CVE-2017-13673 in QEMU
Summary
by MITRE
The vga display update in Qemu 2.8.0 through 2.9.0 mis-calculated the region for the dirty bitmap snapshot in case split screen mode is used causing a denial of service (assertion failure) in the "cpu_physical_memory_snapshot_get_dirty" function.
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Analysis
by VulDB Data Team • 12/27/2022
The vulnerability identified as CVE-2017-13673 affects the QEMU virtualization platform version 2.8.0 through 2.9.0, specifically within the VGA display update mechanism. This issue manifests as a miscalculation in the dirty bitmap snapshot region handling when operating in split screen mode configurations. The dirty bitmap functionality serves as a critical component for tracking memory changes during virtual machine snapshots, enabling efficient incremental backup operations by identifying which memory pages have been modified since the last snapshot. The flaw occurs during the "cpu_physical_memory_snapshot_get_dirty" function execution, where the system fails to properly account for memory regions when split screen mode is active, leading to a critical assertion failure that terminates the virtual machine operation.
The technical root cause of this vulnerability stems from improper memory region boundary calculations within the VGA subsystem's dirty bitmap management code. When split screen mode is enabled, the display subsystem divides the memory space into multiple regions, but the calculation logic fails to correctly handle the boundaries between these regions during snapshot operations. This misalignment results in the system attempting to access memory regions that either do not exist or are not properly mapped, triggering an assertion failure within the memory management subsystem. The issue is classified under CWE-129 Input Validation and Error Handling, specifically related to improper handling of memory boundaries and validation checks during snapshot operations. The vulnerability represents a classic case of buffer overflow prevention mechanisms being bypassed through incorrect boundary calculations, where the assertion failure occurs due to attempting to access memory outside of valid allocated regions.
The operational impact of this vulnerability extends beyond simple denial of service, as it affects the reliability and stability of virtualized environments that utilize QEMU's snapshot functionality. Virtual machine administrators and cloud service providers relying on incremental backup strategies using QEMU's dirty bitmap feature would experience unexpected termination of virtual machines during snapshot operations when split screen mode is active. This could lead to significant operational disruptions in production environments, particularly in scenarios where automated backup processes are configured to run during peak usage periods. The vulnerability affects systems that depend on snapshot-based recovery mechanisms, potentially causing data consistency issues and service interruptions. From an attacker perspective, this represents a reliable denial of service vector that can be exploited without requiring special privileges, as the condition occurs during normal operation of the virtualization platform.
Mitigation strategies for CVE-2017-13673 primarily focus on upgrading to QEMU versions that contain the patched implementation, specifically versions beyond 2.9.0 where the dirty bitmap calculation logic has been corrected. System administrators should implement immediate patch management procedures to upgrade affected QEMU installations, particularly in environments where split screen mode functionality is utilized. Additionally, organizations should consider implementing monitoring solutions to detect and alert on assertion failures within the QEMU process, enabling rapid response to potential exploitation attempts. The vulnerability aligns with ATT&CK technique T1499.004, specifically targeting virtualization and container environments through denial of service mechanisms. Organizations should also review their backup and snapshot procedures to avoid triggering the vulnerable code path when split screen mode is active, potentially implementing alternative backup strategies or ensuring that virtual machines are configured to use single screen modes during critical backup operations. The fix implemented in subsequent versions addresses the core calculation error by properly validating memory region boundaries and ensuring correct handling of multi-screen configurations during snapshot operations.