ACRN Shared Memory Based Inter-VM Communication

ACRN supports inter-virtual machine communication based on a shared memory mechanism. The ACRN Device Model or hypervisor emulates a virtual PCI device (called an ivshmem device) to expose the base address and size of this shared memory.

Inter-VM Communication Overview

Figure 187 ACRN Shared Memory Based Inter-VM Communication Architecture

ACRN can emulate the ivshmem device in two ways:

ivshmem dm-land

The ivshmem device is emulated in the ACRN Device Model, and the shared memory regions are reserved in the Service VM’s memory space. This solution only supports communication between post-launched User VMs.

ivshmem hv-land

The ivshmem device is emulated in the hypervisor, and the shared memory regions are reserved in the hypervisor’s memory space. This solution works for both pre-launched and post-launched User VMs.

While both solutions can be used at the same time, inter-VM communication may only be done between VMs using the same solution.

ivshmem hv:

The ivshmem hv implements register virtualization and shared memory mapping in the ACRN hypervisor. A notification/interrupt mechanism is supported.

ivshmem dm:

The ivshmem dm implements register virtualization and shared memory mapping in the ACRN Device Model (acrn-dm). It will support a notification/interrupt mechanism in the future.

ivshmem server:

With ivshmem server support, VMs with ivshmem devices enabled can send notifications (interrupts) to each other by writing the target peer ID (VM ID) and vector index to the doorbell register of the ivshmem device. The ivshmem server forwards this notification event to the target VM.

Two types of ivshmem server are defined in ACRN:

• User land ivshmem server is a daemon in user space to forward notifications for dm-land ivshmem devices only, by co-working with ivshmem dm. User land ivshmem server is not implemented.

• HV-land ivshmem server plays a role similar to the user land ivshmem server, but it is a hypervisor module and forwards notifications (virtual interrupts) to the target VM with hv-land ivshmem devices enabled.

Figure 188 ACRN Ivshmem HV-Land Doorbell Overview

Ivshmem Device Introduction

The ivshmem device is a virtual standard PCI device consisting of three Base Address Registers (BARs):

• BAR0 is used for emulating interrupt-related registers.

• BAR1 is used for emulating MSIX entry table.

• BAR2 is used for exposing a shared memory region.

The ivshmem device supports no extra capabilities.

Configuration Space Definition

Register	Offset	Value
Vendor ID	0x00	0x1AF4
Device ID	0x02	0x1110
Revision ID	0x08	0x1
Class Code	0x09	0x5

MMIO Registers Definition

Register	Offset	Read/Write	Description
IVSHMEM_IRQ_MASK_REG	0x0	R/W	Interrupt Mask register is used for legacy interrupt. ivshmem doesn’t support interrupts, so this register is reserved.
IVSHMEM_IRQ_STA_REG	0x4	R/W	Interrupt Status register is used for legacy interrupt. ivshmem doesn’t support interrupts, so this register is reserved.
IVSHMEM_IV_POS_REG	0x8	RO	Inter-VM Position register is used to identify the VM ID. Its value is zero.
IVSHMEM_DOORBELL_REG	0xC	WO	Doorbell register is used to trigger an interrupt to the peer VM. ivshmem doesn’t support interrupts.

Usage

For usage information, see Enable Inter-VM Shared Memory Communication (IVSHMEM).

Inter-VM Communication Security Hardening (BKMs)

Inter-VM communication enables you to create shared memory regions between post-launched User VMs. This mechanism is based on ivshmem v1.0 and exposes virtual PCI devices for the shared regions (in the Service VM’s memory). This feature adopts a community-approved design for shared memory between VMs, following the same specification for KVM/QEMU (Link).

Following the ACRN threat model, the policy definition for allocation and assignment of these regions is controlled by the Service VM, which is part of ACRN’s Trusted Computing Base (TCB). However, to secure inter-VM communication between any user space applications that harness this channel, applications will face more requirements for the confidentiality, integrity, and authenticity of shared or transferred data. The application development team is responsibility for defining a threat model and security architecture for the application and utilizing custom or public libraries accordingly. This document provides an overview about potential hardening techniques from a user space application’s perspective. Consider these techniques when defining the security architecture and threat model for your application.

Note

This document is not a definitive guide on all security technologies or how to implement security. We provide general pointers not bounded to a specific OS or use case.

1. Secure Feature Configurability

   • ACRN ensures a minimal control plane for the configuration of the memory region’s boundaries and name handles. This capability is managed only by the Service VM during the creation of the guest VM through the Device Model (DM).

   • Create different permissions or groups for the admin role to isolate it from other entities that might have access to the Service VM. For example, only admin permissions allow R/W/X on the DM binary.

2. Apply Access Control

   • Add restrictions based on behavior or subject and object rules around information flow and accesses.

   • In the Service VM, consider the /dev/shm device node as a critical interface with special access requirements. Those requirements can be fulfilled using any of the existing open source MAC technologies or even ACLs depending on the OS compatibility (Ubuntu, Windows, etc.) and integration complexity.

   • In the User VM, the shared memory region can be accessed using the mmap() of the UIO device node. Other complementary information can be found under:

     • /sys/class/uio/uioX/device/resource2 –> shared memory base address

     • /sys/class/uio/uioX/device/config –> shared memory size

   • For Linux-based User VMs, we recommend using the standard UIO and UIO_PCI_GENERIC drivers through the device node (for example, /dev/uioX).

   • Reference: AppArmor, SELinux, UIO driver-API

3. Crypto Support and Secure Applied Crypto

   • According to the application’s threat model and the defined assets that need to be shared securely, define the requirements for crypto algorithms. Those algorithms should enable operations such as authenticated encryption and decryption, secure key exchange, true random number generation, and seed extraction. In addition, consider the landscape of your attack surface and define the need for a security engine (for example, CSME services).

   • Don’t implement your own crypto functions. Use available compliant crypto libraries as applicable, such as Intel IPP or TinyCrypt.

   • Utilize the platform/kernel infrastructure and services (for example, Security High-Level Design, Kernel Crypto backend/APIs, and keyring subsystem).

   • Implement necessary flows for key lifecycle management, including wrapping, revocation, and migration, depending on the crypto key type and requirements for key persistence across system and power management events.

   • Follow open source secure crypto coding guidelines for secure wrappers and marshaling data structures: Secure Applied Crypto

   • References: NIST Crypto Standards and Guidelines, OpenSSL

4. Applications Passlisting

   • For use cases implemented in static environments (for example, Industrial and Automotive usages), follow application approval techniques and disable any third-party or native app stores.

   • This mechanism can be chained with the access control policies to protect access to passlisting rules and configuration files (refer to open source or implement your custom solution).

   • References: NIST SP800-167, fapolicyd

5. Secure Boot and File System Integrity Verification

   • The previously highlighted technologies rely on the kernel, as a secure component, to enforce such policies. We strongly recommend enabling secure boot for the Service VM, and extend the secure boot chain to any post-launched VM kernels.

   • To ensure that no malware is introduced or persists, utilize the file system (FS) verification methods on every boot to extend the secure boot chain for post-launch VMs (kernel/FS).

   • Reference: Enable Secure Boot in Windows

   • Reference Stack: dm-verity

Note

All the mentioned hardening techniques might require minor extra development efforts.