CVE-2021-47321 in Linux
Summary
by MITRE • 05/21/2024
In the Linux kernel, the following vulnerability has been resolved:
watchdog: Fix possible use-after-free by calling del_timer_sync()
This driver's remove path calls del_timer(). However, that function does not wait until the timer handler finishes. This means that the timer handler may still be running after the driver's remove function has finished, which would result in a use-after-free.
Fix by calling del_timer_sync(), which makes sure the timer handler has finished, and unable to re-schedule itself.
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Analysis
by VulDB Data Team • 12/26/2024
The vulnerability identified as CVE-2021-47321 resides within the Linux kernel's watchdog driver subsystem, representing a critical use-after-free condition that could lead to system instability and potential exploitation. This flaw specifically affects the driver's remove path where improper timer management creates a race condition between the timer handler execution and the driver unloading process. The issue manifests when the watchdog driver attempts to clean up its resources by removing a timer without ensuring that any ongoing timer handler execution has completed, creating a window where memory access occurs after the associated data structures have been freed.
The technical root cause of this vulnerability stems from the improper use of kernel timer management functions within the driver's cleanup routine. When del_timer() is invoked during the driver removal process, it merely cancels the timer from the kernel's timer queue but does not guarantee that any currently executing timer handler has finished its execution. This asynchronous behavior creates a dangerous race condition where the timer handler might still be accessing memory locations that have been deallocated by the driver's remove function, resulting in a use-after-free scenario. The vulnerability is classified under CWE-416 as an improper access to freed memory, specifically manifesting in kernel space where such conditions can lead to system crashes or privilege escalation.
The operational impact of CVE-2021-47321 extends beyond simple system instability, potentially enabling attackers to exploit the use-after-free condition for privilege escalation or denial-of-service attacks. In kernel space, memory corruption from use-after-free vulnerabilities can be leveraged to execute arbitrary code with kernel privileges, making this a particularly dangerous flaw. The vulnerability affects systems running affected Linux kernel versions where watchdog drivers are loaded and actively used, particularly in embedded systems, servers, and industrial control environments where watchdog functionality is critical for system reliability and fault tolerance. The race condition makes this vulnerability difficult to predict and reproduce, as it depends on timing factors between the driver removal process and the timer handler execution.
Mitigation strategies for CVE-2021-47321 focus on implementing proper timer synchronization during driver cleanup operations. The recommended fix involves replacing del_timer() with del_timer_sync() in the driver's remove path, which ensures that the timer handler completes execution before the function returns. This approach aligns with the ATT&CK framework's defensive techniques for kernel-level protection, specifically addressing the persistence and privilege escalation tactics that could leverage such memory corruption vulnerabilities. System administrators should prioritize applying kernel updates that include the patched watchdog driver implementation, while security teams should monitor for potential exploitation attempts in environments where watchdog functionality is actively used. The fix demonstrates proper kernel programming practices as outlined in the Linux kernel documentation, emphasizing the importance of synchronization primitives when managing kernel resources during cleanup operations. Organizations should also consider implementing runtime monitoring to detect unusual timer behavior patterns that might indicate exploitation attempts targeting similar kernel vulnerabilities.