Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751249AbaA1FWH (ORCPT ); Tue, 28 Jan 2014 00:22:07 -0500 Received: from mail-ve0-f179.google.com ([209.85.128.179]:53200 "EHLO mail-ve0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750747AbaA1FWF (ORCPT ); Tue, 28 Jan 2014 00:22:05 -0500 MIME-Version: 1.0 In-Reply-To: <52E72573.9090108@intel.com> References: <1390727338-20487-1-git-send-email-qiaowei.ren@intel.com> <1390727338-20487-3-git-send-email-qiaowei.ren@intel.com> <52E6C33C.8050706@amacapital.net> <52E72573.9090108@intel.com> From: Andy Lutomirski Date: Mon, 27 Jan 2014 21:21:43 -0800 Message-ID: Subject: Re: [PATCH v3 2/4] x86, mpx: hook #BR exception handler to allocate bound tables To: Ren Qiaowei Cc: "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , x86@kernel.org, "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 27, 2014 at 7:35 PM, Ren Qiaowei wrote: > On 01/28/2014 04:36 AM, Andy Lutomirski wrote: >>> >>> + bd_entry = status & MPX_BNDSTA_ADDR_MASK; >>> + if ((bd_entry >= bd_base) && (bd_entry < bd_base + bd_size)) >>> + allocate_bt(bd_entry); >> >> >> What happens if this fails? Retrying forever isn't very nice. >> > If allocation of the bound table fail, the related entry in the bound > directory is still invalid. The following access to this entry still produce > #BR fault. > By the "following access" I think you mean the same instruction that just trapped -- it will trap again because the exception hasn't been fixed up. Then mmap will fail again, and you'll retry again, leading to an infinite loop. I think that failure to fix up the exception should either let the normal bounds error through or should raise SIGBUS. > >>> + if (!user_mode(regs)) { >>> + if (!fixup_exception(regs)) { >>> + tsk->thread.error_code = error_code; >>> + tsk->thread.trap_nr = X86_TRAP_BR; >>> + die("bounds", regs, error_code); >>> + } >> >> >> Why the fixup? Unless I'm missing something, the kernel has no business >> getting #BR on access to a user address. >> >> Or are you adding code to allow the kernel to use MPX itself? If so, >> shouldn't this use an MPX-specific fixup to allow normal C code to use >> this stuff? >> > It checks whether #BR come from user-space. You can see do_trap_no_signal(). Wasn't #BR using do_trap before? do_trap doesn't call fixup_exception. I don't see why it should do it now. (I also don't think it should come from kernel space until someone adds kernel-mode MPX support.) > > >>> + goto exit; >>> + } >>> + >>> + if (!boot_cpu_has(X86_FEATURE_MPX)) { >>> + do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code, >>> NULL); >>> + goto exit; >> >> >> This, as well as the status == 0 case, should probably document that the >> exception is from BOUND, not MPX. >> > Ok. I will add one comment for this. > > > >>> + break; >>> + >>> + case 1: /* Bound violation. */ >>> + case 0: /* No MPX exception. */ >>> + do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code, >>> NULL); >>> + break; >> >> >> What does "No Intel MPX exception" mean? Surely that has business >> sending #BR. >> > Oh. It comes from spec, and just mean it is not from MPX. :) I will change > it to be accurate. > > >>> + >>> + default: >>> + break; >> >> >> What does status 3 mean? The docs say "reserved". Presumably this >> should log and kill the process. > > I guess it should be a good suggestion. > > Thanks, > Qiaowei > -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/