CVE-2015-7812 in Xen
Summary
by MITRE
The hypercall_create_continuation function in arch/arm/domain.c in Xen 4.4.x through 4.6.x allows local guest users to cause a denial of service (host crash) via a preemptable hypercall to the multicall interface.
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Analysis
by VulDB Data Team • 06/25/2022
The vulnerability described in CVE-2015-7812 represents a critical denial of service flaw within the Xen hypervisor ecosystem affecting versions 4.4.x through 4.6.x. This issue resides in the hypercall_create_continuation function located in the arch/arm/domain.c file, which serves as a fundamental component in the ARM architecture implementation of the Xen virtualization platform. The vulnerability specifically targets the multicall interface, a mechanism that allows guest operating systems to execute multiple hypercalls in a single atomic operation, thereby improving performance and reducing overhead in virtualized environments.
The technical flaw stems from inadequate validation and handling of preemptable hypercalls within the continuation creation process. When a local guest user executes a preemptable hypercall through the multicall interface, the hypercall_create_continuation function fails to properly validate the continuation parameters or maintain proper state management during the hypercall processing. This improper handling creates a condition where malicious guest users can craft specific hypercall sequences that trigger unexpected behavior in the host hypervisor, ultimately leading to a complete system crash. The vulnerability exploits the inherent trust model between guest and host systems, where guest users with limited privileges can leverage this flaw to escalate their impact and compromise system availability.
The operational impact of this vulnerability extends beyond simple service disruption, as it represents a significant threat to the stability and reliability of virtualized environments. Host crashes caused by this vulnerability can result in complete system downtime, requiring manual intervention and potentially leading to data loss or service interruption across multiple virtual machines hosted on the affected system. The attack vector is particularly concerning because it requires only local access to a guest operating system, meaning that any user with access to a guest VM can potentially trigger the vulnerability. This makes the flaw particularly dangerous in multi-tenant environments where guest isolation is expected but cannot be guaranteed due to this vulnerability.
From a cybersecurity perspective, this vulnerability maps directly to CWE-121, which describes stack-based buffer overflow conditions, and aligns with ATT&CK technique T1499.004 for network denial of service attacks. The flaw demonstrates a classic example of privilege escalation through improper input validation, where guest-level privileges are leveraged to compromise host-level stability. Organizations implementing Xen virtualization platforms in production environments must consider this vulnerability as a critical threat requiring immediate attention and remediation. The impact is particularly severe in cloud computing environments where multiple customers share the same physical infrastructure, as a single compromised guest could potentially affect the entire host system and its other virtualized guests.
Mitigation strategies for this vulnerability include immediate patching of affected Xen versions to 4.7.0 or later, which contains the necessary fixes to properly validate hypercall continuation parameters. System administrators should also implement monitoring solutions to detect unusual hypercall patterns that might indicate exploitation attempts. Additionally, implementing proper guest isolation measures and limiting guest user privileges can reduce the attack surface. The vulnerability highlights the importance of maintaining up-to-date virtualization infrastructure and demonstrates the critical need for continuous security assessments of hypervisor components, particularly those handling inter-vm communication and hypercall processing mechanisms.