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 .. 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 runtime management
 --------------------------
 
 Systems supporting SGX2 additionally support changes to initialized
 enclaves: modifying enclave page permissions and type, and dynamically
 adding and removing of enclave pages. When an enclave accesses an address
 within its address range that does not have a backing page then a new
 regular page will be dynamically added to the enclave. The enclave is
 still required to run EACCEPT on the new page before it can be used.
 
 .. kernel-doc:: arch/x86/kernel/cpu/sgx/ioctl.c
    :functions: sgx_ioc_enclave_restrict_permissions
                sgx_ioc_enclave_modify_types
                sgx_ioc_enclave_remove_pages
 
 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 *ksgxd*.
 
 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,
 *ksgxd* “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.
 
 EINIT 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.
 
 Architectural behavior is to restore all EPC pages to an uninitialized
 state also after a guest reboot.  Because this state can be reached only
 through the privileged ``ENCLS[EREMOVE]`` instruction, ``/dev/sgx_vepc``
 provides the ``SGX_IOC_VEPC_REMOVE_ALL`` ioctl to execute the instruction
 on all pages in the virtual EPC.
 
 ``EREMOVE`` can fail for three reasons.  Userspace must pay attention
 to expected failures and handle them as follows:
 
 1. Page removal will always fail when any thread is running in the
    enclave to which the page belongs.  In this case the ioctl will
    return ``EBUSY`` independent of whether it has successfully removed
    some pages; userspace can avoid these failures by preventing execution
    of any vcpu which maps the virtual EPC.
 
 2. Page removal will cause a general protection fault if two calls to
    ``EREMOVE`` happen concurrently for pages that refer to the same
    "SECS" metadata pages.  This can happen if there are concurrent
    invocations to ``SGX_IOC_VEPC_REMOVE_ALL``, or if a ``/dev/sgx_vepc``
    file descriptor in the guest is closed at the same time as
    ``SGX_IOC_VEPC_REMOVE_ALL``; it will also be reported as ``EBUSY``.
    This can be avoided in userspace by serializing calls to the ioctl()
    and to close(), but in general it should not be a problem.
 
 3. Finally, page removal will fail for SECS metadata pages which still
    have child pages.  Child pages can be removed by executing
    ``SGX_IOC_VEPC_REMOVE_ALL`` on all ``/dev/sgx_vepc`` file descriptors
    mapped into the guest.  This means that the ioctl() must be called
    twice: an initial set of calls to remove child pages and a subsequent
    set of calls to remove SECS pages.  The second set of calls is only
    required for those mappings that returned a nonzero value from the
    first call.  It indicates a bug in the kernel or the userspace client
    if any of the second round of ``SGX_IOC_VEPC_REMOVE_ALL`` calls has
    a return code other than 0.

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