From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: [PATCH v5 14/32] x86/mm/64: Enable vmapped stacks Date: Wed, 13 Jul 2016 11:42:35 -0700 Message-ID: References: <8d36dd9b2430b61db64333af7b911d0bca7d5d2f.1468270393.git.luto@kernel.org> <20160713075314.GA32700@gmail.com> Reply-To: kernel-hardening@lists.openwall.com Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Return-path: List-Post: List-Help: List-Unsubscribe: List-Subscribe: In-Reply-To: <20160713075314.GA32700@gmail.com> To: Ingo Molnar Cc: Andy Lutomirski , X86 ML , "linux-kernel@vger.kernel.org" , linux-arch , Borislav Petkov , Nadav Amit , Kees Cook , Brian Gerst , "kernel-hardening@lists.openwall.com" , Linus Torvalds , Josh Poimboeuf , Jann Horn , Heiko Carstens List-Id: linux-arch.vger.kernel.org On Wed, Jul 13, 2016 at 12:53 AM, Ingo Molnar wrote: > > * Andy Lutomirski wrote: > >> This allows x86_64 kernels to enable vmapped stacks. There are a >> couple of interesting bits. > >> --- a/arch/x86/Kconfig >> +++ b/arch/x86/Kconfig >> @@ -92,6 +92,7 @@ config X86 >> select HAVE_ARCH_TRACEHOOK >> select HAVE_ARCH_TRANSPARENT_HUGEPAGE >> select HAVE_EBPF_JIT if X86_64 >> + select HAVE_ARCH_VMAP_STACK if X86_64 > > So what is the performance impact? Seems to be a very slight speedup (0.5 =C2=B5s or so) on my silly benchmark (pthread_create, pthread_join in a loop). It should be a small slowdown on workloads that create many threads all at once, thus defeating the stack cache. It should be a *large* speedup on any workload that would trigger compaction on clone() to satisfy the high-order allocation. > > Because I think we should consider enabling this feature by default on x8= 6 - but > the way it's selected here it will be default-off. > > On the plus side: the debuggability and reliability improvements are real= and > making it harder for exploits to use kernel stack overflows is a nice bon= us as > well. There's two performance effects: Agreed. At the very least, I want to wait until after net-next gets pulled to flip the default to y. I'm also a bit concerned about more random driver issues that I haven't found yet. I suppose we could flip the default to y for a few -rc releases and see what, if anything, shakes loose. --Andy From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-vk0-f46.google.com ([209.85.213.46]:32773 "EHLO mail-vk0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750923AbcGMSnV convert rfc822-to-8bit (ORCPT ); Wed, 13 Jul 2016 14:43:21 -0400 Received: by mail-vk0-f46.google.com with SMTP id x130so78318147vkc.0 for ; Wed, 13 Jul 2016 11:43:14 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20160713075314.GA32700@gmail.com> References: <8d36dd9b2430b61db64333af7b911d0bca7d5d2f.1468270393.git.luto@kernel.org> <20160713075314.GA32700@gmail.com> From: Andy Lutomirski Date: Wed, 13 Jul 2016 11:42:35 -0700 Message-ID: Subject: Re: [PATCH v5 14/32] x86/mm/64: Enable vmapped stacks Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-arch-owner@vger.kernel.org List-ID: To: Ingo Molnar Cc: Andy Lutomirski , X86 ML , "linux-kernel@vger.kernel.org" , linux-arch , Borislav Petkov , Nadav Amit , Kees Cook , Brian Gerst , "kernel-hardening@lists.openwall.com" , Linus Torvalds , Josh Poimboeuf , Jann Horn , Heiko Carstens Message-ID: <20160713184235.Ij86E0rGBOMlbAkKLYOxvlLsO8dNsZ_7q_3CnHZsCb8@z> On Wed, Jul 13, 2016 at 12:53 AM, Ingo Molnar wrote: > > * Andy Lutomirski wrote: > >> This allows x86_64 kernels to enable vmapped stacks. There are a >> couple of interesting bits. > >> --- a/arch/x86/Kconfig >> +++ b/arch/x86/Kconfig >> @@ -92,6 +92,7 @@ config X86 >> select HAVE_ARCH_TRACEHOOK >> select HAVE_ARCH_TRANSPARENT_HUGEPAGE >> select HAVE_EBPF_JIT if X86_64 >> + select HAVE_ARCH_VMAP_STACK if X86_64 > > So what is the performance impact? Seems to be a very slight speedup (0.5 µs or so) on my silly benchmark (pthread_create, pthread_join in a loop). It should be a small slowdown on workloads that create many threads all at once, thus defeating the stack cache. It should be a *large* speedup on any workload that would trigger compaction on clone() to satisfy the high-order allocation. > > Because I think we should consider enabling this feature by default on x86 - but > the way it's selected here it will be default-off. > > On the plus side: the debuggability and reliability improvements are real and > making it harder for exploits to use kernel stack overflows is a nice bonus as > well. There's two performance effects: Agreed. At the very least, I want to wait until after net-next gets pulled to flip the default to y. I'm also a bit concerned about more random driver issues that I haven't found yet. I suppose we could flip the default to y for a few -rc releases and see what, if anything, shakes loose. --Andy