CVE-2021-47073 in Linux
Summary
by MITRE • 03/02/2024
In the Linux kernel, the following vulnerability has been resolved:
platform/x86: dell-smbios-wmi: Fix oops on rmmod dell_smbios
init_dell_smbios_wmi() only registers the dell_smbios_wmi_driver on systems where the Dell WMI interface is supported. While exit_dell_smbios_wmi() unregisters it unconditionally, this leads to the following oops:
[ 175.722921] ------------[ cut here ]------------
[ 175.722925] Unexpected driver unregister!
[ 175.722939] WARNING: CPU: 1 PID: 3630 at drivers/base/driver.c:194 driver_unregister+0x38/0x40
... [ 175.723089] Call Trace:
[ 175.723094] cleanup_module+0x5/0xedd [dell_smbios]
... [ 175.723148] ---[ end trace 064c34e1ad49509d ]---
Make the unregister happen on the same condition the register happens to fix this.
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Analysis
by VulDB Data Team • 01/09/2025
The vulnerability CVE-2021-47073 represents a kernel-level issue within the Linux operating system affecting the dell_smbios_wmi driver module. This flaw manifests as a kernel oops condition that occurs during module removal operations, specifically when the rmmod command is executed to unload the dell_smbios module. The problem stems from an inconsistent registration and unregistration pattern within the driver's initialization and cleanup functions, creating a scenario where the kernel attempts to unregister a driver that was never properly registered.
The technical root cause resides in the platform/x86 subsystem of the Linux kernel where the dell_smbios_wmi driver is implemented. The init_dell_smbios_wmi() function employs conditional logic to register the dell_smbios_wmi_driver only on systems that support the Dell WMI interface, ensuring proper hardware compatibility. However, the corresponding exit_dell_smbios_wmi() cleanup function executes an unconditional driver_unregister() operation regardless of whether the driver was actually registered. This mismatch creates a kernel panic condition that violates fundamental driver management principles and results in system instability.
This vulnerability directly impacts system stability and reliability by introducing a potential kernel crash condition during normal module lifecycle operations. The oops condition occurs at the driver.c kernel file level at line 194 in the driver_unregister function, indicating a critical failure in the kernel's device driver framework. When the rmmod command is executed, the kernel's driver subsystem detects an attempt to unregister a driver that does not exist in the registration table, triggering the warning message and subsequent system trace. Such behavior represents a classic case of improper resource management and violates the expected operational semantics of kernel driver modules.
The fix for CVE-2021-47073 implements a straightforward but critical correction to align the unregister operation with the same conditional logic used during registration. This remediation ensures that driver_unregister() is only called when the driver was actually registered during initialization, preventing the kernel from attempting to unregister non-existent driver entries. The solution addresses a fundamental flaw in the module's lifecycle management that aligns with security best practices and kernel design principles outlined in the Linux kernel documentation. From an ATT&CK framework perspective, this vulnerability could potentially be exploited to cause denial of service conditions, though the impact is limited to system stability rather than privilege escalation or data compromise. The fix demonstrates proper defensive programming practices and adherence to the principle of least privilege in kernel module development, ensuring that all driver operations occur only when appropriate for the target system configuration. This vulnerability represents a common class of issues in kernel development where conditional initialization logic is not consistently applied to cleanup operations, making it a prime example of how seemingly minor oversights can result in significant system stability impacts. The resolution maintains the intended functionality while eliminating the risk of kernel panics during normal system operation.