From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot0-f198.google.com (mail-ot0-f198.google.com [74.125.82.198]) by kanga.kvack.org (Postfix) with ESMTP id 749FB6B0007 for ; Fri, 8 Jun 2018 00:10:00 -0400 (EDT) Received: by mail-ot0-f198.google.com with SMTP id r58-v6so7875822otr.0 for ; Thu, 07 Jun 2018 21:10:00 -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 i25-v6sor7071722otc.94.2018.06.07.21.09.59 for (Google Transport Security); Thu, 07 Jun 2018 21:09:59 -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 21:09:58 -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 4:01 PM, Andy Lutomirski wrote: > On Thu, Jun 7, 2018 at 3:02 PM H.J. Lu wrote: >> >> 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. > > Yeah, I got that. No one has explained *why*. It is to prevent malicious code from disabling CET. > (Also, shouldn't the vDSO itself be marked as supporting CET?) No. vDSO is loaded by kernel. vDSO in CET kernel is CET compatible. -- H.J.