From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f200.google.com (mail-pf0-f200.google.com [209.85.192.200]) by kanga.kvack.org (Postfix) with ESMTP id DF5A76B0006 for ; Thu, 7 Jun 2018 17:01:33 -0400 (EDT) Received: by mail-pf0-f200.google.com with SMTP id a13-v6so5037650pfo.22 for ; Thu, 07 Jun 2018 14:01:33 -0700 (PDT) Received: from mail.kernel.org (mail.kernel.org. [198.145.29.99]) by mx.google.com with ESMTPS id i33-v6si53514444pld.546.2018.06.07.14.01.32 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 07 Jun 2018 14:01:32 -0700 (PDT) Received: from mail-wm0-f48.google.com (mail-wm0-f48.google.com [74.125.82.48]) (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 439CF2088F for ; Thu, 7 Jun 2018 21:01:32 +0000 (UTC) Received: by mail-wm0-f48.google.com with SMTP id v16-v6so20429435wmh.5 for ; Thu, 07 Jun 2018 14:01:32 -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: <1528403417.5265.35.camel@2b52.sc.intel.com> From: Andy Lutomirski Date: Thu, 7 Jun 2018 14:01:18 -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: Yu-cheng Yu Cc: Andrew Lutomirski , LKML , linux-doc@vger.kernel.org, Linux-MM , linux-arch , X86 ML , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , "H. J. Lu" , "Shanbhogue, Vedvyas" , "Ravi V. Shankar" , Dave Hansen , Jonathan Corbet , Oleg Nesterov , Arnd Bergmann , mike.kravetz@oracle.com On Thu, Jun 7, 2018 at 1:33 PM Yu-cheng Yu wrote: > > On Thu, 2018-06-07 at 11:48 -0700, Andy Lutomirski wrote: > > On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu wrote: > > > > > > The following operations are provided. > > > > > > ARCH_CET_STATUS: > > > return the current CET status > > > > > > ARCH_CET_DISABLE: > > > disable CET features > > > > > > ARCH_CET_LOCK: > > > lock out CET features > > > > > > ARCH_CET_EXEC: > > > set CET features for exec() > > > > > > ARCH_CET_ALLOC_SHSTK: > > > allocate a new shadow stack > > > > > > ARCH_CET_PUSH_SHSTK: > > > put a return address on shadow stack > > > > > > ARCH_CET_ALLOC_SHSTK and ARCH_CET_PUSH_SHSTK are intended only for > > > the implementation of GLIBC ucontext related APIs. > > > > Please document exactly what these all do and why. I don't understand > > what purpose ARCH_CET_LOCK and ARCH_CET_EXEC serve. CET is opt in for > > each ELF program, so I think there should be no need for a magic > > override. > > CET is initially enabled if the loader has CET capability. Then the > loader decides if the application can run with CET. If the application > cannot run with CET (e.g. a dependent library does not have CET), then > the loader turns off CET before passing to the application. When the > loader is done, it locks out CET and the feature cannot be turned off > anymore until the next exec() call. 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 :) > When the next exec() is called, CET > feature is turned on/off based on the values set by ARCH_CET_EXEC. And why do we need ARCH_CET_EXEC? For background, I really really dislike adding new state that persists across exec(). It's nice to get as close to a clean slate as possible after exec() so that programs can run in a predictable environment. exec() is also a security boundary, and anything a task can do to affect itself after exec() needs to have its security implications considered very carefully. (As a trivial example, you should not be able to use cetcmd ... sudo [malicious options here] to cause sudo to run with CET off and then try to exploit it via the malicious options. If a shutoff is needed for testing, how about teaching ld.so to parse LD_CET=no or similar and protect it the same way as LD_PRELOAD is protected. Or just do LD_PRELOAD=/lib/libdoesntsupportcet.so. --Andy