From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751775AbaJATuU (ORCPT ); Wed, 1 Oct 2014 15:50:20 -0400 Received: from mail-la0-f43.google.com ([209.85.215.43]:35873 "EHLO mail-la0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751615AbaJATuS (ORCPT ); Wed, 1 Oct 2014 15:50:18 -0400 MIME-Version: 1.0 In-Reply-To: <395749a5d39a29bd3e4b35899cf3a3c1340e5595.1412189265.git.luto@amacapital.net> References: <395749a5d39a29bd3e4b35899cf3a3c1340e5595.1412189265.git.luto@amacapital.net> From: Andy Lutomirski Date: Wed, 1 Oct 2014 12:49:56 -0700 Message-ID: Subject: Re: [PATCH v4 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace To: Thomas Gleixner , X86 ML , Ingo Molnar , "H. Peter Anvin" Cc: Sebastian Lackner , Anish Bhatt , "linux-kernel@vger.kernel.org" , Chuck Ebbert , Andy Lutomirski , stable Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. --Andy