On Sat, Nov 03, 2018 at 01:11:22AM +0200, Jarkko Sakkinen wrote: > Documentation of the features of the Software Guard eXtensions used > by the Linux kernel and basic design choices for the core and driver > and functionality. > > Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx> > --- > Documentation/index.rst | 1 + > Documentation/x86/intel_sgx.rst | 185 ++++++++++++++++++++++++++++++++ > 2 files changed, 186 insertions(+) > create mode 100644 Documentation/x86/intel_sgx.rst > > diff --git a/Documentation/index.rst b/Documentation/index.rst > index 5db7e87c7cb1..1cdc139adb40 100644 > --- a/Documentation/index.rst > +++ b/Documentation/index.rst > @@ -104,6 +104,7 @@ implementation. > :maxdepth: 2 > > sh/index > + x86/index It seems there is no Documentation/x86/index.rst, probably you'd want to create one and link intel_sgx.rst there > > Filesystem Documentation > ------------------------ > diff --git a/Documentation/x86/intel_sgx.rst b/Documentation/x86/intel_sgx.rst > new file mode 100644 > index 000000000000..f6b7979c41f2 > --- /dev/null > +++ b/Documentation/x86/intel_sgx.rst > @@ -0,0 +1,185 @@ > +=================== > +Intel(R) SGX driver > +=================== > + > +Introduction > +============ > + > +Intel(R) SGX is a set of CPU instructions that can be used by applications to > +set aside private regions of code and data. The code outside the enclave is > +disallowed to access the memory inside the enclave by the CPU access control. > +In a way you can think that SGX provides inverted sandbox. It protects the > +application from a malicious host. > + > +You can tell if your CPU supports SGX by looking into ``/proc/cpuinfo``: > + > + ``cat /proc/cpuinfo | grep sgx`` > + > +Overview of SGX > +=============== > + > +SGX has a set of data structures to maintain information about the enclaves and > +their security properties. BIOS reserves a fixed size region of physical memory > +for these structures by setting Processor Reserved Memory Range Registers > +(PRMRR). > + > +This memory range is protected from outside access by the CPU and all the data > +coming in and out of the CPU package is encrypted by a key that is generated for > +each boot cycle. > + > +Enclaves execute in ring-3 in a special enclave submode using pages from the > +reserved memory range. A fixed logical address range for the enclave is reserved > +by ENCLS(ECREATE), a leaf instruction used to create enclaves. It is referred in > +the documentation commonly as the ELRANGE. > + > +Every memory access to the ELRANGE is asserted by the CPU. If the CPU is not > +executing in the enclave mode inside the enclave, #GP is raised. On the other > +hand enclave code can make memory accesses both inside and outside of the comma ^ > +ELRANGE. > + > +Enclave can only execute code inside the ELRANGE. Instructions that may cause > +VMEXIT, IO instructions and instructions that require a privilege change are > +prohibited inside the enclave. Interrupts and exceptions always cause enclave > +to exit and jump to an address outside the enclave given when the enclave is > +entered by using the leaf instruction ENCLS(EENTER). > + > +Data types > +---------- > + > +The protected memory range contains the following data: > + > +* **Enclave Page Cache (EPC):** protected pages > +* **Enclave Page Cache Map (EPCM):** a database that describes the state of the > + pages and link them to an enclave. I think it's better to use "definition list" here http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html#definition-lists > +EPC has a number of different types of pages: > + > +* **SGX Enclave Control Structure (SECS)**: describes the global > + properties of an enclave. > +* **Regular (REG):** code and data pages in the ELRANGE. > +* **Thread Control Structure (TCS):** pages that define entry points inside an > + enclave. The enclave can only be entered through these entry points and each > + can host a single hardware thread at a time. > +* **Version Array (VA)**: 64-bit version numbers for pages that have been > + swapped outside the enclave. Each page contains 512 version numbers. ditto > +Launch control > +-------------- > + > +To launch an enclave, two structures must be provided for ENCLS(EINIT): > + > +1. **SIGSTRUCT:** signed measurement of the enclave binary. > +2. **EINITTOKEN:** a cryptographic token CMAC-signed with a AES256-key called > + *launch key*, which is re-generated for each boot cycle. ditto > +The CPU holds a SHA256 hash of a 3072-bit RSA public key inside > +IA32_SGXLEPUBKEYHASHn MSRs. Enclaves with a SIGSTRUCT that is signed with this > +key do not require a valid EINITTOKEN and can be authorized with special > +privileges. One of those privileges is ability to acquire the launch key with > +ENCLS(EGETKEY). > + > +**IA32_FEATURE_CONTROL[17]** is used by the BIOS configure whether > +IA32_SGXLEPUBKEYHASH MSRs are read-only or read-write before locking the > +feature control register and handing over control to the operating system. > + > +Enclave construction > +-------------------- > + > +The construction is started by filling out the SECS that contains enclave > +address range, privileged attributes and measurement of TCS and REG pages (pages > +that will be mapped to the address range) among the other things. This structure > +is passed out to the ENCLS(ECREATE) together with a physical address of a page > +in EPC that will hold the SECS. > + > +The pages are added with ENCLS(EADD) and measured with ENCLS(EEXTEND) i.e. > +SHA256 hash MRENCLAVE residing in the SECS is extended with the page data. > + > +After all of the pages have been added, the enclave is initialized with > +ENCLS(EINIT). ENCLS(INIT) checks that the SIGSTRUCT is signed with the contained EINIT? > +public key. If the given EINITTOKEN has the valid bit set, the CPU checks that > +the token is valid (CMAC'd with the launch key). If the token is not valid, > +the CPU will check whether the enclave is signed with a key matching to the > +IA32_SGXLEPUBKEYHASHn MSRs. > + > +Swapping pages > +-------------- > + > +Enclave pages can be swapped out with ENCLS(EWB) to the unprotected memory. In > +addition to the EPC page, ENCLS(EWB) takes in a VA page and address for PCMD > +structure (Page Crypto MetaData) as input. The VA page will seal a version > +number for the page. PCMD is 128 byte structure that contains tracking > +information for the page, most importantly its MAC. With these structures the > +enclave is sealed and rollback protected while it resides in the unprotected > +memory. > + > +Before the page can be swapped out it must not have any active TLB references. > +ENCLS(EBLOCK) instruction moves a page to the *blocked* state, which means > +that no new TLB entries can be created to it by the hardware threads. > + > +After this a shootdown sequence is started with ENCLS(ETRACK), which sets an > +increased counter value to the entering hardware threads. ENCLS(EWB) will > +return SGX_NOT_TRACKED error while there are still threads with the earlier > +couner value because that means that there might be hardware thread inside > +the enclave with TLB entries to pages that are to be swapped. > + > +Kernel internals > +================ > + > +Requirements > +------------ > + > +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. > + > + > +Roles and responsibilities > +-------------------------- > + > +SGX introduces system resources, e.g. EPC memory, that must be accessible to > +multiple entities, e.g. the native kernel driver (to expose SGX to userspace) > +and KVM (to expose SGX to VMs), ideally without introducing any dependencies > +between each SGX entity. To that end, the kernel owns and manages the shared > +system resources, i.e. the EPC and Launch Control MSRs, and defines functions > +that provide appropriate access to the shared resources. SGX support for > +user space and VMs is left to the SGX platform driver and KVM respectively. > + > +Launching enclaves > +------------------ > + > +The current kernel implementation supports only unlocked MSRs i.e. > +FEATURE_CONTROL_SGX_LE_WR must be set. The launch is performed by setting the > +MSRs to the hash of the public key modulus of the enclave signer, which is one > +f the fields in the SIGSTRUCT. of > + > +EPC management > +-------------- > + > +Due to the unique requirements for swapping EPC pages, and because EPC pages > +(currently) do not have associated page structures, management of the EPC is > +not handled by the standard Linux swapper. SGX directly handles swapping > +of EPC pages, including a kthread to initiate reclaim and a rudimentary LRU > +mechanism. The consumers of EPC pages, e.g. the SGX driver, are required to > +implement function callbacks that can be invoked by the kernel to age, > +swap, and/or forcefully reclaim a target EPC page. In effect, the kernel > +controls what happens and when, while the consumers (driver, KVM, etc..) do > +the actual work. > + > +SGX uapi > +======== > + > +.. kernel-doc:: drivers/platform/x86/intel_sgx/sgx_ioctl.c > + :functions: sgx_ioc_enclave_create > + sgx_ioc_enclave_add_page > + sgx_ioc_enclave_init > + > +.. kernel-doc:: arch/x86/include/uapi/asm/sgx.h > + > +References > +========== > + > +* System Programming Manual: 39.1.4 Intel® SGX Launch Control Configuration > -- > 2.19.1 > -- Sincerely yours, Mike.