Return-path: Received: from mail-ot0-f196.google.com ([74.125.82.196]:46359 "EHLO mail-ot0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932702AbeARQ6K (ORCPT ); Thu, 18 Jan 2018 11:58:10 -0500 Received: by mail-ot0-f196.google.com with SMTP id t35so14011370otd.13 for ; Thu, 18 Jan 2018 08:58:10 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <20180118131837.GA20783@arm.com> References: <151571798296.27429.7166552848688034184.stgit@dwillia2-desk3.amr.corp.intel.com> <20180118131837.GA20783@arm.com> From: Dan Williams Date: Thu, 18 Jan 2018 08:58:08 -0800 Message-ID: (sfid-20180118_175935_872078_1535C6D2) Subject: Re: [PATCH v2 00/19] prevent bounds-check bypass via speculative execution To: Will Deacon Cc: Linus Torvalds , Linux Kernel Mailing List , Mark Rutland , kernel-hardening@lists.openwall.com, Peter Zijlstra , Alan Cox , Alexei Starovoitov , Solomon Peachy , "H. Peter Anvin" , Christian Lamparter , Elena Reshetova , linux-arch@vger.kernel.org, Andi Kleen , "James E.J. Bottomley" , Linux SCSI List , Jonathan Corbet , "the arch/x86 maintainers" , Russell King , Ingo Molnar , Catalin Marinas , Alexey Kuznetsov , Linux Media Mailing List , Tom Lendacky , Kees Cook , Jan Kara , Al Viro , qla2xxx-upstream@qlogic.com, Thomas Gleixner , Mauro Carvalho Chehab , Kalle Valo , Alan Cox , "Martin K. Petersen" , Hideaki YOSHIFUJI , Greg KH , Linux Wireless List , "Eric W. Biederman" , Network Development , Andrew Morton , "David S. Miller" , Laurent Pinchart Content-Type: text/plain; charset="UTF-8" Sender: linux-wireless-owner@vger.kernel.org List-ID: On Thu, Jan 18, 2018 at 5:18 AM, Will Deacon wrote: > Hi Dan, Linus, > > On Thu, Jan 11, 2018 at 05:41:08PM -0800, Dan Williams wrote: >> On Thu, Jan 11, 2018 at 5:19 PM, Linus Torvalds >> wrote: >> > On Thu, Jan 11, 2018 at 4:46 PM, Dan Williams wrote: >> >> >> >> This series incorporates Mark Rutland's latest ARM changes and adds >> >> the x86 specific implementation of 'ifence_array_ptr'. That ifence >> >> based approach is provided as an opt-in fallback, but the default >> >> mitigation, '__array_ptr', uses a 'mask' approach that removes >> >> conditional branches instructions, and otherwise aims to redirect >> >> speculation to use a NULL pointer rather than a user controlled value. >> > >> > Do you have any performance numbers and perhaps example code >> > generation? Is this noticeable? Are there any microbenchmarks showing >> > the difference between lfence use and the masking model? >> >> I don't have performance numbers, but here's a sample code generation >> from __fcheck_files, where the 'and; lea; and' sequence is portion of >> array_ptr() after the mask generation with 'sbb'. >> >> fdp = array_ptr(fdt->fd, fd, fdt->max_fds); >> 8e7: 8b 02 mov (%rdx),%eax >> 8e9: 48 39 c7 cmp %rax,%rdi >> 8ec: 48 19 c9 sbb %rcx,%rcx >> 8ef: 48 8b 42 08 mov 0x8(%rdx),%rax >> 8f3: 48 89 fe mov %rdi,%rsi >> 8f6: 48 21 ce and %rcx,%rsi >> 8f9: 48 8d 04 f0 lea (%rax,%rsi,8),%rax >> 8fd: 48 21 c8 and %rcx,%rax >> >> >> > Having both seems good for testing, but wouldn't we want to pick one in the end? >> >> I was thinking we'd keep it as a 'just in case' sort of thing, at >> least until the 'probably safe' assumption of the 'mask' approach has >> more time to settle out. > > From the arm64 side, the only concern I have (and this actually applies to > our CSDB sequence as well) is the calculation of the array size by the > caller. As Linus mentioned at the end of [1], if the determination of the > size argument is based on a conditional branch, then masking doesn't help > because you bound within the wrong range under speculation. > > We ran into this when trying to use masking to protect our uaccess routines > where the conditional bound is either KERNEL_DS or USER_DS. It's possible > that a prior conditional set_fs(KERNEL_DS) could defeat the masking and so > we'd need to throw some heavy barriers in set_fs to make it robust. At least in the conditional mask case near set_fs() usage the approach we are taking is to use a barrier. I.e. the following guidance from Linus: "Basically, the rule is trivial: find all 'stac' users, and use address masking if those users already integrate the limit check, and lfence they don't." ...which translates to narrow the pointer for get_user() and use a barrier for __get_user().