CVE-2023-46842 - Critical Vulnerability in Xen Hypervisor Affecting HVM Guests Can Cause System Crashes

A critical vulnerability (CVE-2023-46842) has been identified in the Xen Hypervisor, specifically affecting HVM guests which permit switching between 64-bit and other modes. The flaw exists in the processing of hypercalls that take considerable time, causing inconsistent register values and leading to hypervisor crashes. In this post, we will provide a detailed explanation of the vulnerability, a sample code snippet to demonstrate the issue, links to original references, and resources on how to exploit and mitigate the risk.

Vulnerability Overview

The issue revolves around HVM guests that are allowed to switch freely between 64-bit and other modes. This flexibility enables them to set registers for 32-bit-mode hypercall arguments to values that would otherwise be impossible for 32-bit code.

During the processing of extensive hypercalls, the hypervisor may choose to invoke a continuation to avoid blocking the system. This operation involves placing updated hypercall arguments in the respective registers. For guests not running in 64-bit mode, this further involves translating the values.

# Sample code snippet demonstrating the issue
def process_hypercall(args):
    # ... Do some processing
    if lengthy_processing_needed(args):
        args = update_args(args)
        return hypercall_continuation(args)
    else:
        return process_hypercall_directly(args)

def hypercall_continuation(args):
    translated_args = translate_args(args)
    # Check for high halves of registers
    if any(arg & xFFFFFFFF00000000 for arg in translated_args):
        raise ValueError("Inconsistent register value")
    return translated_args

The internal sanity checks of the Xen Hypervisor are not stringent enough, as they assume that the high halves of the registers should always be clear when invoking a hypercall. When this condition fails, it triggers a consistency check in the hypervisor, leading to a crash.

Exploit Details

An attacker with control of a guest can exploit this vulnerability to crash the entire host system, causing a Denial of Service (DoS) condition. The attacker can craft maliciously crafted hypercall arguments to trigger the crash.

Here are some resources detailing the vulnerability and its exploitation

Original Advisory: Xen Security Advisory 397 (CVE-2023-46842)

Exploit Code: N/A (Not publicly available)

Mitigation

Applying the appropriate patch is the most effective way to mitigate this vulnerability. You can download the patches here:

- Xen 4.14.x
- Xen 4.13.x

Conclusion

The CVE-2023-46842 vulnerability in the Xen Hypervisor poses a significant risk for systems utilizing HVM guests that can switch between 64-bit and other modes. Proper understanding of the vulnerability and applying the necessary patches are crucial steps to protect affected systems from potential exploits and system crashes.

Timeline

Published on: 05/16/2024 14:15:08 UTC
Last modified on: 12/04/2024 17:15:11 UTC