From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 11EAFC433DF for ; Tue, 16 Jun 2020 14:40:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id ECFF420786 for ; Tue, 16 Jun 2020 14:40:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729273AbgFPOk2 (ORCPT ); Tue, 16 Jun 2020 10:40:28 -0400 Received: from mga04.intel.com ([192.55.52.120]:22009 "EHLO mga04.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728448AbgFPOk1 (ORCPT ); Tue, 16 Jun 2020 10:40:27 -0400 IronPort-SDR: DN6jn5jKeblUYcoyTfTUi6pBgSxdvFtp3lPWwv7xpnPQVwBZ1TrdH+YO6KLTfEtQxkK8BvxU93 CCmhJI6fI/0Q== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga006.jf.intel.com ([10.7.209.51]) by fmsmga104.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 16 Jun 2020 07:40:21 -0700 IronPort-SDR: qRAAkVPgstsP4BiNwiJ4ZWhpOs0JhFEgky588U/lbruil5tOAuN+ZQw758XauRakiPDe+wl0vn 2O84ffZConlw== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.73,518,1583222400"; d="scan'208";a="276937447" Received: from osowole-mobl.amr.corp.intel.com (HELO [10.255.5.242]) ([10.255.5.242]) by orsmga006.jf.intel.com with ESMTP; 16 Jun 2020 07:40:17 -0700 Subject: Re: [PATCH 4/8] seccomp: Implement constant action bitmaps To: Kees Cook , linux-kernel@vger.kernel.org Cc: Christian Brauner , Sargun Dhillon , Tycho Andersen , Jann Horn , "zhujianwei (C)" , Dave Hansen , Matthew Wilcox , Andy Lutomirski , Will Drewry , Shuah Khan , Matt Denton , Chris Palmer , Jeffrey Vander Stoep , Aleksa Sarai , Hehuazhen , x86@kernel.org, Linux Containers , linux-security-module@vger.kernel.org, linux-api@vger.kernel.org References: <20200616074934.1600036-1-keescook@chromium.org> <20200616074934.1600036-5-keescook@chromium.org> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; keydata= xsFNBE6HMP0BEADIMA3XYkQfF3dwHlj58Yjsc4E5y5G67cfbt8dvaUq2fx1lR0K9h1bOI6fC oAiUXvGAOxPDsB/P6UEOISPpLl5IuYsSwAeZGkdQ5g6m1xq7AlDJQZddhr/1DC/nMVa/2BoY 2UnKuZuSBu7lgOE193+7Uks3416N2hTkyKUSNkduyoZ9F5twiBhxPJwPtn/wnch6n5RsoXsb ygOEDxLEsSk/7eyFycjE+btUtAWZtx+HseyaGfqkZK0Z9bT1lsaHecmB203xShwCPT49Blxz VOab8668QpaEOdLGhtvrVYVK7x4skyT3nGWcgDCl5/Vp3TWA4K+IofwvXzX2ON/Mj7aQwf5W iC+3nWC7q0uxKwwsddJ0Nu+dpA/UORQWa1NiAftEoSpk5+nUUi0WE+5DRm0H+TXKBWMGNCFn c6+EKg5zQaa8KqymHcOrSXNPmzJuXvDQ8uj2J8XuzCZfK4uy1+YdIr0yyEMI7mdh4KX50LO1 pmowEqDh7dLShTOif/7UtQYrzYq9cPnjU2ZW4qd5Qz2joSGTG9eCXLz5PRe5SqHxv6ljk8mb ApNuY7bOXO/A7T2j5RwXIlcmssqIjBcxsRRoIbpCwWWGjkYjzYCjgsNFL6rt4OL11OUF37wL QcTl7fbCGv53KfKPdYD5hcbguLKi/aCccJK18ZwNjFhqr4MliQARAQABzShEYXZpZCBDaHJp c3RvcGhlciBIYW5zZW4gPGRhdmVAc3I3MS5uZXQ+wsF7BBMBAgAlAhsDBgsJCAcDAgYVCAIJ CgsEFgIDAQIeAQIXgAUCTo3k0QIZAQAKCRBoNZUwcMmSsMO2D/421Xg8pimb9mPzM5N7khT0 2MCnaGssU1T59YPE25kYdx2HntwdO0JA27Wn9xx5zYijOe6B21ufrvsyv42auCO85+oFJWfE K2R/IpLle09GDx5tcEmMAHX6KSxpHmGuJmUPibHVbfep2aCh9lKaDqQR07gXXWK5/yU1Dx0r VVFRaHTasp9fZ9AmY4K9/BSA3VkQ8v3OrxNty3OdsrmTTzO91YszpdbjjEFZK53zXy6tUD2d e1i0kBBS6NLAAsqEtneplz88T/v7MpLmpY30N9gQU3QyRC50jJ7LU9RazMjUQY1WohVsR56d ORqFxS8ChhyJs7BI34vQusYHDTp6PnZHUppb9WIzjeWlC7Jc8lSBDlEWodmqQQgp5+6AfhTD kDv1a+W5+ncq+Uo63WHRiCPuyt4di4/0zo28RVcjtzlGBZtmz2EIC3vUfmoZbO/Gn6EKbYAn rzz3iU/JWV8DwQ+sZSGu0HmvYMt6t5SmqWQo/hyHtA7uF5Wxtu1lCgolSQw4t49ZuOyOnQi5 f8R3nE7lpVCSF1TT+h8kMvFPv3VG7KunyjHr3sEptYxQs4VRxqeirSuyBv1TyxT+LdTm6j4a mulOWf+YtFRAgIYyyN5YOepDEBv4LUM8Tz98lZiNMlFyRMNrsLV6Pv6SxhrMxbT6TNVS5D+6 UorTLotDZKp5+M7BTQRUY85qARAAsgMW71BIXRgxjYNCYQ3Xs8k3TfAvQRbHccky50h99TUY sqdULbsb3KhmY29raw1bgmyM0a4DGS1YKN7qazCDsdQlxIJp9t2YYdBKXVRzPCCsfWe1dK/q 66UVhRPP8EGZ4CmFYuPTxqGY+dGRInxCeap/xzbKdvmPm01Iw3YFjAE4PQ4hTMr/H76KoDbD cq62U50oKC83ca/PRRh2QqEqACvIH4BR7jueAZSPEDnzwxvVgzyeuhwqHY05QRK/wsKuhq7s UuYtmN92Fasbxbw2tbVLZfoidklikvZAmotg0dwcFTjSRGEg0Gr3p/xBzJWNavFZZ95Rj7Et db0lCt0HDSY5q4GMR+SrFbH+jzUY/ZqfGdZCBqo0cdPPp58krVgtIGR+ja2Mkva6ah94/oQN lnCOw3udS+Eb/aRcM6detZr7XOngvxsWolBrhwTQFT9D2NH6ryAuvKd6yyAFt3/e7r+HHtkU kOy27D7IpjngqP+b4EumELI/NxPgIqT69PQmo9IZaI/oRaKorYnDaZrMXViqDrFdD37XELwQ gmLoSm2VfbOYY7fap/AhPOgOYOSqg3/Nxcapv71yoBzRRxOc4FxmZ65mn+q3rEM27yRztBW9 AnCKIc66T2i92HqXCw6AgoBJRjBkI3QnEkPgohQkZdAb8o9WGVKpfmZKbYBo4pEAEQEAAcLB XwQYAQIACQUCVGPOagIbDAAKCRBoNZUwcMmSsJeCEACCh7P/aaOLKWQxcnw47p4phIVR6pVL e4IEdR7Jf7ZL00s3vKSNT+nRqdl1ugJx9Ymsp8kXKMk9GSfmZpuMQB9c6io1qZc6nW/3TtvK pNGz7KPPtaDzvKA4S5tfrWPnDr7n15AU5vsIZvgMjU42gkbemkjJwP0B1RkifIK60yQqAAlT YZ14P0dIPdIPIlfEPiAWcg5BtLQU4Wg3cNQdpWrCJ1E3m/RIlXy/2Y3YOVVohfSy+4kvvYU3 lXUdPb04UPw4VWwjcVZPg7cgR7Izion61bGHqVqURgSALt2yvHl7cr68NYoFkzbNsGsye9ft M9ozM23JSgMkRylPSXTeh5JIK9pz2+etco3AfLCKtaRVysjvpysukmWMTrx8QnI5Nn5MOlJj 1Ov4/50JY9pXzgIDVSrgy6LYSMc4vKZ3QfCY7ipLRORyalFDF3j5AGCMRENJjHPD6O7bl3Xo 4DzMID+8eucbXxKiNEbs21IqBZbbKdY1GkcEGTE7AnkA3Y6YB7I/j9mQ3hCgm5muJuhM/2Fr OPsw5tV/LmQ5GXH0JQ/TZXWygyRFyyI2FqNTx4WHqUn3yFj8rwTAU1tluRUYyeLy0ayUlKBH ybj0N71vWO936MqP6haFERzuPAIpxj2ezwu0xb1GjTk4ynna6h5GjnKgdfOWoRtoWndMZxbA z5cecg== Message-ID: Date: Tue, 16 Jun 2020 07:40:17 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: <20200616074934.1600036-5-keescook@chromium.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/16/20 12:49 AM, Kees Cook wrote: > + /* Mark the second page as untouched (i.e. "old") */ > + preempt_disable(); > + set_pte_at(&init_mm, vaddr, ptep, pte_mkold(*(READ_ONCE(ptep)))); > + local_flush_tlb_kernel_range(vaddr, vaddr + PAGE_SIZE); > + preempt_enable(); If you can, I'd wrap that nugget up in a helper. I'd also suggest being very explicit in a comment about what it is trying to do: ensure no TLB entries exist so that a future access will always set the Accessed bit. > + /* Make sure the PTE agrees that it is untouched. */ > + if (WARN_ON_ONCE(sd_touched(ptep))) > + return; > + /* Read a portion of struct seccomp_data from the second page. */ > + check = sd->instruction_pointer; > + /* First, verify the contents are zero from vzalloc(). */ > + if (WARN_ON_ONCE(check)) > + return; > + /* Now make sure the ACCESSED bit has been set after the read. */ > + if (!sd_touched(ptep)) { > + /* > + * If autodetection fails, fall back to standard beahavior by > + * clearing the entire "allow" bitmap. > + */ > + pr_warn_once("seccomp: cannot build automatic syscall filters\n"); > + bitmap_zero(bitmaps->allow, NR_syscalls); > + return; > + } I can't find any big holes with this. It's the kind of code that makes me nervous, but mostly because it's pretty different that anything else we have in the kernel. It's also clear to me here that you probably have a slightly different expectation of what the PTE accessed flag means versus the hardware guys. What you are looking for it to mean is roughly: "a retired instruction touched this page". The hardware guys would probably say it's closer to "a TLB entry was established for this page." Remember that TLB entries can be established speculatively or from things like prefetchers. While I don't know of anything microarchitectural today which would trip this mechanism, it's entirely possible that something in the future might. Accessing close to the page boundary is the exact kind of place folks might want to optimize. *But*, at least it would err in the direction of being conservative. It would say "somebody touched the page!" more often than it should, but never _less_ often than it should. One thing about the implementation (which is roughly): // Touch the data: check = sd->instruction_pointer; // Examine the PTE mapping that data: if (!sd_touched(ptep)) { // something } There aren't any barriers in there, which could lead to the sd_touched() check being ordered before the data touch. I think a rmb() will suffice. You could even do it inside sd_touched(). Was there a reason you chose to export a ranged TLB flush? I probably would have just used the single-page flush_tlb_one_kernel() for this purpose if I were working in arch-specific code.