CVE-2017-12134 in Xen
Summary
by MITRE
The xen_biovec_phys_mergeable function in drivers/xen/biomerge.c in Xen might allow local OS guest users to corrupt block device data streams and consequently obtain sensitive memory information, cause a denial of service, or gain host OS privileges by leveraging incorrect block IO merge-ability calculation.
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Analysis
by VulDB Data Team • 01/09/2021
The vulnerability identified as CVE-2017-12134 resides within the Xen hypervisor's block I/O subsystem, specifically in the xen_biovec_phys_mergeable function located in drivers/xen/biomerge.c. This flaw represents a critical security issue that affects virtualized environments where multiple guest operating systems share the same host infrastructure. The vulnerability stems from an incorrect calculation of block I/O merge-ability, which allows local users within guest operating systems to manipulate how I/O operations are merged and processed. Such manipulation can lead to severe consequences including data corruption, information disclosure, and potential privilege escalation to host OS level access. The issue is particularly concerning because it operates at the hypervisor level where guest users typically have limited privileges, yet can exploit this vulnerability to compromise the integrity of the entire virtualization environment.
The technical root cause of this vulnerability lies in the improper handling of physical memory merge operations within the Xen hypervisor's block device driver. When the xen_biovec_phys_mergeable function incorrectly determines whether two block I/O operations can be safely merged, it creates a scenario where malicious input from guest users can cause the hypervisor to process data streams in unintended ways. This incorrect mergeability calculation can result in overlapping memory regions, improper data alignment, or invalid memory access patterns that ultimately lead to memory corruption. The flaw operates under CWE-129, which describes improper validation of array indices, and specifically relates to improper handling of memory merge operations in virtualized environments. Attackers can exploit this by carefully crafting block I/O requests that, when processed through the flawed merge logic, cause the hypervisor to write data to incorrect memory locations or access memory that should not be accessible to guest users.
The operational impact of this vulnerability extends far beyond simple data corruption, as it represents a potential path to complete system compromise. Local users within guest operating systems can leverage this flaw to perform memory disclosure attacks that expose sensitive information stored in the hypervisor's memory space, including kernel memory addresses, credentials, or other confidential data. The vulnerability also enables denial of service conditions where legitimate I/O operations may be disrupted or corrupted, leading to system instability or complete system crashes. More critically, the flaw could potentially allow privilege escalation to host OS level access, enabling attackers to break out of their virtual machine sandbox and gain control over the underlying physical hardware. This represents a fundamental breach of virtualization security principles where guest isolation is compromised, affecting all virtual machines running on the same host system. The attack vector is particularly dangerous because it requires no special privileges within the guest system, making it accessible to any local user who can submit block I/O operations.
Mitigation strategies for CVE-2017-12134 focus on both immediate patching and operational security improvements. The primary solution involves applying the official Xen hypervisor patches that correct the block I/O mergeability calculation logic in the affected driver. Organizations should prioritize patching their Xen installations as this vulnerability can be exploited remotely if guests have access to block I/O operations. Network segmentation and access controls should be implemented to limit guest user privileges and reduce the attack surface. Additionally, monitoring systems should be deployed to detect unusual I/O patterns that might indicate exploitation attempts. Security teams should implement regular vulnerability assessments and penetration testing to identify similar flaws in virtualization infrastructure. The ATT&CK framework categorizes this vulnerability under T1059 for command and control, and T1068 for exploit for privilege escalation, highlighting the need for comprehensive monitoring of system calls and memory access patterns. Organizations should also consider implementing hypervisor-level security controls such as memory protection units and kernel address space layout randomization to reduce the impact of potential exploitation. Regular security audits of virtualization environments are essential to maintain the integrity of the virtualized infrastructure and prevent similar vulnerabilities from being exploited in the future.