From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f71.google.com (mail-pg0-f71.google.com [74.125.83.71]) by kanga.kvack.org (Postfix) with ESMTP id 141BE6B0005 for ; Tue, 12 Jun 2018 12:34:37 -0400 (EDT) Received: by mail-pg0-f71.google.com with SMTP id e2-v6so7967869pgq.4 for ; Tue, 12 Jun 2018 09:34:37 -0700 (PDT) Received: from mail.kernel.org (mail.kernel.org. [198.145.29.99]) by mx.google.com with ESMTPS id d2-v6si415351pge.342.2018.06.12.09.34.35 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 12 Jun 2018 09:34:35 -0700 (PDT) Received: from mail-wm0-f49.google.com (mail-wm0-f49.google.com [74.125.82.49]) (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 1A317208BA for ; Tue, 12 Jun 2018 16:34:35 +0000 (UTC) Received: by mail-wm0-f49.google.com with SMTP id j15-v6so200929wme.0 for ; Tue, 12 Jun 2018 09:34:35 -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 Message-ID: Subject: Re: [PATCH 06/10] x86/cet: Add arch_prctl functions for shadow stack Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: 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 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.