CVE-2021-28693 in Xen
Summary
by MITRE • 06/30/2021
xen/arm: Boot modules are not scrubbed The bootloader will load boot modules (e.g. kernel, initramfs...) in a temporary area before they are copied by Xen to each domain memory. To ensure sensitive data is not leaked from the modules, Xen must "scrub" them before handing the page over to the allocator. Unfortunately, it was discovered that modules will not be scrubbed on Arm.
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Analysis
by VulDB Data Team • 07/04/2021
The vulnerability identified as CVE-2021-28693 represents a critical security flaw in the Xen hypervisor implementation specifically affecting arm architecture systems. This issue stems from inadequate memory sanitization practices during the boot process where sensitive data could potentially leak between virtual machines due to improper handling of temporary boot module storage. The root cause lies in the failure of the Xen hypervisor to properly scrub memory pages containing boot modules before these pages are made available for reuse within the system's memory allocator. This vulnerability directly impacts the security isolation guarantees that virtualization platforms are expected to maintain between different domains.
The technical implementation flaw occurs within the arm-specific boot module handling code where the bootloader loads kernel images, initramfs components, and other critical boot data into temporary memory regions. These temporary areas are subsequently copied by Xen to each domain's memory space for execution. However, the scrubbing mechanism that should clear sensitive data from these temporary pages before they are reallocated is absent or malfunctioning specifically on arm platforms. This creates a scenario where remnants of previous boot modules could persist in memory and potentially be accessed by malicious actors or compromised domains, violating fundamental security principles of memory isolation and data sanitization.
From an operational perspective, this vulnerability presents significant risks to virtualized environments running on arm architecture hardware. Attackers could potentially exploit this flaw to extract sensitive information from memory regions that should have been cleared, including kernel memory layouts, cryptographic keys, or other confidential data that might have been present in previous boot modules. The impact extends beyond simple information disclosure as it undermines the core security model of virtualization, potentially allowing privilege escalation attacks or cross-domain data leakage. Systems running Xen hypervisors on arm processors are particularly vulnerable, with the risk being most pronounced in multi-tenant cloud environments where isolation between different virtual machines is paramount.
The vulnerability aligns with CWE-242, which addresses "Use of Inherently Dangerous Function," and relates to memory management security issues that could enable information disclosure through improper data sanitization practices. From an ATT&CK framework perspective, this vulnerability maps to techniques involving privilege escalation and information gathering through memory inspection. The most effective mitigations involve implementing proper memory scrubbing routines that ensure all temporary boot module data is completely cleared before memory pages are returned to the allocator. Organizations should apply the relevant security patches provided by Xen project maintainers, which typically include enhanced memory sanitization procedures specifically for arm architecture platforms. Additionally, system administrators should consider implementing additional monitoring and detection measures to identify potential exploitation attempts and ensure that all virtualization environments are updated with the latest security patches to prevent unauthorized access to sensitive data through memory leakage mechanisms.