ecfe10cb88@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Return-path: In-Reply-To: <978d5f1a-ec4d-f747-93fd-27ecfe10cb88-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org 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-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org" , "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , Linux API List-Id: linux-api@vger.kernel.org On Thu, Jan 5, 2017 at 12:49 PM, Dave Hansen wrote: > On 01/05/2017 12:14 PM, Andy Lutomirski wrote: >>> 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. > > The MPX bounds tables are indexed by virtual address. They need to grow > if the virtual address space grows. There's an MSR that controls > whether we use the 48-bit or 57-bit layout. It basically decides > whether we need a 2GB (48-bit) or 1TB (57-bit) bounds directory. > > The question is what we do with legacy MPX applications. We obviously > can't let them just allocate a 2GB table and then go let the hardware > pretend it's 1TB in size. We also can't hand the hardware using a 2GB > table an address >48-bits. > > Ideally, I'd like to make sure that legacy MPX can't be enabled if this > RLIMIT is set over 48-bits (really 47). I'd also like to make sure that > legacy MPX is active, that the RLIMIT can't be raised because all hell > will break loose when the new addresses show up. > > Remember, we already have (legacy MPX) binaries in the wild that have no > knowledge of this stuff. So, we can implicitly have the kernel bump > this rlimit around, but we can't expect userspace to do it, ever. If you s/rlimit/prctl, then I think this all makes sense with one exception. It would be a bit sad if the personality-setting tool didn't work if compiled with MPX. So what if we had a second prctl field that is the value that kicks in after execve()? --Andy