Introduction to SGX¶
The Gramine project uses the Intel SGX (Software Guard Extensions) technology to protect software running on untrusted hosts. SGX is a complicated topic, which may be hard to learn, because the documentation is scattered through official/reference documentation, blogposts and academic papers. This page is an attempt to curate a dossier of available reading material.
SGX is an umbrella name of technology that comprises several parts:
- CPU/platform hardware features: the new instruction set, new microarchitecture with the PRM (EPC) memory region and some new MSRs and some new logic in the MMU and so on;
- the SGX Remote Attestation infrastructure, online services provided by Intel and/or third parties (see DCAP);
- SDK and assorted software.
SGX is still being developed. The current (March 2020) version of CPU features is referred to as “SGX1” or simply “SGX” and is more or less finalized. All new/changed instructions from original SGX are informally referred to as “SGX2”.
Features which might be considered part of SGX2:
- EDMM (Enclave Dynamic Memory Management) is part of SGX2
- FLC (Flexible Launch Control), not strictly part of SGX2, but was not part of original SGX hardware either
- KSS (Key Separation and Sharing), also not part of SGX2, but was not part of original SGX hardware either
As of now there is hardware support (on a limited set of CPUs) for FLC and (on an even more limited set of CPUs) SGX2/EDMM. Most of the literature available (especially introduction-level) concerns original SGX1 only.
- Quarkslab’s two-part “Overview of Intel SGX”:
- MIT’s deep dive in SGX architecture.
- Intel’s whitepapers:
- Hardware compatibility list (unofficial)
- Intel® 64 and IA-32 Architectures Software Developer’s Manual Volume 3D: System Programming Guide, Part 4
- SDK for Linux (download of both the binaries and the documentation)
- Intel’s collection of academic papers, likely the most comprehensive list of references
Linux kernel drivers¶
For historical reasons, there are three SGX drivers currently (January 2021):
https://github.com/intel/linux-sgx-driver – old one, does not support DCAP, deprecated
https://github.com/intel/SGXDataCenterAttestationPrimitives/tree/master/driver – out-of-tree, supports both non-DCAP software infrastructure (with old EPID remote-attestation technique) and the new DCAP (with new ECDSA and more “normal” PKI infrastructure). Deprecated in favor of the upstreamed driver (see below).
SGX support was upstreamed to the Linux mainline starting from 5.11. It currently supports only DCAP attestation. The driver is accessible through /dev/sgx_enclave and /dev/sgx_provision.
The following udev rules are recommended for users to access the SGX node:
groupadd -r sgx gpasswd -a USERNAME sgx groupadd -r sgx_prv gpasswd -a USERNAME sgx_prv cat > /etc/udev/rules.d/65-gramine-sgx.rules << EOF SUBSYSTEM=="misc",KERNEL=="sgx_enclave",MODE="0660",GROUP="sgx" SUBSYSTEM=="misc",KERNEL=="sgx_provision",MODE="0660",GROUP="sgx_prv" EOF udevadm trigger
Also it will not require IAS and kernel maintainers consider non-writable FLC MSRs as non-functional SGX: https://lore.kernel.org/lkml/20191223094614.GB16710@zn.tnic/
The chronicle of kernel patchset:
- v1 (2016-04-25)
- v4 (2017-10-16)
- v5 (2017-11-13)
- v6 (2017-11-25)
- v7 (2017-12-07)
- v8 (2017-12-15)
- v9 (2017-12-16)
- v10 (2017-12-24)
- v11 (2018-06-08)
- v12 (2018-07-03)
- v13 (2018-08-27)
- v14 (2018-09-25)
- v15 (2018-11-03)
- v16 (2018-11-06)
- v17 (2018-11-16)
- v18 (2018-12-22)
- v19 (2019-03-20)
- v20 (2019-04-17)
- v21 (2019-07-13)
- v22 (2019-09-03)
- v23 (2019-10-28)
- v24 (2019-11-30)
- v25 (2020-02-04)
- v26 (2020-02-09)
- v27 (2020-02-23)
- v28 (2020-04-04)
- v29 (2020-04-22)
- v30 (2020-05-15)
- Architectural Enclaves
Architectural Enclaves (AEs) are a set of “system” enclaves concerned with starting and attesting other enclaves. Intel provides reference implementations of these enclaves, though other companies may write their own implementations.
- Architectural Enclave Service Manager
- The Architectural Enclave Service Manager is responsible for providing SGX
applications with access to the Architectural Enclaves. It consists
of the Architectural Enclave Service Manager Daemon, which hosts the enclaves,
and a component of the SGX SDK, which communicates with the daemon over a Unix
socket with the fixed path
Attestation is a mechanism to prove the trustworthiness of the SGX enclave to a local or remote party. More specifically, SGX attestation proves that the enclave runs on a real hardware in an up-to-date TEE with the expected initial state. There are two types of the attestation: Local Attestation and Remote Attestation. For local attestation, the attesting SGX enclave collects attestation evidence in the form of an SGX Report using the EREPORT hardware instruction. For remote attestation, the attesting SGX enclave collects attestation evidence in the form of an SGX Quote using the Quoting Enclave (and the Provisioning Enclave if required). The enclave then may send the collected attestation evidence to the local or remote party, which will verify the evidence and confirm the correctness of the attesting enclave. After this, the local or remote party trusts the enclave and may establish a secure channel with the enclave and send secrets to it.
- Data Center Attestation Primitives
A software infrastructure provided by Intel as a reference implementation for the new ECDSA/PCS-based remote attestation. Relies on the Flexible Launch Control hardware feature. In principle this is a special version of SDK/PSW that has a reference launch enclave and is backed by the DCAP-enabled SGX driver.
This allows for launching enclaves with Intel’s remote infrastructure only involved in the initial setup. Naturally however, this requires deployment of own infrastructure, so is operationally more complicated. Therefore it is intended for server environments (where you control all the machines).
- Orientation Guide
- A way to launch enclaves with Intel’s infrastructure, intended for client machines.
- Enclave Dynamic Memory Management
- A hardware feature of SGX2, allows for dynamic (in enclave runtime) addition and removal of enclave memory, as well as changing memory permissions and type. This in turn allows dynamic thread creation.
- Enclave Page Cache
- Enclave Page Cache Map
- Enhanced Privacy Identification
- Enhanced Privacy Identifier
- Enhanced Privacy Identifier
EPID is the attestation protocol originally shipped with SGX. Unlike DCAP, a remote verifier making use of the EPID protocol needs to contact the Intel Attestation Service each time it wishes to attest an enclave.
Contrary to DCAP, EPID may be understood as “opinionated”, with most moving parts fixed and tied to services provided by Intel. This is intended for client enclaves and deprecated for server environments.
EPID attestation can operate in two modes: fully-anonymous (unlinkable) quotes and pseudonymous (linkable) quotes. Unlike fully-anonymous quotes, pseudonymous quotes include an identifier dependent on the identity of the CPU and the developer of the enclave being quoted, which allows determining whether two instances of your enclave are running on the same hardware or not.
If your security model depends on enforcing that the identifiers are different (e.g. because you want to prevent sybil attacks), keep in mind that the enclave host can generate a new identity by performing an epoch reset. The previous identity will then become inaccessible, though.
The attestation mode being used can be chosen by the application enclave, but it must match what was chosen when generating the SPID.
- Flexible Launch Control
Hardware (CPU) feature that allows substituting Launch Enclave for one not signed by Intel. A change in SGX’s EINIT logic to not require the EINITTOKEN from the Intel-based Launch Enclave. An MSR, which can be locked at boot time, keeps the hash of the public key of the “launching” entity.
With FLC, Launch Enclave can be written by other companies (other than Intel) and must be signed with the key corresponding to the one locked in the MSR (a reference Launch Enclave simply allows all enclaves to run). The MSR can also stay unlocked and then it can be modified at run-time by the VMM or the OS kernel.
Support for FLC can be detected using
CPUID.07H:ECX.SGX_LC[bit 30] == 1(SDM vol. 2A calls this “SGX Launch Control”).
- Key Separation and Sharing
A feature that lets developer define additional enclave identity attributes and configuration identifier. Extended enclave identity is defined by the developer on enclave build. Enclave configuration is defined on enclave launch and cannot be modified afterwards.
In addition to the calculated enclave and signer measurements, developer is expected to define a product ID and SVN for her enclaves. These identifiers are part of the SGX Report and are expected to be used in Attestation. They are also used by SGX key derivation to derive different keys per configuration.
KSS adds two more attributes for enclave build and two new ones for enclave launch, which are part of the SGX Report. Additionally, key policy attributes are extended to provide fine-grained control over key derivation.
New build attributes:
- Extended product ID
- Family ID
New enclave launch attributes:
- Config ID
- Config SVN
This feature was not part of original SGX and therefore not supported by all SGX-enabled hardware.
- Launch Enclave
- Local Attestation
In local attestation, the attesting SGX enclave collects attestation evidence in the form of an SGX Report using the EREPORT hardware instruction. This form of attestation is used to send the attestation evidence to a local party (on the same physical machine).
- Intel Attestation Service
Internet service provided by Intel for “old” EPID-based remote attestation. Enclaves send SGX quotes to the client/verifier who will forward them to IAS to check their validity.
- Provisioning Certification Service, another Internet service provided by Intel.
- Memory Encryption Engine
- SGX Platform Software
- Software infrastructure provided by Intel with all special Architectural Enclaves (Provisioning Enclave, Quoting Enclave, Launch Enclave). This mainly refers to the “old” EPID/IAS-based remote attestation.
- Processor Reserved Memory
- Provisioning Enclave
One of the Architectural Enclaves of the Intel SGX software infrastructure. It is part of the SGX Platform Software. The Provisioning Enclave is used in EPID based remote attestation. This enclave communicates with the Intel Provisioning Service (IPS) to perform EPID provisioning. The result of this provisioning procedure is the private EPID key securely accessed by the Provisioning Enclave. This procedure happens only during the first deployment of the SGX machine (or, in rare cases, to provision a new EPID key after TCB upgrade). The main user of the Provisioning Enclave is the Quoting Enclave.
- Provisioning Certification Enclave
One of the Architectural Enclaves of the Intel SGX software infrastructure. It is part of the SGX Platform Software and DCAP. The Provisioning Certification Enclave is used in DCAP based remote attestation. This enclave communicates with the Intel Provisioning Certification Service (PCS) to perform DCAP provisioning. The result of this provisioning procedure is the DCAP/ECDSA attestation collateral (mainly the X.509 certificate chains rooted in a well-known Intel certificate and Certificate Revocation Lists). This procedure happens during the first deployment of the SGX machine and then periodically to refresh the cached attestation collateral. Typically, to reduce the dependency on PCS, a cloud service provider introduces an intermediate caching service (Provisioning Certification Caching Service, or PCCS) that stores all the attestation collateral obtained from Intel. The main user of the Provisioning Certification Enclave is the Quoting Enclave.
- Intel Provisioning Service
- Internet service provided by Intel for EPID-based remote attestation. This service provides the corresponding EPID key to the Provisioning Enclave on a remote SGX machine.
- Intel Provisioning Certification Service
New internet service provided by Intel for new ECDSA-based remote attestation. Enclave provider creates its own internal Attestation Service where it caches PKI collateral from Intel’s PCS, and the verifier gets the certificate chain from the enclave provider to check validity.
- Intel Attestation Service, another Internet service.
- Quoting Enclave
One of the Architectural Enclaves of the Intel SGX software infrastructure. It is part of the SGX Platform Software. The Quoting Enclave receives an SGX Report and produces a corresponding SGX Quote. The identity of the Quoting Enclave is publicly known (it signer, its measurement and its attributes) and is vetted by public companies such as Intel (in the form of the certificate chain ending in a publicly known root certificate of the company).
- Remote Attestation
In remote attestation, the attesting SGX enclave collects attestation evidence in the form of an SGX Quote using the Quoting Enclave (and the Provisioning Enclave if required). This form of attestation is used to send the attestation evidence to a remote party (not on the same physical machine).
- Intel SGX Software Development Kit
- Intel SGX SDK
- SGX SDK
- Intel SGX SDK
- In the context of SGX, this means a specific piece of software
supplied by Intel which helps people write enclaves packed into
.sofiles to be accessible like normal libraries (at least on Linux). Available together with a kernel module and documentation.
- SGX Enclave Control Structure
- SGX Quote
- The SGX quote is the proof of trustworthiness of the enclave and is used during Remote Attestation. The attesting enclave generates the enclave-specific SGX Report, sends the request to the Quoting Enclave using Local Attestation, and the Quoting Enclave returns back the SGX quote with the SGX report embedded in it. The resulting SGX quote contains the enclave’s measurement, attributes and other security-relevant fields, and is tied to the identity of the Quoting Enclave to prove its authenticity. The obtained SGX quote may be later sent to the verifying remote party, which examines the SGX quote and gains trust in the remote enclave.
- SGX Report
- The SGX report is a data structure that contains the enclave’s measurement,
signer identity, attributes and a user-defined 64B string. The SGX report
is generated using the
EREPORThardware instruction. It is used during Local Attestation. The SGX report is embedded into the SGX Quote.
This refers to all new SGX instructions and other hardware features that were introduced after the release of the original SGX1.
Encompasses at least EDMM, but is still work in progress.
- Service Provider ID
See EPID for a description of the difference between linkable and unlinkable quotes.
- State Save Area
- Security Version Number
- Each element of the SGX TCB is assigned a Security Version Number (SVN). For the hardware, these SVNs are referred to collectively as CPU_SVN, and for software referred as ISV_SVN. A TCB is considered up to date if all components of the TCB have SVNs greater than or equal to a threshold published by the author of the component.
- Trusted Execution Environment
- A Trusted Execution Environment (TEE) is an environment where the code executed and the data accessed are isolated and protected in terms of confidentiality (no one has access to the data except the code running inside the TEE) and integrity (no one can change the code and its behavior).
- Trusted Computing Base
In context of SGX this has the usual meaning: the set of all components that are critical to security. Any vulnerability in TCB compromises security. Any problem outside TCB is not a vulnerability, i.e. should not compromise security.
In context of Gramine there is also a different meaning (Thread Control Block). Those two should not be confused.
- Thread Control Structure