From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f69.google.com (mail-pg0-f69.google.com [74.125.83.69]) by kanga.kvack.org (Postfix) with ESMTP id 2CD846B000C for ; Tue, 12 Jun 2018 12:02:05 -0400 (EDT) Received: by mail-pg0-f69.google.com with SMTP id r8-v6so7241681pgq.2 for ; Tue, 12 Jun 2018 09:02:05 -0700 (PDT) Received: from mail.kernel.org (mail.kernel.org. [198.145.29.99]) by mx.google.com with ESMTPS id h8-v6si444643pls.69.2018.06.12.09.02.03 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 12 Jun 2018 09:02:03 -0700 (PDT) Received: from mail-wm0-f47.google.com (mail-wm0-f47.google.com [74.125.82.47]) (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 1F53A208CB for ; Tue, 12 Jun 2018 16:02:03 +0000 (UTC) Received: by mail-wm0-f47.google.com with SMTP id v16-v6so21575501wmh.5 for ; Tue, 12 Jun 2018 09:02:03 -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 09:01:48 -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: Thomas Gleixner , Andrew Lutomirski , 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 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.