From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,MAILING_LIST_MULTI,T_DKIMWL_WL_HIGH,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by aws-us-west-2-korg-lkml-1.web.codeaurora.org (Postfix) with ESMTP id 1E6C8C5CFF1 for ; Tue, 12 Jun 2018 16:34:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BD728208B2 for ; Tue, 12 Jun 2018 16:34:39 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="EXURu2tj" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BD728208B2 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934712AbeFLQei (ORCPT ); Tue, 12 Jun 2018 12:34:38 -0400 Received: from mail.kernel.org ([198.145.29.99]:53132 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933640AbeFLQef (ORCPT ); Tue, 12 Jun 2018 12:34:35 -0400 Received: from mail-wm0-f50.google.com (mail-wm0-f50.google.com [74.125.82.50]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 0AE8D208B2 for ; Tue, 12 Jun 2018 16:34:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1528821275; bh=EzzfG0a8cu2r/pCtb2ro4nWK49y1X5ywdX3P6HHd85Y=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=EXURu2tjqcluA73MwGSzhl3UGrN0Qh3K+9N9V3OMxPO0Jo7gEI5iYWIfkaHt6CLNW /R+gP9okergUCM3dxXCvqX2E0XR5nMQGitjNLUvKdnVj5LPpH1InXqLFQ36+5jzfl8 I6yi63J+T6TlUf2nsQ0Frz2JhfaTRyWgju0J1RaI= Received: by mail-wm0-f50.google.com with SMTP id 69-v6so161744wmf.3 for ; Tue, 12 Jun 2018 09:34:34 -0700 (PDT) X-Gm-Message-State: APt69E0eNy8pHSZqJRuoQLUrVKFMJipaL0c+0uZVoHpXqC10rgRRzJSQ 1haAUUlpES2Srk1cnLHyE2/sDjVc4WX4AAUshJZZSw== X-Google-Smtp-Source: ADUXVKL98ubFXu0LpnRR7ZjC7wwdY0MyzBWVwTr3dYL+lGHZUzp2U6DqUD8BN9BI1wRrQRbARyKYZQq+Ob306zb8Zjg= X-Received: by 2002:a1c:f902:: with SMTP id x2-v6mr709578wmh.116.1528821273566; Tue, 12 Jun 2018 09:34:33 -0700 (PDT) MIME-Version: 1.0 References: <20180607143807.3611-1-yu-cheng.yu@intel.com> <20180607143807.3611-7-yu-cheng.yu@intel.com> <1528403417.5265.35.camel@2b52.sc.intel.com> In-Reply-To: From: Andy Lutomirski Date: Tue, 12 Jun 2018 09:34:21 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 06/10] x86/cet: Add arch_prctl functions for shadow stack To: "H. J. Lu" Cc: Andrew Lutomirski , Thomas Gleixner , Yu-cheng Yu , LKML , linux-doc@vger.kernel.org, Linux-MM , linux-arch , X86 ML , "H. Peter Anvin" , Ingo Molnar , "Shanbhogue, Vedvyas" , "Ravi V. Shankar" , Dave Hansen , Jonathan Corbet , Oleg Nesterov , Arnd Bergmann , mike.kravetz@oracle.com Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 12, 2018 at 9:05 AM H.J. Lu wrote: > > On Tue, Jun 12, 2018 at 9:01 AM, Andy Lutomirski wrote: > > On Tue, Jun 12, 2018 at 4:43 AM H.J. Lu wrote: > >> > >> On Tue, Jun 12, 2018 at 3:03 AM, Thomas Gleixner wrote: > >> > On Thu, 7 Jun 2018, H.J. Lu wrote: > >> >> On Thu, Jun 7, 2018 at 2:01 PM, Andy Lutomirski wrote: > >> >> > Why is the lockout necessary? If user code enables CET and tries to > >> >> > run code that doesn't support CET, it will crash. I don't see why we > >> >> > need special code in the kernel to prevent a user program from calling > >> >> > arch_prctl() and crashing itself. There are already plenty of ways to > >> >> > do that :) > >> >> > >> >> On CET enabled machine, not all programs nor shared libraries are > >> >> CET enabled. But since ld.so is CET enabled, all programs start > >> >> as CET enabled. ld.so will disable CET if a program or any of its shared > >> >> libraries aren't CET enabled. ld.so will lock up CET once it is done CET > >> >> checking so that CET can't no longer be disabled afterwards. > >> > > >> > That works for stuff which loads all libraries at start time, but what > >> > happens if the program uses dlopen() later on? If CET is force locked and > >> > the library is not CET enabled, it will fail. > >> > >> That is to prevent disabling CET by dlopening a legacy shared library. > >> > >> > I don't see the point of trying to support CET by magic. It adds complexity > >> > and you'll never be able to handle all corner cases correctly. dlopen() is > >> > not even a corner case. > >> > >> That is a price we pay for security. To enable CET, especially shadow > >> shack, the program and all of shared libraries it uses should be CET > >> enabled. Most of programs can be enabled with CET by compiling them > >> with -fcf-protection. > > > > If you charge too high a price for security, people may turn it off. > > I think we're going to need a mode where a program says "I want to use > > the CET, but turn it off if I dlopen an unsupported library". There > > are programs that load binary-only plugins. > > You can do > > # export GLIBC_TUNABLES=glibc.tune.hwcaps=-SHSTK > > which turns off shadow stack. > Which exactly illustrates my point. By making your security story too absolute, you'll force people to turn it off when they don't need to. If I'm using a fully CET-ified distro and I'm using a CET-aware program that loads binary plugins, and I may or may not have an old (binary-only, perhaps) plugin that doesn't support CET, then the behavior I want is for CET to be on until I dlopen() a program that doesn't support it. Unless there's some ABI reason why that can't be done, but I don't think there is. I'm concerned that the entire concept of locking CET is there to solve a security problem that doesn't actually exist.