--- /dev/null
+======================================================
+Confidential Computing in Linux for x86 virtualization
+======================================================
+
+.. contents:: :local:
+
+By: Elena Reshetova <elena.reshetova@intel.com> and Carlos Bilbao <carlos.bilbao@amd.com>
+
+Motivation
+==========
+
+Kernel developers working on confidential computing for virtualized
+environments in x86 operate under a set of assumptions regarding the Linux
+kernel threat model that differ from the traditional view. Historically,
+the Linux threat model acknowledges attackers residing in userspace, as
+well as a limited set of external attackers that are able to interact with
+the kernel through various networking or limited HW-specific exposed
+interfaces (USB, thunderbolt). The goal of this document is to explain
+additional attack vectors that arise in the confidential computing space
+and discuss the proposed protection mechanisms for the Linux kernel.
+
+Overview and terminology
+========================
+
+Confidential Computing (CoCo) is a broad term covering a wide range of
+security technologies that aim to protect the confidentiality and integrity
+of data in use (vs. data at rest or data in transit). At its core, CoCo
+solutions provide a Trusted Execution Environment (TEE), where secure data
+processing can be performed and, as a result, they are typically further
+classified into different subtypes depending on the SW that is intended
+to be run in TEE. This document focuses on a subclass of CoCo technologies
+that are targeting virtualized environments and allow running Virtual
+Machines (VM) inside TEE. From now on in this document will be referring
+to this subclass of CoCo as 'Confidential Computing (CoCo) for the
+virtualized environments (VE)'.
+
+CoCo, in the virtualization context, refers to a set of HW and/or SW
+technologies that allow for stronger security guarantees for the SW running
+inside a CoCo VM. Namely, confidential computing allows its users to
+confirm the trustworthiness of all SW pieces to include in its reduced
+Trusted Computing Base (TCB) given its ability to attest the state of these
+trusted components.
+
+While the concrete implementation details differ between technologies, all
+available mechanisms aim to provide increased confidentiality and
+integrity for the VM's guest memory and execution state (vCPU registers),
+more tightly controlled guest interrupt injection, as well as some
+additional mechanisms to control guest-host page mapping. More details on
+the x86-specific solutions can be found in
+:doc:`Intel Trust Domain Extensions (TDX) </arch/x86/tdx>` and
+`AMD Memory Encryption <https://www.amd.com/system/files/techdocs/sev-snp-strengthening-vm-isolation-with-integrity-protection-and-more.pdf>`_.
+
+The basic CoCo guest layout includes the host, guest, the interfaces that
+communicate guest and host, a platform capable of supporting CoCo VMs, and
+a trusted intermediary between the guest VM and the underlying platform
+that acts as a security manager. The host-side virtual machine monitor
+(VMM) typically consists of a subset of traditional VMM features and
+is still in charge of the guest lifecycle, i.e. create or destroy a CoCo
+VM, manage its access to system resources, etc. However, since it
+typically stays out of CoCo VM TCB, its access is limited to preserve the
+security objectives.
+
+In the following diagram, the "<--->" lines represent bi-directional
+communication channels or interfaces between the CoCo security manager and
+the rest of the components (data flow for guest, host, hardware) ::
+
+ +-------------------+ +-----------------------+
+ | CoCo guest VM |<---->| |
+ +-------------------+ | |
+ | Interfaces | | CoCo security manager |
+ +-------------------+ | |
+ | Host VMM |<---->| |
+ +-------------------+ | |
+ | |
+ +--------------------+ | |
+ | CoCo platform |<--->| |
+ +--------------------+ +-----------------------+
+
+The specific details of the CoCo security manager vastly diverge between
+technologies. For example, in some cases, it will be implemented in HW
+while in others it may be pure SW.
+
+Existing Linux kernel threat model
+==================================
+
+The overall components of the current Linux kernel threat model are::
+
+ +-----------------------+ +-------------------+
+ | |<---->| Userspace |
+ | | +-------------------+
+ | External attack | | Interfaces |
+ | vectors | +-------------------+
+ | |<---->| Linux Kernel |
+ | | +-------------------+
+ +-----------------------+ +-------------------+
+ | Bootloader/BIOS |
+ +-------------------+
+ +-------------------+
+ | HW platform |
+ +-------------------+
+
+There is also communication between the bootloader and the kernel during
+the boot process, but this diagram does not represent it explicitly. The
+"Interfaces" box represents the various interfaces that allow
+communication between kernel and userspace. This includes system calls,
+kernel APIs, device drivers, etc.
+
+The existing Linux kernel threat model typically assumes execution on a
+trusted HW platform with all of the firmware and bootloaders included on
+its TCB. The primary attacker resides in the userspace, and all of the data
+coming from there is generally considered untrusted, unless userspace is
+privileged enough to perform trusted actions. In addition, external
+attackers are typically considered, including those with access to enabled
+external networks (e.g. Ethernet, Wireless, Bluetooth), exposed hardware
+interfaces (e.g. USB, Thunderbolt), and the ability to modify the contents
+of disks offline.
+
+Regarding external attack vectors, it is interesting to note that in most
+cases external attackers will try to exploit vulnerabilities in userspace
+first, but that it is possible for an attacker to directly target the
+kernel; particularly if the host has physical access. Examples of direct
+kernel attacks include the vulnerabilities CVE-2019-19524, CVE-2022-0435
+and CVE-2020-24490.
+
+Confidential Computing threat model and its security objectives
+===============================================================
+
+Confidential Computing adds a new type of attacker to the above list: a
+potentially misbehaving host (which can also include some part of a
+traditional VMM or all of it), which is typically placed outside of the
+CoCo VM TCB due to its large SW attack surface. It is important to note
+that this doesn’t imply that the host or VMM are intentionally
+malicious, but that there exists a security value in having a small CoCo
+VM TCB. This new type of adversary may be viewed as a more powerful type
+of external attacker, as it resides locally on the same physical machine
+(in contrast to a remote network attacker) and has control over the guest
+kernel communication with most of the HW::
+
+ +------------------------+
+ | CoCo guest VM |
+ +-----------------------+ | +-------------------+ |
+ | |<--->| | Userspace | |
+ | | | +-------------------+ |
+ | External attack | | | Interfaces | |
+ | vectors | | +-------------------+ |
+ | |<--->| | Linux Kernel | |
+ | | | +-------------------+ |
+ +-----------------------+ | +-------------------+ |
+ | | Bootloader/BIOS | |
+ +-----------------------+ | +-------------------+ |
+ | |<--->+------------------------+
+ | | | Interfaces |
+ | | +------------------------+
+ | CoCo security |<--->| Host/Host-side VMM |
+ | manager | +------------------------+
+ | | +------------------------+
+ | |<--->| CoCo platform |
+ +-----------------------+ +------------------------+
+
+While traditionally the host has unlimited access to guest data and can
+leverage this access to attack the guest, the CoCo systems mitigate such
+attacks by adding security features like guest data confidentiality and
+integrity protection. This threat model assumes that those features are
+available and intact.
+
+The **Linux kernel CoCo VM security objectives** can be summarized as follows:
+
+1. Preserve the confidentiality and integrity of CoCo guest's private
+memory and registers.
+
+2. Prevent privileged escalation from a host into a CoCo guest Linux kernel.
+While it is true that the host (and host-side VMM) requires some level of
+privilege to create, destroy, or pause the guest, part of the goal of
+preventing privileged escalation is to ensure that these operations do not
+provide a pathway for attackers to gain access to the guest's kernel.
+
+The above security objectives result in two primary **Linux kernel CoCo
+VM assets**:
+
+1. Guest kernel execution context.
+2. Guest kernel private memory.
+
+The host retains full control over the CoCo guest resources, and can deny
+access to them at any time. Examples of resources include CPU time, memory
+that the guest can consume, network bandwidth, etc. Because of this, the
+host Denial of Service (DoS) attacks against CoCo guests are beyond the
+scope of this threat model.
+
+The **Linux CoCo VM attack surface** is any interface exposed from a CoCo
+guest Linux kernel towards an untrusted host that is not covered by the
+CoCo technology SW/HW protection. This includes any possible
+side-channels, as well as transient execution side channels. Examples of
+explicit (not side-channel) interfaces include accesses to port I/O, MMIO
+and DMA interfaces, access to PCI configuration space, VMM-specific
+hypercalls (towards Host-side VMM), access to shared memory pages,
+interrupts allowed to be injected into the guest kernel by the host, as
+well as CoCo technology-specific hypercalls, if present. Additionally, the
+host in a CoCo system typically controls the process of creating a CoCo
+guest: it has a method to load into a guest the firmware and bootloader
+images, the kernel image together with the kernel command line. All of this
+data should also be considered untrusted until its integrity and
+authenticity is established via attestation.
+
+The table below shows a threat matrix for the CoCo guest Linux kernel but
+does not discuss potential mitigation strategies. The matrix refers to
+CoCo-specific versions of the guest, host and platform.
+
+.. list-table:: CoCo Linux guest kernel threat matrix
+ :widths: auto
+ :align: center
+ :header-rows: 1
+
+ * - Threat name
+ - Threat description
+
+ * - Guest malicious configuration
+ - A misbehaving host modifies one of the following guest's
+ configuration:
+
+ 1. Guest firmware or bootloader
+
+ 2. Guest kernel or module binaries
+
+ 3. Guest command line parameters
+
+ This allows the host to break the integrity of the code running
+ inside a CoCo guest, and violates the CoCo security objectives.
+
+ * - CoCo guest data attacks
+ - A misbehaving host retains full control of the CoCo guest's data
+ in-transit between the guest and the host-managed physical or
+ virtual devices. This allows any attack against confidentiality,
+ integrity or freshness of such data.
+
+ * - Malformed runtime input
+ - A misbehaving host injects malformed input via any communication
+ interface used by the guest's kernel code. If the code is not
+ prepared to handle this input correctly, this can result in a host
+ --> guest kernel privilege escalation. This includes traditional
+ side-channel and/or transient execution attack vectors.
+
+ * - Malicious runtime input
+ - A misbehaving host injects a specific input value via any
+ communication interface used by the guest's kernel code. The
+ difference with the previous attack vector (malformed runtime input)
+ is that this input is not malformed, but its value is crafted to
+ impact the guest's kernel security. Examples of such inputs include
+ providing a malicious time to the guest or the entropy to the guest
+ random number generator. Additionally, the timing of such events can
+ be an attack vector on its own, if it results in a particular guest
+ kernel action (i.e. processing of a host-injected interrupt).
+ resistant to supplied host input.
+
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