From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f198.google.com (mail-pf0-f198.google.com [209.85.192.198]) by kanga.kvack.org (Postfix) with ESMTP id D3FF66B0003 for ; Thu, 7 Jun 2018 19:01:38 -0400 (EDT) Received: by mail-pf0-f198.google.com with SMTP id j25-v6so4068606pfi.9 for ; Thu, 07 Jun 2018 16:01:38 -0700 (PDT) Received: from mail.kernel.org (mail.kernel.org. [198.145.29.99]) by mx.google.com with ESMTPS id s1-v6si8908548pgb.486.2018.06.07.16.01.37 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 07 Jun 2018 16:01:37 -0700 (PDT) Received: from mail-wr0-f174.google.com (mail-wr0-f174.google.com [209.85.128.174]) (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 109F6208AF for ; Thu, 7 Jun 2018 23:01:37 +0000 (UTC) Received: by mail-wr0-f174.google.com with SMTP id o12-v6so11435003wrm.12 for ; Thu, 07 Jun 2018 16:01:36 -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: Thu, 7 Jun 2018 16:01:23 -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: Andrew Lutomirski , 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 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*. (Also, shouldn't the vDSO itself be marked as supporting CET?)