CVE-2017-12136 in Xen
Summary
by MITRE
Race condition in the grant table code in Xen 4.6.x through 4.9.x allows local guest OS administrators to cause a denial of service (free list corruption and host crash) or gain privileges on the host via vectors involving maptrack free list handling.
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Analysis
by VulDB Data Team • 11/07/2019
The vulnerability identified as CVE-2017-12136 represents a critical race condition within the grant table implementation of the Xen hypervisor affecting versions 4.6.x through 4.9.x. This flaw exists in the maptrack free list handling mechanism that manages memory mapping operations between guest operating systems and the host hypervisor. The race condition occurs when multiple threads or processes attempt to access and modify the same free list data structure simultaneously, creating a scenario where concurrent operations can corrupt the internal data structures used for tracking memory mappings. The grant table functionality serves as a crucial component for memory sharing between guests and the host, making this vulnerability particularly dangerous as it directly impacts the hypervisor's memory management subsystem.
The technical exploitation of this race condition involves local guest OS administrators who can manipulate the grant table operations to trigger concurrent access patterns that lead to memory corruption. When the maptrack free list handling code executes concurrently with multiple mapping operations, the lack of proper synchronization mechanisms allows for inconsistent state updates within the free list structure. This results in corrupted free list entries that can cause the hypervisor to attempt to free memory that has already been freed or to allocate memory from corrupted free lists. The vulnerability specifically targets the interaction between guest memory mapping operations and the host's grant table management, creating a pathway for privilege escalation or system instability.
The operational impact of CVE-2017-12136 extends beyond simple denial of service scenarios to include potential privilege escalation opportunities that could allow guest administrators to execute code with host-level privileges. The corruption of free lists within the hypervisor's memory management system can lead to unpredictable behavior including host crashes, system instability, and potential data corruption across all virtual machines running on the compromised host. This vulnerability is particularly concerning in cloud computing environments where multiple tenants share the same physical hardware, as a malicious guest administrator could potentially compromise the entire host system and affect other virtual machines. The race condition nature means that successful exploitation is not guaranteed but becomes more likely under specific concurrent access patterns, making it a persistent threat in multi-tenant environments.
Mitigation strategies for this vulnerability require immediate patching of Xen hypervisor versions to address the synchronization issues in the grant table implementation. Organizations should prioritize updating to patched versions of Xen 4.6.x through 4.9.x, as the fix typically involves implementing proper locking mechanisms around the maptrack free list operations to prevent concurrent access. System administrators should also implement monitoring solutions to detect unusual memory management patterns that might indicate exploitation attempts. The vulnerability aligns with CWE-362, which describes race conditions in concurrent programming, and maps to ATT&CK technique T1055 for privilege escalation through hypervisor manipulation. Additional defensive measures include implementing virtual machine isolation controls, monitoring guest-to-host memory mapping operations, and conducting regular vulnerability assessments of virtualization environments to identify similar race condition vulnerabilities in other hypervisor components.