CVE-2021-28703 : Security Advisory and Response

Grant table v2 status pages may remain accessible after de-allocation in Xen Project versions up to Branch 4.13 unless patched for XSA-378. This vulnerability could allow a malicious guest to escalate privileges, leading to Denial of Service (DoS) or information leaks.

Understanding CVE-2021-28703

This CVE refers to a security flaw in Xen Project that could enable a guest to retain access to memory pages even after deallocation, potentially compromising the host system.

What is CVE-2021-28703?

Guests in Xen Project are granted access to specific memory pages, some of which are deallocated when guests switch back from grant table v2 to v1. However, due to potential race conditions, guests may retain access to freed pages, leading to security risks.

The Impact of CVE-2021-28703

The vulnerability could empower a malicious guest to raise its privileges to the host's level, trigger DoS on the host or guests, and even result in sensitive information leaks.

Technical Details of CVE-2021-28703

This security issue affects Xen Project versions up to Branch 4.13 that have not applied the necessary patches for XSA-378. Only specific x86 guests leveraging grant table version 2 interfaces are susceptible.

Vulnerability Description

The bug allows guests to maintain access to deallocated memory pages due to improper handling during guest switches, potentially leading to memory corruption and unauthorized access.

Affected Systems and Versions

Xen Project versions up to Branch 4.13 are vulnerable if not patched for XSA-378. Versions 4.13.4, 4.14.x, and 4.15.x are not affected by this flaw.

Exploitation Mechanism

A malicious guest could exploit this vulnerability by racing requests to insert mappings of deallocated pages, allowing them to retain access to freed memory.

Mitigation and Prevention

To address CVE-2021-28703, immediate actions such as running only PV guests or suppressing grant table v2 interfaces for HVM or PVH guests are recommended.

Immediate Steps to Take

Admins should apply relevant patches for XSA-378 and closely monitor guest memory allocations to prevent unauthorized access or memory leaks.

Long-Term Security Practices

Regularly updating Xen Project to security-supported versions and maintaining a robust vulnerability management process can help mitigate similar risks in the long run.

Patching and Updates

Ensure timely application of patches for known vulnerabilities like XSA-378 and stay informed about security advisories to protect your Xen Project deployments effectively.