Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752438AbaJAO4L (ORCPT ); Wed, 1 Oct 2014 10:56:11 -0400 Received: from mail-oi0-f42.google.com ([209.85.218.42]:46189 "EHLO mail-oi0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751501AbaJAO4I (ORCPT ); Wed, 1 Oct 2014 10:56:08 -0400 Date: Wed, 1 Oct 2014 09:56:03 -0500 From: Chuck Ebbert To: Andy Lutomirski Cc: Thomas Gleixner , X86 ML , Ingo Molnar , "H. Peter Anvin" , Sebastian Lackner , Anish Bhatt , "linux-kernel@vger.kernel.org" , stable Subject: Re: [PATCH v2 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace Message-ID: <20141001095603.3d5103d9@as> In-Reply-To: References: <0e906bdeba3660c9766248d3d7229e78a423ca5b.1412138935.git.luto@amacapital.net> <20141001090915.16c8b1db@as> <20141001093208.79bb0891@as> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 1 Oct 2014 07:46:54 -0700 Andy Lutomirski wrote: > On Wed, Oct 1, 2014 at 7:32 AM, Chuck Ebbert wrote: > > On Wed, 1 Oct 2014 09:09:13 -0500 > > Chuck Ebbert wrote: > > > >> On Tue, 30 Sep 2014 21:51:27 -0700 > >> 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. > >> > > >> > Entry via hardware or software interrupt clears NT automatically, so > >> > the only relevant entries are fast syscalls. > >> > > >> > If user code causes kernel code to run with NT set, then there's at > >> > least some (small) chance that it could cause trouble. For example, > >> > user code could cause a call to EFI code with NT set, and who knows > >> > what would happen? Apparently some games on Wine sometimes do > >> > this (!), and, if an IRET return happens, they will segfault. That > >> > segfault cannot be handled, because signal delivery fails, too. > >> > > >> > This patch programs the CPU to clear NT on entry via SYSCALL (both > >> > 32-bit and 64-bit, by my reading of the AMD APM), and it clears NT > >> > in software on entry via SYSENTER. > >> > > >> > To save a few cycles, this borrows a trick from Jan Beulich in Xen: > >> > it checks whether NT is set before trying to clear it. As a result, > >> > it seems to have very little effect on SYSENTER performance on my > >> > machine. > >> > > >> > Testers beware: on Xen, SYSENTER with NT set turns into a GPF. > >> > > >> > I haven't touched anything on 32-bit kernels. > >> > > >> > The syscall mask change comes from a variant of this patch by Anish > >> > Bhatt. > >> > > >> > Cc: stable@vger.kernel.org > >> > Reported-by: Anish Bhatt > >> > Signed-off-by: Andy Lutomirski > >> > --- > >> > arch/x86/ia32/ia32entry.S | 12 ++++++++++++ > >> > arch/x86/kernel/cpu/common.c | 2 +- > >> > 2 files changed, 13 insertions(+), 1 deletion(-) > >> > > >> > diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S > >> > index 4299eb05023c..44d1dd371454 100644 > >> > --- a/arch/x86/ia32/ia32entry.S > >> > +++ b/arch/x86/ia32/ia32entry.S > >> > @@ -151,6 +151,18 @@ ENTRY(ia32_sysenter_target) > >> > 1: movl (%rbp),%ebp > >> > _ASM_EXTABLE(1b,ia32_badarg) > >> > ASM_CLAC > >> > + > >> > + /* > >> > + * 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 */ > >> > + jz 1f > >> > + pushq_cfi $(X86_EFLAGS_IF|X86_EFLAGS_FIXED) > >> > + popfq_cfi > >> > +1: > >> > + > >> > >> I think you've gone backwards with this version. The earlier one got > >> some of the performance loss back by not needing to do the "cld" insn. > >> > >> You should just replace that "cld" (line 146) with > >> > >> pushfq_cfi $2 > >> popfq_cfi > >> > >> Unfortunately I'm not set up to test that yet. But I did look at > >> the SDM and can't see a need to preserve any of the flags. > >> > > > > > > that's: > > > > pushfw_cfi $0x202 > > > > IF needs to stay on because we've already enabled interrupts after > > sysenter. > > I tried exactly this. It was much slower than the version I sent. > Yeah, it looks like a new paravirt op that enables interrupts and clears all the other flags would be the only way to do this without at least some impact on performance. -- 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/