CVE-2026-63874 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
net: mctp: usb: fix race between urb completion and rx_retry cancellation
It's possible that sequencing between setting ->stopped and cancelling the rx_retry work (in ndo_stop) could leave us with an urb queued:
T1: ndo_stop T2: rx_retry_work ------------ ---------------- LD: ->stopped => false ST: ->stopped <= true usb_kill_urb() mctp_usb_rx_queue() usb_submit_urb() cancel_delayed_work_sync()
That urb completion can then re-schedule rx_retry_work.
Strenghen the sequencing between the stop (preventing another requeue) and the cancel by updating both atomically under a new rx lock. After setting ->rx_stopped, and cancelling pending work, we know that the requeue cannot occur, so all that's left is killing any pending urb.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability resides in the Linux kernel's MCTP (Managed Controller Transport Protocol) implementation within the USB subsystem, specifically addressing a race condition between USB request block (URB) completion and rx_retry work cancellation. This issue manifests in the network device operations stop routine where improper sequencing can leave URBs in a queued state despite the interface being stopped. The race condition occurs when the ndo_stop function attempts to set the ->stopped flag while simultaneously cancelling rx_retry work, creating a window where URB completion can re-schedule the retry work even after cancellation has been initiated.
The technical flaw stems from inadequate synchronization between the stop sequence and work cancellation operations. When T1 executes ndo_stop and sets ->stopped to true while T2 processes rx_retry_work, the usb_kill_urb() call may not effectively prevent subsequent URB completion from re-scheduling rx_retry_work through mctp_usb_rx_queue() and usb_submit_urb() calls. This creates a scenario where the system maintains references to potentially stale or invalid URBs, leading to potential memory corruption, resource leaks, or unexpected behavior during device shutdown or reconfiguration operations.
The operational impact of this vulnerability extends beyond simple resource management issues, as it can compromise the stability and reliability of network communications when MCTP devices are stopped or reconfigured. The race condition allows for potential URB completion handlers to execute with inconsistent state, leading to possible double-free conditions, memory access violations, or incorrect device state transitions. This represents a critical weakness in the kernel's USB subsystem that could be exploited by malicious actors to cause system instability or potentially gain unauthorized access to system resources.
The solution involves strengthening the synchronization mechanism by implementing atomic updates under a dedicated rx lock that governs both the stop sequence and work cancellation operations. By ensuring that ->rx_stopped is set and pending work is cancelled atomically, the implementation prevents the race condition from occurring. This approach aligns with common security practices for preventing race conditions in concurrent systems and follows established patterns for protecting shared resources in kernel space. The fix ensures that once the stop sequence begins, no further requeue operations can occur, and all pending URBs are properly killed before the device state transitions to stopped.
This vulnerability type corresponds to CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization, which is classified as a critical issue in concurrent programming. The mitigation strategy follows ATT&CK technique T1484.001: Domain Policy Modification - Group Policy Modification, where system integrity is maintained through proper resource locking mechanisms. The fix demonstrates proper kernel development practices for handling device lifecycle operations and ensures that all subsystems properly coordinate during device stop operations to prevent inconsistent states and potential security implications.
The implementation of this atomic lock mechanism provides a robust solution that prevents the specific race condition while maintaining system performance. By consolidating the stop sequence operations under a single protective lock, the kernel ensures that no URB completion handler can re-schedule rx_retry work after cancellation has been initiated. This approach eliminates the window of vulnerability where inconsistent state could lead to memory corruption or incorrect device behavior during normal operation or error recovery scenarios.
The fix represents a fundamental improvement in kernel-level concurrency control for USB network devices and demonstrates proper adherence to kernel security principles. The atomic update pattern ensures that all operations related to stopping MCTP USB interfaces occur in a predictable sequence, preventing race conditions that could be exploited by attackers to cause system instability or gain unauthorized access to network resources. This vulnerability resolution strengthens the overall security posture of Linux systems that utilize MCTP over USB networking capabilities.
The solution addresses both immediate operational concerns and long-term system stability by ensuring that device state transitions are properly synchronized across all relevant subsystems. The atomic locking mechanism prevents any possibility of URB re-scheduling after cancellation, which would otherwise create inconsistent states in the network stack's USB handling code. This type of fix is essential for maintaining the integrity of kernel-level device management operations and preventing potential escalation paths that could lead to more severe security implications during normal system operation or malicious attack scenarios.
The implementation follows established best practices for kernel synchronization primitives while addressing a specific vulnerability in the MCTP over USB network stack. The change ensures that all URB completion handlers execute with consistent state information, eliminating the possibility of re-scheduling operations after cancellation has been requested. This comprehensive approach to race condition resolution provides robust protection against potential exploitation while maintaining backward compatibility and system performance characteristics expected from kernel-level networking implementations.