From: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> 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, puiterwijk@redhat.com, linux-doc@vger.kernel.org, Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> Subject: [PATCH v24 24/24] docs: x86/sgx: Document kernel internals Date: Sat, 30 Nov 2019 01:13:26 +0200 Message-ID: <20191129231326.18076-25-jarkko.sakkinen@linux.intel.com> (raw) In-Reply-To: <20191129231326.18076-1-jarkko.sakkinen@linux.intel.com> From: Sean Christopherson <sean.j.christopherson@intel.com> Document some of the more tricky parts of the kernel implementation internals. Cc: linux-doc@vger.kernel.org Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> Co-developed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com> --- Documentation/x86/sgx/2.Kernel-internals.rst | 78 ++++++++++++++++++++ Documentation/x86/sgx/index.rst | 1 + 2 files changed, 79 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..7bfd5cb19b8e --- /dev/null +++ b/Documentation/x86/sgx/2.Kernel-internals.rst @@ -0,0 +1,78 @@ +.. 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 force 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 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 the than amount of available EPC by providing +a subset of leaf instructions 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. + +If the MSRs were read-only, the platform would need to provide a launch enclave +(LE), which would be signed with the key matching the MSRs. The LE creates +cryptographic tokens for other enclaves that they can pass together with their +signature to the ENCLS(EINIT) opcode, which is used to initialize enclaves. + +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 legitimate enclave. This could be used by a malware network to ensure +that its nodes are running inside legitimate 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
prev parent reply index Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-11-29 23:13 [PATCH v24 00/24] Intel SGX foundations Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 01/24] x86/sgx: Update MAINTAINERS Jarkko Sakkinen 2019-11-30 1:38 ` Neil Horman 2019-11-30 14:37 ` Dr. Greg 2019-12-02 9:24 ` Jethro Beekman 2019-12-06 21:23 ` Jarkko Sakkinen 2019-12-06 21:24 ` Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 02/24] x86/cpufeatures: x86/msr: Add Intel SGX hardware bits Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 03/24] x86/cpufeatures: x86/msr: Intel SGX Launch Control " Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 04/24] x86/mm: x86/sgx: Signal SIGSEGV with PF_SGX Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 05/24] x86/sgx: Add SGX microarchitectural data structures Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 06/24] x86/sgx: Add wrappers for ENCLS leaf functions Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 07/24] x86/cpu/intel: Detect SGX supprt Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 08/24] x86/sgx: Enumerate and track EPC sections Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 09/24] x86/sgx: Add functions to allocate and free EPC pages Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 10/24] x86/sgx: Add sgx_einit() for wrapping ENCLS[EINIT] Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 11/24] mm: Introduce vm_ops->may_mprotect() Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 12/24] x86/sgx: Linux Enclave Driver Jarkko Sakkinen 2019-12-02 15:48 ` Haitao Huang 2019-12-02 18:21 ` Sean Christopherson 2019-12-09 19:38 ` Jarkko Sakkinen 2019-12-09 19:08 ` Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 13/24] selftests/x86: Recurse into subdirectories Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 14/24] selftests/x86: Add a selftest for SGX Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 15/24] x86/sgx: Add provisioning Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 16/24] x86/sgx: Add a page reclaimer Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 17/24] x86/sgx: ptrace() support for the SGX driver Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 18/24] x86/vdso: Add support for exception fixup in vDSO functions Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 19/24] x86/fault: Add helper function to sanitize error code Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 20/24] x86/traps: Attempt to fixup exceptions in vDSO before signaling Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 21/24] x86/vdso: Add __vdso_sgx_enter_enclave() to wrap SGX enclave transitions Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 22/24] selftests/x86: Add vDSO selftest for SGX Jarkko Sakkinen 2019-11-29 23:13 ` [PATCH v24 23/24] docs: x86/sgx: Document microarchitecture Jarkko Sakkinen 2019-11-29 23:13 ` Jarkko Sakkinen [this message]
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