From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot0-f197.google.com (mail-ot0-f197.google.com [74.125.82.197]) by kanga.kvack.org (Postfix) with ESMTP id ED6336B0003 for ; Thu, 7 Jun 2018 18:02:47 -0400 (EDT) Received: by mail-ot0-f197.google.com with SMTP id b1-v6so7117684otf.22 for ; Thu, 07 Jun 2018 15:02:47 -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 z9-v6sor863904oig.132.2018.06.07.15.02.46 for (Google Transport Security); Thu, 07 Jun 2018 15:02:46 -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: Thu, 7 Jun 2018 15:02:44 -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: Yu-cheng Yu , LKML , linux-doc@vger.kernel.org, Linux-MM , linux-arch , X86 ML , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , "Shanbhogue, Vedvyas" , "Ravi V. Shankar" , Dave Hansen , Jonathan Corbet , Oleg Nesterov , Arnd Bergmann , mike.kravetz@oracle.com On Thu, Jun 7, 2018 at 2:01 PM, Andy Lutomirski wrote: > 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 :) 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. >> 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. > I will take a look. -- H.J.