CVE-2026-80854 in Linux
Summary
by MITRE • 09/04/2026
In the Linux kernel, the following vulnerability has been resolved:
usb: gadget: f_tcm: keep port count until LUN teardown completes
tcm_usbg_drop_nexus() permits session removal once tpg_port_count reaches zero. However, usbg_port_unlink() currently decrements that count from the fabric_pre_unlink() callback, before core_dev_del_lun() waits for active se_lun references to drain.
If removal of the last LUN races a nexus removal, the latter can observe a zero port count and call target_remove_session(). This frees sess_cmd_map while an in-flight struct usbg_cmd, including its work item, can still be accessed.
Overlapping the last-LUN unlink with nexus removal reproduces this lifetime violation as a DEBUG_OBJECTS "free active" warning for usbg_cmd_work, followed by a target-core BUG/Oops.
The generic target-core unlink path has no callback after core_dev_del_lun() completes. Add an optional fabric_post_unlink() callback and use it for the f_tcm port count. The count now remains nonzero until core_dev_del_lun() has finished draining active LUN references, preventing nexus removal from freeing the session during command completion.
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Analysis
by VulDB Data Team • 09/04/2026
The Linux kernel USB gadget function driver for Target Core Module contains a race condition vulnerability related to resource lifecycle management and reference counting. This issue arises within the interaction between the fabric-specific port tracking mechanisms and the core device deletion logic. Specifically, the tcm_usbg_drop_nexus function is designed to permit session removal only after the target portal group port count reaches zero. However, the current implementation decrements this port count during the usbg_port_unlink operation, which occurs within the fabric_pre_unlink callback. This timing creates a critical window where the port count drops to zero before the core device deletion process has fully completed its cleanup tasks.
The core of the vulnerability lies in the asynchronous nature of reference counting and command completion. When a Logical Unit Number is unlinked, the system must wait for all active references to that LUN to drain completely. This ensures that any ongoing operations or commands associated with that LUN are safely finished before resources are freed. In the flawed implementation, because the port count is decremented prematurely in fabric_pre_unlink, an external event triggering nexus removal can observe a zero port count while internal structures related to active commands are still being processed. Consequently, target_remove_session may be invoked, leading to the premature freeing of sess_cmd_map and other session-related data structures.
This race condition results in a severe use-after-free vulnerability where in-flight struct usbg_cmd objects, including their associated work items, remain accessible after the underlying memory has been freed. If an attacker or automated testing tool reproduces this scenario by overlapping the unlinking of the last LUN with nexus removal, it triggers a DEBUG_OBJECTS warning indicating that active debug objects are being freed while still in use. This is followed by a kernel BUG or Oops due to invalid memory access when the system attempts to interact with the already-freed command structures. The vulnerability effectively allows for denial of service through kernel panic and potentially arbitrary code execution if an attacker can control the state of these dangling pointers, although exploitation typically requires precise timing conditions that may be difficult to achieve in practice without specific environmental factors.
From a standards perspective, this flaw aligns with CWE-416: Use After Free, as it involves accessing memory after it has been made available for reuse by another part of the program or system. Additionally, the race condition aspect relates to CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization. In terms of attack vectors and techniques, this vulnerability could be leveraged in contexts where an attacker can influence USB gadget configurations rapidly, potentially mapping to ATT&CK technique T1059: Command and Scripting Interpreter if the resulting kernel crash is used as part of a broader exploitation chain for privilege escalation or system disruption.
The resolution involves modifying the generic target-core unlink path by introducing an optional fabric_post_unlink callback. This new callback allows the f_tcm driver to defer the decrement of the port count until after core_dev_del_lun has completed and all active LUN references have been drained. By ensuring that the port count remains nonzero during the entire duration of command completion, the system prevents nexus removal from prematurely freeing the session data structures while commands are still in flight. This synchronization fix ensures proper ordering of resource deallocation events, eliminating the race condition and preventing the associated kernel instability.
To mitigate this vulnerability, systems running affected versions of the Linux kernel should be updated to a patched release that includes the fabric_post_unlink implementation for USB gadget targets. Administrators managing virtualized storage environments or using USB-based target configurations should verify their kernel version against known vulnerable ranges. Until patches are applied, limiting concurrent operations on LUNs and avoiding rapid reconfiguration of USB gadget sessions may reduce the likelihood of triggering this race condition, although such workarounds do not provide complete protection given the asynchronous nature of the underlying issue.