CVE-2026-93214 in Linux
Summary
by MITRE • 09/24/2026
In the Linux kernel, the following vulnerability has been resolved:
usb: gadget: f_tcm: fix deadlock in usbg_make_tpg()
usbg_make_tpg() held dep_lock while calling configfs_depend_item_unlocked(), which acquires the configfs root inode lock when operating across subsystems. This creates a circular lock dependency with configfs_rmdir():
dep_lock -> configfs root inode lock -> su_mutex -> dep_lock
In usbg_make_tpg(), dep_lock only serialized the read of opts->ready, which is a monotonic flag that transitions from false to true exactly once (in tcm_set_name()) and never reverts. Remove dep_lock from usbg_make_tpg() entirely and use READ_ONCE/WRITE_ONCE to access opts->ready locklessly instead.
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Analysis
by VulDB Data Team • 09/24/2026
The Linux kernel vulnerability identified in the USB gadget function TCM driver involves a critical deadlock condition within the usbg_make_tpg function, which is responsible for creating target portal groups. This issue stems from an improper locking strategy that introduces circular dependencies between kernel synchronization primitives. Specifically, the function acquires dep_lock before invoking configfs_depend_item_unlocked. When this operation spans across different subsystems, it necessitates acquiring the configfs root inode lock. However, another code path involving configfs_rmdir holds the su_mutex and subsequently attempts to acquire dep_lock, creating a circular wait condition where one thread waits for a lock held by another, which in turn is waiting for the first thread's resource. This classic deadlock scenario can cause system hangs or require manual intervention via kernel panic mechanisms to recover, severely impacting system availability and stability during USB gadget configuration operations.
From a technical perspective, the root cause lies in the unnecessary serialization of reading the opts->ready flag using dep_lock. Analysis reveals that this flag is monotonic, meaning it transitions from false to true exactly once within tcm_set_name() and never reverts back to false. Because the state change is unidirectional and does not require complex mutual exclusion beyond ensuring visibility across CPUs, holding a heavy mutex like dep_lock for such an operation is both inefficient and dangerous when combined with other locks in different subsystems. The circular dependency chain involves dep_lock leading to the configfs root inode lock, which leads to su_mutex, and finally back to dep_lock. This intricate locking hierarchy violates standard kernel concurrency best practices by creating potential cycles that are difficult to detect during static analysis but manifest as fatal runtime errors under specific timing conditions involving concurrent configuration changes and removals.
The operational impact of this vulnerability is significant for systems utilizing USB gadget functionality with Target Configuration Management (TCM) backstores. An attacker or a misconfigured application could trigger the deadlock by rapidly creating and removing target portal groups, effectively causing a denial of service against the host system. In server environments where high availability is critical, such kernel-level deadlocks can lead to complete node failures within clustered configurations. Furthermore, because this involves USB gadget drivers often used in embedded systems or specialized hardware appliances, the reliability of these devices is compromised whenever dynamic reconfiguration occurs. The lack of proper lock ordering and unnecessary locking granularity reduces the robustness of the kernel's resource management subsystems, making them susceptible to simple race conditions that escalate into system-wide freezes.
To mitigate this vulnerability, the recommended approach involves removing dep_lock from usbg_make_tpg entirely and replacing direct accesses to opts->ready with READ_ONCE and WRITE_ONCE macros. This change ensures memory visibility across CPUs without introducing heavy locking overhead or creating dependency cycles. By treating the ready flag as a simple atomic state transition rather than a resource requiring mutual exclusion, the code eliminates the circular lock dependency while maintaining correctness. Developers should adhere strictly to kernel locking rules that prevent circular dependencies by enforcing a strict global order on lock acquisition and avoiding holding locks across subsystem boundaries unless absolutely necessary. Additionally, employing READ_ONCE/WRITE_ONCE for single-writer multi-reader flags is a standard pattern in Linux kernel development for optimizing performance and preventing deadlocks associated with unnecessary serialization of monotonic state changes. This fix aligns with general principles found in CWE-833 regarding improper locking mechanisms and mitigates risks associated with concurrency control failures that lead to resource exhaustion or system unresponsiveness.