CVE-2021-47144 in Linux
Summary
by MITRE • 03/25/2024
In the Linux kernel, the following vulnerability has been resolved:
drm/amd/amdgpu: fix refcount leak
[Why]
the gem object rfb->base.obj[0] is get according to num_planes
in amdgpufb_create, but is not put according to num_planes
[How]
put rfb->base.obj[0] in amdgpu_fbdev_destroy according to num_planes
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Analysis
by VulDB Data Team • 11/23/2025
The vulnerability in question represents a reference count leak within the AMDGPU direct rendering manager subsystem of the Linux kernel. This issue occurs specifically within the drm/amd/amdgpu driver where improper handling of graphics object references leads to resource management failures that can ultimately impact system stability and security posture. The flaw manifests when creating framebuffer objects through the amdgpufb_create function, which retrieves graphics memory objects but fails to properly release them during cleanup operations.
The technical implementation of this vulnerability stems from a mismatch in reference counting operations within the AMDGPU driver's framebuffer handling code. During the initialization phase of framebuffer creation, the system correctly acquires references to graphics memory objects through the gem object rfb->base.obj[0] based on the number of planes specified in the configuration. However, during the destruction phase handled by amdgpu_fbdev_destroy function, the cleanup routine fails to iterate through all the acquired references according to the num_planes parameter, resulting in leaked reference counts that prevent proper memory deallocation.
This reference count leak creates a persistent resource consumption issue that can accumulate over time, particularly in systems with frequent framebuffer operations or multiple display configurations. The vulnerability operates at the kernel level within the graphics subsystem, making it particularly concerning as it can lead to memory exhaustion and potential denial of service conditions. From an operational perspective, this flaw represents a classic resource management error that violates fundamental principles of proper reference counting and object lifecycle management in kernel space code.
The security implications of this vulnerability extend beyond simple resource exhaustion, as improper reference counting can create pathways for privilege escalation or system instability. Attackers could potentially exploit this weakness to consume system resources progressively until the system becomes unresponsive, effectively creating a denial of service condition. The flaw also represents a deviation from established kernel programming best practices and security guidelines that mandate proper resource cleanup operations throughout all code paths.
Mitigation strategies should focus on implementing proper reference counting in the amdgpu_fbdev_destroy function to ensure that each graphics object acquired during initialization is properly released according to the num_planes parameter. This correction aligns with common vulnerability remediation patterns identified in industry standards such as CWE-401, which specifically addresses improper resource release or cleanup vulnerabilities. The fix requires careful synchronization with the existing reference counting logic and thorough testing to ensure no regressions in functionality occur during the cleanup process.
The resolution of this vulnerability demonstrates the importance of maintaining consistent reference counting practices throughout kernel subsystems, particularly in graphics drivers where complex object relationships and lifecycle management are common. This issue serves as a reminder of the critical nature of resource management in kernel space code, where improper handling can lead to cascading failures that impact system stability and security. The fix pattern recommended follows established ATT&CK framework principles for kernel-level vulnerability remediation, emphasizing proper object lifecycle management and resource cleanup operations.
This vulnerability type falls under the category of memory management flaws that are particularly dangerous in operating system kernels due to their potential for causing system instability and resource exhaustion. The solution requires careful attention to ensure that all acquired references are properly released regardless of the execution path taken during framebuffer creation or destruction, thereby maintaining proper reference counting semantics throughout the driver's operation.