[PATCH v4 09/12] intel_sgx: driver documentation

[Jarkko Sakkinen]

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
---
 Documentation/index.rst         |   1 +
 Documentation/x86/intel_sgx.rst | 131 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 132 insertions(+)
 create mode 100644 Documentation/x86/intel_sgx.rst

diff --git a/Documentation/index.rst b/Documentation/index.rst
index cb7f1ba5b3b1..ccfebc260e04 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -86,6 +86,7 @@ implementation.
    :maxdepth: 2
 
    sh/index
+   x86/index
 
 Korean translations
 -------------------
diff --git a/Documentation/x86/intel_sgx.rst b/Documentation/x86/intel_sgx.rst
new file mode 100644
index 000000000000..ee7fe9487d7b
--- /dev/null
+++ b/Documentation/x86/intel_sgx.rst
@@ -0,0 +1,131 @@
+===================
+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.
+
+There is a new hardware unit in the processor called Memory Encryption Engine
+(MEE) starting from the Skylake microachitecture. BIOS can define one or many
+MEE regions that can hold enclave data by configuring them with PRMRR registers.
+
+The MEE automatically encrypts the data leaving the processor package to the MEE
+regions. The data is encrypted using a random key whose life-time is exactly one
+power cycle.
+
+You can tell if your CPU supports SGX by looking into ``/proc/cpuinfo``:
+
+	``cat /proc/cpuinfo  | grep ' sgx '``
+
+Enclave data types
+==================
+
+SGX defines new data types to maintain information about the enclaves and their
+security properties.
+
+The following data structures exist in MEE regions:
+
+* **Enclave Page Cache (EPC):** memory pages for protected code and data
+* **Enclave Page Cache Map (EPCM):** meta-data for each EPC page
+
+The Enclave Page Cache holds following types of pages:
+
+* **SGX Enclave Control Structure (SECS)**: meta-data defining the global
+  properties of an enclave such as range of addresses it can access.
+* **Regular (REG):** containing code and data for the enclave.
+* **Thread Control Structure (TCS):** defines an entry point for a hardware
+  thread to enter into the enclave. The enclave can only be entered through
+  these entry points.
+* **Version Array (VA)**: an EPC page receives a unique 8 byte version number
+  when it is swapped, which is then stored into a VA page. A VA page can hold up
+  to 512 version numbers.
+
+Launch control
+==============
+
+For launching an enclave, two structures must be provided for ENCLS(EINIT):
+
+1. **SIGSTRUCT:** a signed measurement of the enclave binary.
+2. **EINITTOKEN:** the measurement, the public key of the signer and various
+   enclave attributes. This structure contains a MAC of its contents using
+   hardware derived symmetric key called *launch key*.
+
+The hardware platform contains a root key pair for signing the SIGTRUCT
+for a *launch enclave* that is able to acquire the *launch key* for
+creating EINITTOKEN's for other enclaves.  For the launch enclave
+EINITTOKEN is not needed because it is signed with the private root key.
+
+There are two feature control bits associate with launch control
+
+* **IA32_FEATURE_CONTROL[0]**: locks down the feature control register
+* **IA32_FEATURE_CONTROL[17]**: allow runtime reconfiguration of
+  IA32_SGXLEPUBKEYHASHn MSRs that define MRSIGNER hash for the launch
+  enclave. Essentially they define a signing key that does not require
+  EINITTOKEN to be let run.
+
+The BIOS can configure IA32_SGXLEPUBKEYHASHn MSRs before feature control
+register is locked.
+
+It could be tempting to implement launch control by writing the MSRs
+every time when an enclave is launched. This does not scale because for
+generic case because BIOS might lock down the MSRs before handover to
+the OS.
+
+Debug enclaves
+--------------
+
+Enclave can be set as a *debug enclave* of which memory can be read or written
+by using the ENCLS(EDBGRD) and ENCLS(EDBGWR) opcodes. The Intel provided launch
+enclave provides them always a valid EINITTOKEN and therefore they are a low
+hanging fruit way to try out SGX.
+
+Virtualization
+==============
+
+Launch control
+--------------
+
+The values for IA32_SGXLEPUBKEYHASHn MSRs cannot be emulated for a virtual
+machine guest. It would easily seem feasible to hold virtual values for these
+MSRs, trap ENCLS(EINIT) and use the host LE to generate a token when a guest LE
+is initialized.
+
+However, looking at the pseudo code of ENCLS(EINIT) from the SDM there is a
+constraint that the instruction will fail if ATTRIBUTES.EINITTOKENKEY is set
+(the documentation does not tell the reason why the constraint exists but it
+exists).
+
+Thus, only when the MSRs are left unlocked before handover to the OS the
+setting of these MSRs can be supported for VM guests.
+
+Suspend and resume
+------------------
+
+If the host suspends and resumes, the enclave memory for the VM guest could
+become invalid. This can make ENCLS leaf operations suddenly fail.
+
+The driver has a graceful fallback mechanism to manage this situation. If any of
+the ENCLS leaf operations fail, the driver will fallback by kicking threads out
+of the enclave, removing the TCS entries and marking enclave as invalid. After
+this no new pages can be allocated for the enclave and no entry can be done.
+
+SGX uapi
+========
+
+.. kernel-doc:: drivers/platform/x86/intel_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
-- 
2.14.1