From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot0-f200.google.com (mail-ot0-f200.google.com [74.125.82.200]) by kanga.kvack.org (Postfix) with ESMTP id C08B86B0005 for ; Tue, 12 Jun 2018 12:51:57 -0400 (EDT) Received: by mail-ot0-f200.google.com with SMTP id r26-v6so9023717otk.17 for ; Tue, 12 Jun 2018 09:51:57 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id y4-v6sor279707oty.159.2018.06.12.09.51.56 for (Google Transport Security); Tue, 12 Jun 2018 09:51:56 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: 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> From: "H.J. Lu" Date: Tue, 12 Jun 2018 09:51:55 -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: Andy Lutomirski Cc: 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 9:34 AM, Andy Lutomirski wrote: > 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. We can make it opt-in via GLIBC_TUNABLES. But by default, the legacy shared object is disallowed when CET is enabled. > I'm concerned that the entire concept of locking CET is there to solve > a security problem that doesn't actually exist. We don't know that. -- H.J.