CVE-2021-47123 in Linux
Summary
by MITRE • 03/15/2024
In the Linux kernel, the following vulnerability has been resolved:
io_uring: fix ltout double free on completion race
Always remove linked timeout on io_link_timeout_fn() from the master request link list, otherwise we may get use-after-free when first io_link_timeout_fn() puts linked timeout in the fail path, and then will be found and put on master's free.
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Analysis
by VulDB Data Team • 01/14/2025
The vulnerability CVE-2021-47123 represents a critical double free condition within the Linux kernel's io_uring subsystem that stems from improper handling of timeout objects during asynchronous I/O operations. This issue specifically affects the io_link_timeout_fn() function which manages linked timeout objects in the io_uring completion path. The flaw occurs when a timeout object is linked to a master request and subsequently processed through the fail path, leading to a scenario where the same memory location may be freed twice, creating a use-after-free condition that can be exploited for arbitrary code execution or system instability.
The technical root cause of this vulnerability lies in the io_uring implementation's handling of linked timeout objects within the completion race condition scenario. When io_link_timeout_fn() processes timeout objects, it fails to consistently remove linked timeouts from the master request link list, creating a race condition where the same timeout object can be processed multiple times. This improper cleanup mechanism allows for a situation where the first invocation of io_link_timeout_fn() places the linked timeout in a fail path, but subsequent processing attempts to free the same object again during the master request's cleanup phase, resulting in the double free condition that violates fundamental memory management principles.
This vulnerability operates at the kernel level and presents significant operational impact for systems utilizing io_uring for high-performance asynchronous I/O operations. The use-after-free condition can lead to system crashes, data corruption, or potentially privilege escalation depending on the exploitation vector. Systems running Linux kernels with io_uring functionality enabled are particularly at risk, especially those handling high volumes of asynchronous I/O operations where the race condition is more likely to manifest. The vulnerability affects the kernel's memory management subsystem and can compromise the integrity of the entire operating system, making it a critical security concern for enterprise environments relying on high-performance I/O operations.
The fix for CVE-2021-47123 involves ensuring that linked timeouts are always removed from the master request link list within the io_link_timeout_fn() function, regardless of the execution path taken. This remediation addresses the core race condition by guaranteeing proper cleanup of timeout objects and prevents the double free scenario that leads to use-after-free conditions. The solution aligns with CWE-415 which specifically addresses double free conditions in memory management, and follows the ATT&CK technique T1068 for privilege escalation through kernel vulnerabilities. Organizations should prioritize applying kernel updates that include this fix to protect against potential exploitation, particularly in environments where io_uring is actively utilized for performance-critical applications. The mitigation strategy should also include monitoring for anomalous system behavior or crashes that might indicate exploitation attempts targeting this specific memory management vulnerability.