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 Co-developed-by: Sean Christopherson Signed-off-by: Sean Christopherson --- Documentation/index.rst | 1 + Documentation/x86/index.rst | 8 ++ Documentation/x86/intel_sgx.rst | 233 ++++++++++++++++++++++++++++++++ 3 files changed, 242 insertions(+) create mode 100644 Documentation/x86/index.rst create mode 100644 Documentation/x86/intel_sgx.rst diff --git a/Documentation/index.rst b/Documentation/index.rst index c858c2e66e36..63864826dcd6 100644 --- a/Documentation/index.rst +++ b/Documentation/index.rst @@ -101,6 +101,7 @@ implementation. :maxdepth: 2 sh/index + x86/index Filesystem Documentation ------------------------ diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst new file mode 100644 index 000000000000..11d5b18d9537 --- /dev/null +++ b/Documentation/x86/index.rst @@ -0,0 +1,8 @@ +====================== +x86 Architecture Guide +====================== + +.. toctree:: + :maxdepth: 2 + + intel_sgx diff --git a/Documentation/x86/intel_sgx.rst b/Documentation/x86/intel_sgx.rst new file mode 100644 index 000000000000..f51b43f9e125 --- /dev/null +++ b/Documentation/x86/intel_sgx.rst @@ -0,0 +1,233 @@ +=================== +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 +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). + +Protected memory +---------------- + +Enclave Page Cache (EPC) + Physical pages used with enclaves that are protected by the CPU from + unauthorized access. + +Enclave Page Cache Map (EPCM) + A database that describes the properties and state of the pages e.g. their + permissions or to which enclave they belong to. + +Memory Encryption Engine (MEE) integrity tree + Autonomously updated integrity tree. The root of the tree located in on-die + SRAM. + +EPC data types +-------------- + +SGX Enclave Control Structure (SECS) + Describes the global properties of an enclave. Will not be mapped to the + ELRANGE. + +Regular (REG) + These pages contain code and data. + +Thread Control Structure (TCS) + The pages that define the entry points inside an enclave. An enclave can + only be entered through these entry points and each can host a single + hardware thread at a time. + +Version Array (VA) + The pages contain 64-bit version numbers for pages that have been swapped + outside the enclave. Each page has the capacity of 512 version numbers. + +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. + +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). It will check that the SIGSTRUCT is signed with the contained +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 +of the fields in the SIGSTRUCT. + +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. + +Exception handling +------------------ + +The PF_SGX bit is set if and only if the #PF is detected by the SGX Enclave Page +Cache Map (EPCM). The EPCM is a hardware-managed table that enforces accesses to +an enclave's EPC pages in addition to the software-managed kernel page tables, +i.e. the effective permissions for an EPC page are a logical AND of the kernel's +page tables and the corresponding EPCM entry. + +The EPCM is consulted only after an access walks the kernel's page tables, i.e.: + +1. the access was allowed by the kernel +2. the kernel's tables have become less restrictive than the EPCM +3. the kernel cannot fixup the cause of the fault + +Noteably, (2) implies that either the kernel has botched the EPC mappings or the +EPCM has been invalidated (see below). Regardless of why the fault occurred, +userspace needs to be alerted so that it can take appropriate action, e.g. +restart the enclave. This is reinforced by (3) as the kernel doesn't really +have any other reasonable option, i.e. signalling SIGSEGV is actually the least +severe action possible. + +Although the primary purpose of the EPCM is to prevent a malicious or +compromised kernel from attacking an enclave, e.g. by modifying the enclave's +page tables, do not WARN on a #PF w/ PF_SGX set. The SGX architecture +effectively allows the CPU to invalidate all EPCM entries at will and requires +that software be prepared to handle an EPCM fault at any time. The architecture +defines this behavior because the EPCM is encrypted with an ephemeral key that +isn't exposed to software. As such, the EPCM entries cannot be preserved across +transitions that result in a new key being used, e.g. CPU power down as part of +an S3 transition or when a VM is live migrated to a new physical system. + +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 +========== + +* A Memory Encryption Engine Suitable for General Purpose Processors + +* System Programming Manual: 39.1.4 Intel® SGX Launch Control Configuration -- 2.19.1