CVE-2026-98693 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
thunderbolt: Hold a router reference for each allocated HopID
tb_stop() drops the reference to all DP tunnels but does not deactivate them, thus nothing cancels a dprx_work still in flight (which holds its own tunnel reference) and the tunnel can outlive tb_switch_remove(). The HopID releases in tb_path_free() then operate on freed IDAs and trigger warnings like
ida_free called for id=8 which is not allocated.
This can be triggered by unbinding the driver while a DP tunnel is still waiting for the DPRX capabilities read to finish. On the Apple NHI unplugging the cable runs into just that reliably because the read can never finish right now and because the unplug powers down the entire USB4 complex and removes the NHI device.
Take a router reference whenever an input or output HopID is allocated and drop it again after the HopID is released. This keeps the ports and their HopID IDAs alive for as long as they are used.
The KUnit tests allocate routers without ever registering their devices so initialize the embedded struct device there as well and drop its initial reference when the test is finished.
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Analysis
by VulDB Data Team • 10/18/2026
This vulnerability represents a use-after-free condition within the Thunderbolt subsystem of the Linux kernel, specifically affecting how router references are managed during Dynamic Port tunnel lifecycle operations. The core technical flaw lies in an imbalance between resource allocation and deallocation for DisplayPort tunnels. When tb_stop is invoked to terminate DP tunnels, it releases the reference counts associated with those tunnels but fails to actively deactivate them or cancel pending asynchronous work items such as dprx_work. This oversight creates a race condition where background tasks retain references to tunnel structures that may have already been freed by higher-level removal processes like tb_switch_remove. Consequently, when the system attempts to free HopIDs via tb_path_free later in the teardown sequence, it operates on invalid or freed IDA (ID allocator) resources, triggering kernel warnings such as ida_free called for id=8 which is not allocated and potentially leading to memory corruption or undefined behavior if the race condition results in actual access of freed memory.
The operational impact of this vulnerability is most pronounced during driver unbinding scenarios where a DisplayPort tunnel remains active while its capabilities are being read from the DPRX (Display Port Receive) register. This scenario occurs reliably on Apple NHI hardware when a cable is unplugged, as the physical disconnection powers down the entire USB4 complex and triggers immediate removal of the NHI device. Because the DPRX capability read cannot complete under these power-down conditions, the associated work item remains in flight indefinitely or until forced termination after the underlying structures have been freed. This leads to kernel instability, potential crashes, or security implications if an attacker can manipulate timing to exploit the use-after-free state for arbitrary code execution, although the primary manifestation observed is a denial of service through kernel warnings and potential system hangs during device removal.
To mitigate this vulnerability, the fix implements strict reference counting semantics by holding a router reference whenever an input or output HopID is allocated. This ensures that the Thunderbolt switch structure remains valid for as long as any active tunnel relies on it, preventing premature deallocation of ports and their associated IDAs. The reference count is properly decremented only after the HopID is explicitly released, thereby synchronizing the lifecycle of the router with its dependent resources. Additionally, adjustments were made to KUnit tests which allocate routers without registering devices; these tests now initialize the embedded struct device structure and correctly drop the initial reference upon test completion to maintain consistency in resource management practices across both production code and testing frameworks.
From a vulnerability classification perspective, this issue aligns with CWE-416 Use After Free, as it involves accessing memory or resources after they have been freed due to improper lifecycle management. In terms of ATT&CK mapping, while primarily a stability issue rather than an exploit vector for lateral movement, the underlying mechanism relates to resource manipulation and could be categorized under T1078 Valid Accounts if leveraged in conjunction with privilege escalation techniques that rely on kernel memory corruption. The fix emphasizes robust state machine management and reference counting integrity within the Linux kernel's Thunderbolt driver stack, ensuring that asynchronous operations are properly synchronized with device removal events to prevent race conditions during hardware unplug or driver unload scenarios.