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Kernel developers working on confidential computing for virtualized environments in x86 operate under a set of assumptions regarding the Linux kernel threat model that differs 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 networking or limited HW-specific exposed interfaces (e.g. USB, thunderbolt). The goal of this document is to explain additional attack vectors that arise in the virtualized confidential computing space. Reviewed-by: Larry Dewey <larry.dewey@amd.com> Reviewed-by: David Kaplan <david.kaplan@amd.com> Co-developed-by: Elena Reshetova <elena.reshetova@intel.com> Signed-off-by: Elena Reshetova <elena.reshetova@intel.com> Signed-off-by: Carlos Bilbao <carlos.bilbao@amd.com> Message-ID: <98804f27-c2e7-74d6-d671-1eda927e19fe@amd.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Confidential Computing in Linux for x86 virtualization
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======================================================
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.. contents:: :local:
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By: Elena Reshetova <elena.reshetova@intel.com> and Carlos Bilbao <carlos.bilbao@amd.com>
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Motivation
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==========
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Kernel developers working on confidential computing for virtualized
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environments in x86 operate under a set of assumptions regarding the Linux
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kernel threat model that differ from the traditional view. Historically,
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the Linux threat model acknowledges attackers residing in userspace, as
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well as a limited set of external attackers that are able to interact with
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the kernel through various networking or limited HW-specific exposed
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interfaces (USB, thunderbolt). The goal of this document is to explain
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additional attack vectors that arise in the confidential computing space
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and discuss the proposed protection mechanisms for the Linux kernel.
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Overview and terminology
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========================
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Confidential Computing (CoCo) is a broad term covering a wide range of
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security technologies that aim to protect the confidentiality and integrity
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of data in use (vs. data at rest or data in transit). At its core, CoCo
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solutions provide a Trusted Execution Environment (TEE), where secure data
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processing can be performed and, as a result, they are typically further
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classified into different subtypes depending on the SW that is intended
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to be run in TEE. This document focuses on a subclass of CoCo technologies
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that are targeting virtualized environments and allow running Virtual
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Machines (VM) inside TEE. From now on in this document will be referring
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to this subclass of CoCo as 'Confidential Computing (CoCo) for the
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virtualized environments (VE)'.
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CoCo, in the virtualization context, refers to a set of HW and/or SW
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technologies that allow for stronger security guarantees for the SW running
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inside a CoCo VM. Namely, confidential computing allows its users to
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confirm the trustworthiness of all SW pieces to include in its reduced
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Trusted Computing Base (TCB) given its ability to attest the state of these
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trusted components.
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While the concrete implementation details differ between technologies, all
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available mechanisms aim to provide increased confidentiality and
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integrity for the VM's guest memory and execution state (vCPU registers),
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more tightly controlled guest interrupt injection, as well as some
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additional mechanisms to control guest-host page mapping. More details on
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the x86-specific solutions can be found in
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:doc:`Intel Trust Domain Extensions (TDX) </arch/x86/tdx>` and
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`AMD Memory Encryption <https://www.amd.com/system/files/techdocs/sev-snp-strengthening-vm-isolation-with-integrity-protection-and-more.pdf>`_.
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The basic CoCo guest layout includes the host, guest, the interfaces that
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communicate guest and host, a platform capable of supporting CoCo VMs, and
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a trusted intermediary between the guest VM and the underlying platform
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that acts as a security manager. The host-side virtual machine monitor
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(VMM) typically consists of a subset of traditional VMM features and
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is still in charge of the guest lifecycle, i.e. create or destroy a CoCo
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VM, manage its access to system resources, etc. However, since it
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typically stays out of CoCo VM TCB, its access is limited to preserve the
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security objectives.
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In the following diagram, the "<--->" lines represent bi-directional
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communication channels or interfaces between the CoCo security manager and
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the rest of the components (data flow for guest, host, hardware) ::
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+-------------------+ +-----------------------+
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| CoCo guest VM |<---->| |
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+-------------------+ | |
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| Interfaces | | CoCo security manager |
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+-------------------+ | |
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| Host VMM |<---->| |
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+-------------------+ | |
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| |
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+--------------------+ | |
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| CoCo platform |<--->| |
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+--------------------+ +-----------------------+
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The specific details of the CoCo security manager vastly diverge between
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technologies. For example, in some cases, it will be implemented in HW
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while in others it may be pure SW.
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Existing Linux kernel threat model
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==================================
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The overall components of the current Linux kernel threat model are::
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+-----------------------+ +-------------------+
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| |<---->| Userspace |
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| | +-------------------+
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| External attack | | Interfaces |
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| vectors | +-------------------+
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| |<---->| Linux Kernel |
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| | +-------------------+
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+-----------------------+ +-------------------+
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| Bootloader/BIOS |
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+-------------------+
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+-------------------+
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| HW platform |
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+-------------------+
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There is also communication between the bootloader and the kernel during
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the boot process, but this diagram does not represent it explicitly. The
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"Interfaces" box represents the various interfaces that allow
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communication between kernel and userspace. This includes system calls,
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kernel APIs, device drivers, etc.
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The existing Linux kernel threat model typically assumes execution on a
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trusted HW platform with all of the firmware and bootloaders included on
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its TCB. The primary attacker resides in the userspace, and all of the data
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coming from there is generally considered untrusted, unless userspace is
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privileged enough to perform trusted actions. In addition, external
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attackers are typically considered, including those with access to enabled
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external networks (e.g. Ethernet, Wireless, Bluetooth), exposed hardware
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interfaces (e.g. USB, Thunderbolt), and the ability to modify the contents
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of disks offline.
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Regarding external attack vectors, it is interesting to note that in most
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cases external attackers will try to exploit vulnerabilities in userspace
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first, but that it is possible for an attacker to directly target the
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kernel; particularly if the host has physical access. Examples of direct
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kernel attacks include the vulnerabilities CVE-2019-19524, CVE-2022-0435
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and CVE-2020-24490.
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Confidential Computing threat model and its security objectives
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===============================================================
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Confidential Computing adds a new type of attacker to the above list: a
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potentially misbehaving host (which can also include some part of a
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traditional VMM or all of it), which is typically placed outside of the
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CoCo VM TCB due to its large SW attack surface. It is important to note
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that this doesn’t imply that the host or VMM are intentionally
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malicious, but that there exists a security value in having a small CoCo
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VM TCB. This new type of adversary may be viewed as a more powerful type
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of external attacker, as it resides locally on the same physical machine
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(in contrast to a remote network attacker) and has control over the guest
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kernel communication with most of the HW::
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+------------------------+
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| CoCo guest VM |
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+-----------------------+ | +-------------------+ |
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| |<--->| | Userspace | |
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| | | +-------------------+ |
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| External attack | | | Interfaces | |
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| vectors | | +-------------------+ |
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| |<--->| | Linux Kernel | |
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| | | +-------------------+ |
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+-----------------------+ | +-------------------+ |
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| | Bootloader/BIOS | |
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+-----------------------+ | +-------------------+ |
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| |<--->+------------------------+
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| | | Interfaces |
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| | +------------------------+
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| CoCo security |<--->| Host/Host-side VMM |
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| manager | +------------------------+
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| | +------------------------+
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| |<--->| CoCo platform |
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+-----------------------+ +------------------------+
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While traditionally the host has unlimited access to guest data and can
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leverage this access to attack the guest, the CoCo systems mitigate such
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attacks by adding security features like guest data confidentiality and
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integrity protection. This threat model assumes that those features are
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available and intact.
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The **Linux kernel CoCo VM security objectives** can be summarized as follows:
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1. Preserve the confidentiality and integrity of CoCo guest's private
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memory and registers.
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2. Prevent privileged escalation from a host into a CoCo guest Linux kernel.
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While it is true that the host (and host-side VMM) requires some level of
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privilege to create, destroy, or pause the guest, part of the goal of
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preventing privileged escalation is to ensure that these operations do not
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provide a pathway for attackers to gain access to the guest's kernel.
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The above security objectives result in two primary **Linux kernel CoCo
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VM assets**:
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1. Guest kernel execution context.
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2. Guest kernel private memory.
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The host retains full control over the CoCo guest resources, and can deny
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access to them at any time. Examples of resources include CPU time, memory
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that the guest can consume, network bandwidth, etc. Because of this, the
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host Denial of Service (DoS) attacks against CoCo guests are beyond the
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scope of this threat model.
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The **Linux CoCo VM attack surface** is any interface exposed from a CoCo
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guest Linux kernel towards an untrusted host that is not covered by the
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CoCo technology SW/HW protection. This includes any possible
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side-channels, as well as transient execution side channels. Examples of
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explicit (not side-channel) interfaces include accesses to port I/O, MMIO
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and DMA interfaces, access to PCI configuration space, VMM-specific
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hypercalls (towards Host-side VMM), access to shared memory pages,
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interrupts allowed to be injected into the guest kernel by the host, as
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well as CoCo technology-specific hypercalls, if present. Additionally, the
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host in a CoCo system typically controls the process of creating a CoCo
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guest: it has a method to load into a guest the firmware and bootloader
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images, the kernel image together with the kernel command line. All of this
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data should also be considered untrusted until its integrity and
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authenticity is established via attestation.
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The table below shows a threat matrix for the CoCo guest Linux kernel but
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does not discuss potential mitigation strategies. The matrix refers to
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CoCo-specific versions of the guest, host and platform.
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.. list-table:: CoCo Linux guest kernel threat matrix
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:widths: auto
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:align: center
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:header-rows: 1
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* - Threat name
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- Threat description
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* - Guest malicious configuration
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- A misbehaving host modifies one of the following guest's
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configuration:
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1. Guest firmware or bootloader
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2. Guest kernel or module binaries
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3. Guest command line parameters
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This allows the host to break the integrity of the code running
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inside a CoCo guest, and violates the CoCo security objectives.
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* - CoCo guest data attacks
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- A misbehaving host retains full control of the CoCo guest's data
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in-transit between the guest and the host-managed physical or
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virtual devices. This allows any attack against confidentiality,
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integrity or freshness of such data.
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* - Malformed runtime input
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- A misbehaving host injects malformed input via any communication
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interface used by the guest's kernel code. If the code is not
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prepared to handle this input correctly, this can result in a host
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--> guest kernel privilege escalation. This includes traditional
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side-channel and/or transient execution attack vectors.
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* - Malicious runtime input
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- A misbehaving host injects a specific input value via any
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communication interface used by the guest's kernel code. The
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difference with the previous attack vector (malformed runtime input)
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is that this input is not malformed, but its value is crafted to
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impact the guest's kernel security. Examples of such inputs include
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providing a malicious time to the guest or the entropy to the guest
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random number generator. Additionally, the timing of such events can
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be an attack vector on its own, if it results in a particular guest
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kernel action (i.e. processing of a host-injected interrupt).
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resistant to supplied host input.
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