Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752655AbcD3Agv (ORCPT ); Fri, 29 Apr 2016 20:36:51 -0400 Received: from mga02.intel.com ([134.134.136.20]:35053 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752344AbcD3Agu (ORCPT ); Fri, 29 Apr 2016 20:36:50 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.24,553,1455004800"; d="scan'208";a="795452305" Subject: Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows correct xstates copy from kernel to user directly To: Yu-cheng Yu References: <5723BF63.2000100@linux.intel.com> <20160429224338.GA15757@test-lenovo> Cc: x86@kernel.org, "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , linux-kernel@vger.kernel.org, Andy Lutomirski , Borislav Petkov , Sai Praneeth Prakhya , "Ravi V. Shankar" , Fenghua Yu From: Dave Hansen Message-ID: <5723FE20.7000906@linux.intel.com> Date: Fri, 29 Apr 2016 17:36:48 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 MIME-Version: 1.0 In-Reply-To: <20160429224338.GA15757@test-lenovo> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1310 Lines: 30 On 04/29/2016 03:43 PM, Yu-cheng Yu wrote: > On Fri, Apr 29, 2016 at 01:09:07PM -0700, Dave Hansen wrote: >> On 03/04/2016 10:12 AM, Yu-cheng Yu wrote: >>> +static int may_copy_fpregs_to_sigframe(void) >>> +{ >>> + /* >>> + * In signal handling path, the kernel already checks if >>> + * FPU instructions have been used before it calls >>> + * copy_fpstate_to_sigframe(). We check this here again >>> + * to detect any potential mis-use and saving invalid >>> + * register values directly to a signal frame. >>> + */ >>> + WARN_ONCE(!current->thread.fpu.fpstate_active, >>> + "direct FPU save with no math use\n"); >> >> This is probably an OK check for this _particular_ context (since this >> context is all ready to copy_to_user() the fpu state). But is it good >> generally? Why couldn't you have a !fpstate_active thread that _was_ >> fpregs_active? >> >> Such a thread _could_ do a direct XSAVE with no issues. > > But it won't come to this function unless fpstate_active is ture? If may_copy_fpregs_to_sigframe() were called from a slightly different context, or if we change the call-site, what breaks? In other words. if we can still "may_copy_fpregs_to_sigframe()" no matter the state of fpu.fpstate_active, then I don't think we should be checking it in may_copy_fpregs_to_sigframe().