253 lines
10 KiB
ReStructuredText
253 lines
10 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
===============================
|
|
Software Guard eXtensions (SGX)
|
|
===============================
|
|
|
|
Overview
|
|
========
|
|
|
|
Software Guard eXtensions (SGX) hardware enables for user space applications
|
|
to set aside private memory regions of code and data:
|
|
|
|
* Privileged (ring-0) ENCLS functions orchestrate the construction of the.
|
|
regions.
|
|
* Unprivileged (ring-3) ENCLU functions allow an application to enter and
|
|
execute inside the regions.
|
|
|
|
These memory regions are called enclaves. An enclave can be only entered at a
|
|
fixed set of entry points. Each entry point can hold a single hardware thread
|
|
at a time. While the enclave is loaded from a regular binary file by using
|
|
ENCLS functions, only the threads inside the enclave can access its memory. The
|
|
region is denied from outside access by the CPU, and encrypted before it leaves
|
|
from LLC.
|
|
|
|
The support can be determined by
|
|
|
|
``grep sgx /proc/cpuinfo``
|
|
|
|
SGX must both be supported in the processor and enabled by the BIOS. If SGX
|
|
appears to be unsupported on a system which has hardware support, ensure
|
|
support is enabled in the BIOS. If a BIOS presents a choice between "Enabled"
|
|
and "Software Enabled" modes for SGX, choose "Enabled".
|
|
|
|
Enclave Page Cache
|
|
==================
|
|
|
|
SGX utilizes an *Enclave Page Cache (EPC)* to store pages that are associated
|
|
with an enclave. It is contained in a BIOS-reserved region of physical memory.
|
|
Unlike pages used for regular memory, pages can only be accessed from outside of
|
|
the enclave during enclave construction with special, limited SGX instructions.
|
|
|
|
Only a CPU executing inside an enclave can directly access enclave memory.
|
|
However, a CPU executing inside an enclave may access normal memory outside the
|
|
enclave.
|
|
|
|
The kernel manages enclave memory similar to how it treats device memory.
|
|
|
|
Enclave Page Types
|
|
------------------
|
|
|
|
**SGX Enclave Control Structure (SECS)**
|
|
Enclave's address range, attributes and other global data are defined
|
|
by this structure.
|
|
|
|
**Regular (REG)**
|
|
Regular EPC pages contain the code and data of an enclave.
|
|
|
|
**Thread Control Structure (TCS)**
|
|
Thread Control Structure pages define the entry points to an enclave and
|
|
track the execution state of an enclave thread.
|
|
|
|
**Version Array (VA)**
|
|
Version Array pages contain 512 slots, each of which can contain a version
|
|
number for a page evicted from the EPC.
|
|
|
|
Enclave Page Cache Map
|
|
----------------------
|
|
|
|
The processor tracks EPC pages in a hardware metadata structure called the
|
|
*Enclave Page Cache Map (EPCM)*. The EPCM contains an entry for each EPC page
|
|
which describes the owning enclave, access rights and page type among the other
|
|
things.
|
|
|
|
EPCM permissions are separate from the normal page tables. This prevents the
|
|
kernel from, for instance, allowing writes to data which an enclave wishes to
|
|
remain read-only. EPCM permissions may only impose additional restrictions on
|
|
top of normal x86 page permissions.
|
|
|
|
For all intents and purposes, the SGX architecture allows the processor to
|
|
invalidate all EPCM entries at will. This requires that software be prepared to
|
|
handle an EPCM fault at any time. In practice, this can happen on events like
|
|
power transitions when the ephemeral key that encrypts enclave memory is lost.
|
|
|
|
Application interface
|
|
=====================
|
|
|
|
Enclave build functions
|
|
-----------------------
|
|
|
|
In addition to the traditional compiler and linker build process, SGX has a
|
|
separate enclave “build” process. Enclaves must be built before they can be
|
|
executed (entered). The first step in building an enclave is opening the
|
|
**/dev/sgx_enclave** device. Since enclave memory is protected from direct
|
|
access, special privileged instructions are Then used to copy data into enclave
|
|
pages and establish enclave page permissions.
|
|
|
|
.. kernel-doc:: arch/x86/kernel/cpu/sgx/ioctl.c
|
|
:functions: sgx_ioc_enclave_create
|
|
sgx_ioc_enclave_add_pages
|
|
sgx_ioc_enclave_init
|
|
sgx_ioc_enclave_provision
|
|
|
|
Enclave vDSO
|
|
------------
|
|
|
|
Entering an enclave can only be done through SGX-specific EENTER and ERESUME
|
|
functions, and is a non-trivial process. Because of the complexity of
|
|
transitioning to and from an enclave, enclaves typically utilize a library to
|
|
handle the actual transitions. This is roughly analogous to how glibc
|
|
implementations are used by most applications to wrap system calls.
|
|
|
|
Another crucial characteristic of enclaves is that they can generate exceptions
|
|
as part of their normal operation that need to be handled in the enclave or are
|
|
unique to SGX.
|
|
|
|
Instead of the traditional signal mechanism to handle these exceptions, SGX
|
|
can leverage special exception fixup provided by the vDSO. The kernel-provided
|
|
vDSO function wraps low-level transitions to/from the enclave like EENTER and
|
|
ERESUME. The vDSO function intercepts exceptions that would otherwise generate
|
|
a signal and return the fault information directly to its caller. This avoids
|
|
the need to juggle signal handlers.
|
|
|
|
.. kernel-doc:: arch/x86/include/uapi/asm/sgx.h
|
|
:functions: vdso_sgx_enter_enclave_t
|
|
|
|
ksgxd
|
|
=====
|
|
|
|
SGX support includes a kernel thread called *ksgxwapd*.
|
|
|
|
EPC sanitization
|
|
----------------
|
|
|
|
ksgxd is started when SGX initializes. Enclave memory is typically ready
|
|
For use when the processor powers on or resets. However, if SGX has been in
|
|
use since the reset, enclave pages may be in an inconsistent state. This might
|
|
occur after a crash and kexec() cycle, for instance. At boot, ksgxd
|
|
reinitializes all enclave pages so that they can be allocated and re-used.
|
|
|
|
The sanitization is done by going through EPC address space and applying the
|
|
EREMOVE function to each physical page. Some enclave pages like SECS pages have
|
|
hardware dependencies on other pages which prevents EREMOVE from functioning.
|
|
Executing two EREMOVE passes removes the dependencies.
|
|
|
|
Page reclaimer
|
|
--------------
|
|
|
|
Similar to the core kswapd, ksgxd, is responsible for managing the
|
|
overcommitment of enclave memory. If the system runs out of enclave memory,
|
|
*ksgxwapd* “swaps” enclave memory to normal memory.
|
|
|
|
Launch Control
|
|
==============
|
|
|
|
SGX provides a launch control mechanism. After all enclave pages have been
|
|
copied, kernel executes EINIT function, which initializes the enclave. Only after
|
|
this the CPU can execute inside the enclave.
|
|
|
|
ENIT function takes an RSA-3072 signature of the enclave measurement. The function
|
|
checks that the measurement is correct and signature is signed with the key
|
|
hashed to the four **IA32_SGXLEPUBKEYHASH{0, 1, 2, 3}** MSRs representing the
|
|
SHA256 of a public key.
|
|
|
|
Those MSRs can be configured by the BIOS to be either readable or writable.
|
|
Linux supports only writable configuration in order to give full control to the
|
|
kernel on launch control policy. Before calling EINIT function, the driver sets
|
|
the MSRs to match the enclave's signing key.
|
|
|
|
Encryption engines
|
|
==================
|
|
|
|
In order to conceal the enclave data while it is out of the CPU package, the
|
|
memory controller has an encryption engine to transparently encrypt and decrypt
|
|
enclave memory.
|
|
|
|
In CPUs prior to Ice Lake, the Memory Encryption Engine (MEE) is used to
|
|
encrypt pages leaving the CPU caches. MEE uses a n-ary Merkle tree with root in
|
|
SRAM to maintain integrity of the encrypted data. This provides integrity and
|
|
anti-replay protection but does not scale to large memory sizes because the time
|
|
required to update the Merkle tree grows logarithmically in relation to the
|
|
memory size.
|
|
|
|
CPUs starting from Icelake use Total Memory Encryption (TME) in the place of
|
|
MEE. TME-based SGX implementations do not have an integrity Merkle tree, which
|
|
means integrity and replay-attacks are not mitigated. B, it includes
|
|
additional changes to prevent cipher text from being returned and SW memory
|
|
aliases from being Created.
|
|
|
|
DMA to enclave memory is blocked by range registers on both MEE and TME systems
|
|
(SDM section 41.10).
|
|
|
|
Usage Models
|
|
============
|
|
|
|
Shared Library
|
|
--------------
|
|
|
|
Sensitive data and the code that acts on it is partitioned from the application
|
|
into a separate library. The library is then linked as a DSO which can be loaded
|
|
into an enclave. The application can then make individual function calls into
|
|
the enclave through special SGX instructions. A run-time within the enclave is
|
|
configured to marshal function parameters into and out of the enclave and to
|
|
call the correct library function.
|
|
|
|
Application Container
|
|
---------------------
|
|
|
|
An application may be loaded into a container enclave which is specially
|
|
configured with a library OS and run-time which permits the application to run.
|
|
The enclave run-time and library OS work together to execute the application
|
|
when a thread enters the enclave.
|
|
|
|
Impact of Potential Kernel SGX Bugs
|
|
===================================
|
|
|
|
EPC leaks
|
|
---------
|
|
|
|
When EPC page leaks happen, a WARNING like this is shown in dmesg:
|
|
|
|
"EREMOVE returned ... and an EPC page was leaked. SGX may become unusable..."
|
|
|
|
This is effectively a kernel use-after-free of an EPC page, and due
|
|
to the way SGX works, the bug is detected at freeing. Rather than
|
|
adding the page back to the pool of available EPC pages, the kernel
|
|
intentionally leaks the page to avoid additional errors in the future.
|
|
|
|
When this happens, the kernel will likely soon leak more EPC pages, and
|
|
SGX will likely become unusable because the memory available to SGX is
|
|
limited. However, while this may be fatal to SGX, the rest of the kernel
|
|
is unlikely to be impacted and should continue to work.
|
|
|
|
As a result, when this happpens, user should stop running any new
|
|
SGX workloads, (or just any new workloads), and migrate all valuable
|
|
workloads. Although a machine reboot can recover all EPC memory, the bug
|
|
should be reported to Linux developers.
|
|
|
|
|
|
Virtual EPC
|
|
===========
|
|
|
|
The implementation has also a virtual EPC driver to support SGX enclaves
|
|
in guests. Unlike the SGX driver, an EPC page allocated by the virtual
|
|
EPC driver doesn't have a specific enclave associated with it. This is
|
|
because KVM doesn't track how a guest uses EPC pages.
|
|
|
|
As a result, the SGX core page reclaimer doesn't support reclaiming EPC
|
|
pages allocated to KVM guests through the virtual EPC driver. If the
|
|
user wants to deploy SGX applications both on the host and in guests
|
|
on the same machine, the user should reserve enough EPC (by taking out
|
|
total virtual EPC size of all SGX VMs from the physical EPC size) for
|
|
host SGX applications so they can run with acceptable performance.
|