From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753899AbYGXSfM (ORCPT ); Thu, 24 Jul 2008 14:35:12 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752379AbYGXSfA (ORCPT ); Thu, 24 Jul 2008 14:35:00 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:36150 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752406AbYGXSe7 (ORCPT ); Thu, 24 Jul 2008 14:34:59 -0400 Date: Thu, 24 Jul 2008 11:31:42 -0700 (PDT) From: Linus Torvalds To: Suresh Siddha cc: x86@kernel.org, andi@firstfloor.org, Linux Kernel Mailing List , stable@kernel.org, Ingo Molnar Subject: Re: [patch] x64, fpu: fix possible FPU leakage in error conditions In-Reply-To: <20080724180429.GI14380@linux-os.sc.intel.com> Message-ID: References: <20080724180429.GI14380@linux-os.sc.intel.com> User-Agent: Alpine 1.10 (LFD 962 2008-03-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 24 Jul 2008, Suresh Siddha wrote: > > In the error condition for restore_fpu_checking() (especially during > the 64bit signal return), we are doing init_fpu(), which saves the live > FPU register state (possibly belonging to some other process context) into the > thread struct (through unlazy_fpu() in init_fpu()). This is wrong and can leak > the FPU data. > > Remove the unlazy_fpu() from the init_fpu(). init_fpu() will now always > init the FPU data in the thread struct. For the error conditions in > restore_fpu_checking(), restore the initialized FPU data from the thread > struct. Why? The thread struct is guaranteed to contain pointless data. If we cannot restore the FP state from the signal stack, we should not try to restore it from anywhere _else_ either, since nowhere else will have any better results. I suspect we should just reset the x87 state (which was the _intention_ of the code), possibly by just doing "stts + used_math = 0". The signal handling code already checks for errors, and will force a SIGSEGV if this ever happens. (Yes, there is also a restore_fpu_checking() in math_state_restore(), but that one _already_ uses ¤t->thread.xstate->fxsave as the buffer to restore from, so trying to do that _again_ when it fails seems to be really really wrong - we already _did_ that, and that was what failed to begin with) Linus