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=-9.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_GIT autolearn=unavailable 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 849E1C3A5A7 for ; Tue, 3 Sep 2019 14:31:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5DEA723697 for ; Tue, 3 Sep 2019 14:31:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729841AbfICObE (ORCPT ); Tue, 3 Sep 2019 10:31:04 -0400 Received: from mga09.intel.com ([134.134.136.24]:34539 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727107AbfICObE (ORCPT ); Tue, 3 Sep 2019 10:31:04 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga003.jf.intel.com ([10.7.209.27]) by orsmga102.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 03 Sep 2019 07:31:03 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.64,463,1559545200"; d="scan'208";a="184768017" Received: from vkuppusa-mobl2.ger.corp.intel.com (HELO localhost) ([10.252.39.67]) by orsmga003.jf.intel.com with ESMTP; 03 Sep 2019 07:30:54 -0700 From: Jarkko Sakkinen To: linux-kernel@vger.kernel.org, x86@kernel.org, linux-sgx@vger.kernel.org Cc: akpm@linux-foundation.org, dave.hansen@intel.com, sean.j.christopherson@intel.com, nhorman@redhat.com, npmccallum@redhat.com, serge.ayoun@intel.com, shay.katz-zamir@intel.com, haitao.huang@intel.com, andriy.shevchenko@linux.intel.com, tglx@linutronix.de, kai.svahn@intel.com, bp@alien8.de, josh@joshtriplett.org, luto@kernel.org, kai.huang@intel.com, rientjes@google.com, cedric.xing@intel.com, Jarkko Sakkinen Subject: [PATCH v22 24/24] docs: x86/sgx: Document kernel internals Date: Tue, 3 Sep 2019 17:26:55 +0300 Message-Id: <20190903142655.21943-25-jarkko.sakkinen@linux.intel.com> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20190903142655.21943-1-jarkko.sakkinen@linux.intel.com> References: <20190903142655.21943-1-jarkko.sakkinen@linux.intel.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Sean Christopherson Document some of the more tricky parts of the kernel implementation internals. Signed-off-by: Sean Christopherson Co-developed-by: Jarkko Sakkinen Signed-off-by: Jarkko Sakkinen --- Documentation/x86/sgx/2.Kernel-internals.rst | 76 ++++++++++++++++++++ Documentation/x86/sgx/index.rst | 1 + 2 files changed, 77 insertions(+) create mode 100644 Documentation/x86/sgx/2.Kernel-internals.rst diff --git a/Documentation/x86/sgx/2.Kernel-internals.rst b/Documentation/x86/sgx/2.Kernel-internals.rst new file mode 100644 index 000000000000..5c90a65936f2 --- /dev/null +++ b/Documentation/x86/sgx/2.Kernel-internals.rst @@ -0,0 +1,76 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================ +Kernel Internals +================ + +CPU configuration +================= + +Because SGX has an ever evolving and expanding feature set, it's possible for +a BIOS or VMM to configure a system in such a way that not all CPUs are equal, +e.g. where Launch Control is only enabled on a subset of CPUs. Linux does +*not* support such a heterogeneous system configuration, nor does it even +attempt to play nice in the face of a misconfigured system. With the exception +of Launch Control's hash MSRs, which can vary per CPU, Linux assumes that all +CPUs have a configuration that is identical to the boot CPU. + +EPC management +============== + +Because the kernel can't arbitrarily read EPC memory or share RO backing pages +between enclaves, traditional memory models such as CoW and fork() do not work +with enclaves. In other words, the architectural rules of EPC forces it to be +treated as MAP_SHARED at all times. + +The inability to employ traditional memory models also means that EPC memory +must be isolated from normal memory pools, e.g. attempting to use EPC memory +for normal mappings would result in faults and/or perceived data corruption. +Furthermore, EPC is not enumerated by as normal memory, e.g. BIOS enumerates +EPC as reserved memory in the e820 tables, or not at all. As a result, EPC +memory is directly managed by the SGX subsystem, e.g. SGX employs VM_PFNMAP to +manually insert/zap/swap page table entries, and exposes EPC to userspace via +a well known device, /dev/sgx/enclave. + +The net effect is that all enclave VMAs must be MAP_SHARED and are backed by +a single file, /dev/sgx/enclave. + +EPC oversubscription +==================== + +SGX allows to have larger enclaves than amount of available EPC by providing a +subset of leaf instruction for swapping EPC pages to the system memory. The +details of these instructions are discussed in the architecture document. Due +to the unique requirements for swapping EPC pages, and because EPC pages do not +have associated page structures, management of the EPC is not handled by the +standard memory subsystem. + +SGX directly handles swapping of EPC pages, including a thread to initiate the +reclaiming process and a rudimentary LRU mechanism. When the amount of free EPC +pages goes below a low watermark the swapping thread starts reclaiming pages. +The pages that have not been recently accessed (i.e. do not have the A bit set) +are selected as victim pages. Each enclave holds an shmem file as a backing +storage for reclaimed pages. + +Launch Control +============== + +The current kernel implementation supports only writable MSRs. The launch is +performed by setting the MSRs to the hash of the public key modulus of the +enclave signer and a token with the valid bit set to zero. Because kernel makes +ultimately all the launch decisions token are not needed for anything. We +don't need or have a launch enclave for generating them as the MSRs must always +be writable. + +Provisioning +============ + +The use of provisioning must be controlled because it allows to get access to +the provisioning keys to attest to a remote party that the software is running +inside a legit enclave. This could be used by a malware network to ensure that +its nodes are running inside legit enclaves. + +The driver introduces a special device file /dev/sgx/provision and a special +ioctl SGX_IOC_ENCLAVE_SET_ATTRIBUTE to accomplish this. A file descriptor +pointing to /dev/sgx/provision is passed to ioctl from which kernel authorizes +the PROVISION_KEY attribute to the enclave. diff --git a/Documentation/x86/sgx/index.rst b/Documentation/x86/sgx/index.rst index c5dfef62e612..5d660e83d984 100644 --- a/Documentation/x86/sgx/index.rst +++ b/Documentation/x86/sgx/index.rst @@ -14,3 +14,4 @@ potentially malicious. :maxdepth: 1 1.Architecture + 2.Kernel-internals -- 2.20.1