Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753162AbaJFQm0 (ORCPT ); Mon, 6 Oct 2014 12:42:26 -0400 Received: from terminus.zytor.com ([198.137.202.10]:41432 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751731AbaJFQmY (ORCPT ); Mon, 6 Oct 2014 12:42:24 -0400 Message-ID: <5432C65C.4040406@zytor.com> Date: Mon, 06 Oct 2014 09:42:04 -0700 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.1.1 MIME-Version: 1.0 To: Andy Lutomirski , Thomas Gleixner , X86 ML , Ingo Molnar CC: Sebastian Lackner , Anish Bhatt , "linux-kernel@vger.kernel.org" , Chuck Ebbert , stable Subject: Re: [PATCH v4 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace References: <395749a5d39a29bd3e4b35899cf3a3c1340e5595.1412189265.git.luto@amacapital.net> In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/01/2014 12:49 PM, Andy Lutomirski wrote: > On Wed, Oct 1, 2014 at 11:49 AM, Andy Lutomirski wrote: >> The NT flag doesn't do anything in long mode other than causing IRET >> to #GP. Oddly, CPL3 code can still set NT using popf. >> > > [...] > >> + >> + /* >> + * Sysenter doesn't filter flags, so we need to clear NT >> + * ourselves. To save a few cycles, we can check whether >> + * NT was set instead of doing an unconditional popfq. >> + */ >> + testl $X86_EFLAGS_NT,EFLAGS(%rsp) /* saved EFLAGS match cpu */ >> + jnz sysenter_fix_flags >> +sysenter_flags_fixed: >> + > > Because this thread hasn't gone on long enough: > > Do we need to clear IOPL here, too? With patch 2 applied, an IOPL != > 0 program can leak IOPL into another task. It should be cleared on > iret, sysexit (via popf) and sysret (directly), so this shouldn't > matter. Am I missing something? > > Adding IOPL to the test will add no overhead for non-iopl-using tasks, > but it will slighly slow down 32-bit tasks that use iopl. > I don't see why. IOPL has no effect in the kernel. -hpa -- 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/