From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp.codeaurora.org by pdx-caf-mail.web.codeaurora.org (Dovecot) with LMTP id B4aiOR25GVv7DQAAmS7hNA ; Thu, 07 Jun 2018 23:01:42 +0000 Received: by smtp.codeaurora.org (Postfix, from userid 1000) id 0E2D56089E; Thu, 7 Jun 2018 23:01:42 +0000 (UTC) Authentication-Results: smtp.codeaurora.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="cMLV4XiB" X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on pdx-caf-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.0 required=2.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,T_DKIMWL_WL_HIGH autolearn=unavailable autolearn_force=no version=3.4.0 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by smtp.codeaurora.org (Postfix) with ESMTP id 5A713601C3; Thu, 7 Jun 2018 23:01:41 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 5A713601C3 Authentication-Results: pdx-caf-mail.web.codeaurora.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: pdx-caf-mail.web.codeaurora.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752201AbeFGXBj (ORCPT + 25 others); Thu, 7 Jun 2018 19:01:39 -0400 Received: from mail.kernel.org ([198.145.29.99]:33810 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751990AbeFGXBh (ORCPT ); Thu, 7 Jun 2018 19:01:37 -0400 Received: from mail-wr0-f169.google.com (mail-wr0-f169.google.com [209.85.128.169]) (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 07F35208A3 for ; Thu, 7 Jun 2018 23:01:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1528412497; bh=RRA6TxHfG0MrgJ9oyYoyxLRdoG+OMFsQ5PeUsRf7u9I=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=cMLV4XiBhqD7KSaifHoP+0aX7NomKAGLZsl26zYMJEMS77Xd34/L9hVPGDo4mMtWU 7i055cByUOohu1CXkhFl4bcoTP1lG25OLMs16CGH4C5ib4j9xKQ/RDaSpb/5HyO09v EOrPw9qO08x/kUfJTDD9AnFd7ZRBP9HmXS1exaBk= Received: by mail-wr0-f169.google.com with SMTP id v13-v6so11453469wrp.13 for ; Thu, 07 Jun 2018 16:01:36 -0700 (PDT) X-Gm-Message-State: APt69E39KQi8NYwPP8G1puN3yBZgtt3DHfbzAA4FsNwqE0B3HZbamyvF KUi5QEtzb05SyCpwUjkeIaPQyGdsDanUNxC8yzXzEA== X-Google-Smtp-Source: ADUXVKJbqZw0ME+XMu/yadec1IJqxCNDDh20SIGnMaYhgvw7Ywk+vjb+UwvocuNMZF+fhcHpSoBCNwF5Tlc3q+F9dBM= X-Received: by 2002:adf:b1ca:: with SMTP id r10-v6mr2919455wra.221.1528412495562; Thu, 07 Jun 2018 16:01:35 -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 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 06/10] x86/cet: Add arch_prctl functions for shadow stack 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 Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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?)