Date: Thu, 11 Jan 2018 17:41:08 -0800 From: Dan Williams <dan.j.williams@...el.com> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: 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>, Will Deacon <will.deacon@....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 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. > > Also, I do think that there is one particular array load that would > seem to be pretty obvious: the system call function pointer array. > > Yes, yes, the actual call is now behind a retpoline, but that protects > against a speculative BTB access, it's not obvious that it protects > against the mispredict of the __NR_syscall_max comparison in > arch/x86/entry/entry_64.S. > > The act of fetching code is a kind of read too. And retpoline protects > against BTB stuffing etc, but what if the _actual_ system call > function address is wrong (due to mis-prediction of the system call > index check)? > > Should the array access in entry_SYSCALL_64_fastpath be made to use > the masking approach? I'll take a look. I'm firmly in the 'patch first / worry later' stance on these investigations.
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