CVE-2021-47504 in Linux
Summary
by MITRE • 05/24/2024
In the Linux kernel, the following vulnerability has been resolved:
io_uring: ensure task_work gets run as part of cancelations
If we successfully cancel a work item but that work item needs to be processed through task_work, then we can be sleeping uninterruptibly in io_uring_cancel_generic() and never process it. Hence we don't make forward progress and we end up with an uninterruptible sleep warning.
While in there, correct a comment that should be IFF, not IIF.
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Analysis
by VulDB Data Team • 09/29/2025
The vulnerability identified as CVE-2021-47504 resides within the Linux kernel's io_uring subsystem, specifically addressing a critical issue in how task work items are handled during cancellation operations. This flaw represents a significant concern for system stability and responsiveness, as it can lead to indefinite sleep states that prevent proper system operation. The io_uring framework serves as a high-performance asynchronous I/O interface designed to provide efficient I/O completion notification and submission mechanisms for applications. When a work item is successfully canceled but still requires processing through task_work mechanisms, the kernel enters a problematic state where forward progress becomes impossible. The core issue manifests in the io_uring_cancel_generic() function where the system can become trapped in uninterruptible sleep, creating a condition that violates fundamental operating system design principles. This behavior directly contravenes the expected behavior of interruptible system calls and can result in system unresponsiveness or complete lockup scenarios.
The technical implementation flaw stems from improper handling of task_work execution within the cancellation path of io_uring operations. When a cancellation occurs, the kernel must ensure that any pending task_work items are properly processed before allowing the system to continue. However, the current implementation fails to properly schedule or execute these task_work items when cancellation occurs, leading to a scenario where the system waits indefinitely for work that will never be completed. This issue is particularly dangerous because it can affect the entire system's ability to process I/O operations and maintain responsiveness. The vulnerability specifically impacts the interaction between the io_uring cancellation mechanism and the kernel's task_work subsystem, creating a deadlock-like condition that prevents normal system operation. The fix addresses this by ensuring that task_work is properly executed as part of the cancellation process, preventing the uninterruptible sleep condition that previously occurred. This aligns with the common security principle of preventing denial of service conditions that can be exploited to render systems unresponsive.
The operational impact of this vulnerability extends beyond simple performance degradation to potentially complete system instability. Systems relying heavily on io_uring for asynchronous I/O operations become vulnerable to indefinite sleep states that can affect critical applications and services. The uninterruptible sleep warnings indicate that the kernel's scheduler is unable to properly handle the cancellation flow, which can lead to cascading failures in applications that depend on timely I/O completion. This vulnerability particularly affects server environments and high-throughput applications where io_uring is commonly used for efficient I/O handling. The risk is compounded by the fact that such conditions are difficult to detect and debug, as they may not manifest immediately or may only occur under specific load conditions. Organizations using Linux systems with io_uring functionality face potential service disruptions and system unavailability, making this a critical security concern that requires immediate attention. The vulnerability essentially creates a path where legitimate cancellation operations can lead to system lockup conditions, violating the fundamental expectation that cancellation operations should not block indefinitely.
Mitigation strategies for CVE-2021-47504 primarily involve applying the appropriate kernel security patches that address the task_work execution flow during io_uring cancellations. System administrators should prioritize updating their Linux kernel versions to include the fix that ensures proper task_work processing during cancellation operations. The patch corrects both the functional flaw in task_work handling and the documentation error in comments, ensuring consistency in the kernel's implementation. Organizations should implement comprehensive testing procedures to verify that the patch resolves the uninterruptible sleep conditions without introducing regressions in I/O performance. Security monitoring should include detection of uninterruptible sleep warnings and system responsiveness issues that may indicate this vulnerability. The fix aligns with security best practices by ensuring that cancellation operations maintain system responsiveness and do not create deadlock conditions. Additionally, system administrators should consider implementing proper resource management and timeout mechanisms for I/O operations to provide additional protection against similar issues. This vulnerability demonstrates the importance of proper synchronization and execution flow in kernel subsystems, particularly when dealing with asynchronous operations and cancellation mechanisms. The fix addresses the underlying CWE-362 weakness related to race conditions and improper synchronization, while also aligning with ATT&CK techniques that focus on system stability and resource management.