From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f198.google.com (mail-wr0-f198.google.com [209.85.128.198]) by kanga.kvack.org (Postfix) with ESMTP id 874C36B025F for ; Thu, 2 Nov 2017 07:33:42 -0400 (EDT) Received: by mail-wr0-f198.google.com with SMTP id g10so2843494wrg.6 for ; Thu, 02 Nov 2017 04:33:42 -0700 (PDT) Received: from Galois.linutronix.de (Galois.linutronix.de. [2a01:7a0:2:106d:700::1]) by mx.google.com with ESMTPS id d12si2559983wrg.307.2017.11.02.04.33.41 for (version=TLS1_2 cipher=AES128-SHA bits=128/128); Thu, 02 Nov 2017 04:33:41 -0700 (PDT) Date: Thu, 2 Nov 2017 12:33:37 +0100 (CET) From: Thomas Gleixner Subject: Re: [PATCH 02/23] x86, kaiser: do not set _PAGE_USER for init_mm page tables In-Reply-To: Message-ID: References: <20171031223146.6B47C861@viggo.jf.intel.com> <20171031223150.AB41C68F@viggo.jf.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Andy Lutomirski Cc: Linus Torvalds , Dave Hansen , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , moritz.lipp@iaik.tugraz.at, Daniel Gruss , michael.schwarz@iaik.tugraz.at, Kees Cook , Hugh Dickins , X86 ML On Thu, 2 Nov 2017, Andy Lutomirski wrote: > On Wed, Nov 1, 2017 at 3:20 PM, Thomas Gleixner wrote: > > On Wed, 1 Nov 2017, Linus Torvalds wrote: > >> On Wed, Nov 1, 2017 at 2:52 PM, Dave Hansen wrote: > >> > On 11/01/2017 02:28 PM, Thomas Gleixner wrote: > >> >> On Wed, 1 Nov 2017, Andy Lutomirski wrote: > >> >>> The vsyscall page is _PAGE_USER and lives in init_mm via the fixmap. > >> >> > >> >> Groan, forgot about that abomination, but still there is no point in having > >> >> it marked PAGE_USER in the init_mm at all, kaiser or not. > >> > > >> > So shouldn't this patch effectively make the vsyscall page unusable? > >> > Any idea why that didn't show up in any of the x86 selftests? > >> > >> I actually think there may be two issues here: > >> > >> - vsyscall isn't even used much - if any - any more > > > > Only legacy user space uses it. > > > >> - the vsyscall emulation works fine without _PAGE_USER, since the > >> whole point is that we take a fault on it and then emulate. > >> > >> We do expose the vsyscall page read-only to user space in the > >> emulation case, but I'm not convinced that's even required. > > > > I don't see a reason why it needs to be mapped at all for emulation. > > At least a couple years ago, the maintainers of some userspace tracing > tools complained very loudly at the early versions of the patches. > There are programs like pin (semi-open-source IIRC) that parse > instructions, make an instrumented copy, and run it. This means that > the vsyscall page needs to contain text that is semantically > equivalent to what calling it actually does. > > So yes, read access needs to work. I should add a selftest for this. > > This is needed in emulation mode as well as native mode, so removing > native mode is totally orthogonal. Fair enough. I enabled function tracing with emulate_vsyscall as the filter on a couple of machines and so far I have no hit at all. Though I found a VM with a real old user space (~2005) and that actually used it. So for the problem at hand, I'd suggest we disable the vsyscall stuff if CONFIG_KAISER=y and be done with it. Thanks, tglx -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org