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SubjectRe: [PATCH v13 13/13] x86/sgx: Driver documentation
On Mon, Aug 27, 2018 at 12:40:50PM -0700, Randy Dunlap wrote:
> On 08/27/2018 11:53 AM, 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@linux.intel.com>
>
> Hi,
>
> Comments inline...
>
> > ---
> > 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
> >
> > 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
>
> set aside private regions of code and data (which are called enclaves). The ...
>
> > +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
>
> provides an inverted sandbox.
>
> > +application from a malicious host.
> > +
> > +You can tell if your CPU supports SGX by looking into ``/proc/cpuinfo``:
> > +
> > + ``cat /proc/cpuinfo | grep sgx``
>
> or ``grep sgx /proc/cpuinfo``
> ?
>
> > +
> > +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
>
> It is referred to 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
>
> An enclave can
> or
> Enclaves can
>
> > +VMEXIT, IO instructions and instructions that require a privilege change are
> > +prohibited inside the enclave. Interrupts and exceptions always cause enclave
>
> cause the 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.
> > +
> > +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.
> > +
> > +Launch control
> > +--------------
> > +
> > +To launch an enclave, two structures must be provided for ENCLS(EINIT):
> > +
> > +1. **SIGSTRUCT:** signed measurement of the enclave binary.
>
> what does "signed measurement" mean?

Right, I admit that this looks complete nonsense.

It is SHA256 hash of the construction of the enclave signed with the
private RSA key of the enclave signer (the public key is part of the
SIGSTRUCT).

A measurement is constructed by hashing tags for ECREATE, EADD and
EEXTEND together with those parameters that either define ordinal and
cardinal properties of the enclave associated with the operation and for
each EADD the associated data page is also hashed.

> > +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
>
> by the BIOS to 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
>
> what is this "measurement"? how is it done?

I will update this according the above explanation probably adding
section just for explaining what measurement means as it is a concept
that is cuts through the whole stack.

The CPU calculates it to variable inside the SECS page of the enclave
while you construct the enclave and a signing tool calculates equivalent
value and signs it with the private key (i.e. the signed measurement).

> > +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
> > +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
>
> counter
>
> > +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.
>
> userspace
> (as on the second line of that same paragraph)
>
> > +
> > +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.
> > +
> > +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
> >
>
> HTH.
> --

Thank this was really useful feedback.

I've been drowned in SGX long enough to become blind sometimes to what
is not obvious :-)

> ~Randy

/Jarkko

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