Date: Thu, 18 Jan 2018 08:58:08 -0800 From: Dan Williams <dan.j.williams@...el.com> To: Will Deacon <will.deacon@....com> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Mark Rutland <mark.rutland@....com>, kernel-hardening@...ts.openwall.com, Peter Zijlstra <peterz@...radead.org>, Alan Cox <alan.cox@...el.com>, Alexei Starovoitov <ast@...nel.org>, Solomon Peachy <pizza@...ftnet.org>, "H. Peter Anvin" <hpa@...or.com>, Christian Lamparter <chunkeey@...glemail.com>, Elena Reshetova <elena.reshetova@...el.com>, linux-arch@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>, "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>, Linux SCSI List <linux-scsi@...r.kernel.org>, Jonathan Corbet <corbet@....net>, "the arch/x86 maintainers" <x86@...nel.org>, Russell King <linux@...linux.org.uk>, Ingo Molnar <mingo@...hat.com>, Catalin Marinas <catalin.marinas@....com>, Alexey Kuznetsov <kuznet@....inr.ac.ru>, Linux Media Mailing List <linux-media@...r.kernel.org>, Tom Lendacky <thomas.lendacky@....com>, Kees Cook <keescook@...omium.org>, Jan Kara <jack@...e.com>, Al Viro <viro@...iv.linux.org.uk>, qla2xxx-upstream@...gic.com, Thomas Gleixner <tglx@...utronix.de>, Mauro Carvalho Chehab <mchehab@...nel.org>, Kalle Valo <kvalo@...eaurora.org>, Alan Cox <alan@...ux.intel.com>, "Martin K. Petersen" <martin.petersen@...cle.com>, Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>, Greg KH <gregkh@...uxfoundation.org>, Linux Wireless List <linux-wireless@...r.kernel.org>, "Eric W. Biederman" <ebiederm@...ssion.com>, Network Development <netdev@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>, "David S. Miller" <davem@...emloft.net>, Laurent Pinchart <laurent.pinchart@...asonboard.com> Subject: Re: [PATCH v2 00/19] prevent bounds-check bypass via speculative execution On Thu, Jan 18, 2018 at 5:18 AM, Will Deacon <will.deacon@....com> 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 >> <torvalds@...ux-foundation.org> wrote: >> > On Thu, Jan 11, 2018 at 4:46 PM, Dan Williams <dan.j.williams@...el.com> 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 , 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().
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