CVE-2026-89732
Low risk· EPSS 12%Exploitation Probability (EPSS)
Low risk12th percentile - higher than 12% of all known CVEs
Summary
In the Linux kernel, ffs_ep0_read() in the USB gadget FunctionFS interface could deadlock. The function slept while holding ffs->mutex, so when the gadget was torn down via configfs (e.g., echo "" > UDC) while a userspace daemon looped on read(), a permanent deadlock occurred. The fix drops the mutex before sleeping and uses wait_event_interruptible_exclusive().
Risk Assessment
The deadlock can hang the configfs thread in uninterruptible sleep and block the userspace process, freezing the USB gadget stack and requiring a system reboot. This affects systems using FunctionFS (e.g., embedded devices, phones, USB gadget configurations).
Recommendation
Update the Linux kernel to a version containing the fix for CVE-2026-89732. If updating is not possible, avoid tearing down the gadget via configfs while a daemon is actively reading ep0.
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: usb: gadget: f_fs: Prevent deadlock during ep0 read loop Currently, ffs_ep0_read() holds ffs->mutex when it prepares to go to sleep waiting for an event. When no setup events are pending, it calls wait_event_interruptible_exclusive_locked_irq() with the mutex still held. The wait macro deliberately drops the waitqueue spinlock before sleeping but does not drop the mutex. If a userspace daemon is polling ep0 via read() and the gadget is asynchronously torn down via configfs (e.g., echo "" > UDC), a deadlock can occur: 1. The configfs teardown calls functionfs_unbind(), which queues a FUNCTIONFS_UNBIND event. 2. The daemon wakes up, consumes the event, and drops the mutex. 3. However, if the daemon loops and immediately issues another read() before exiting, it reacquires ffs->mutex and again goes into an interruptible sleep. 4. Meanwhile, functionfs_unbind() continues execution and attempts to acquire ffs->mutex to tear down ep0req. 5. The kernel deadlocks because the configfs thread is stuck in an uninterruptible sleep waiting for the mutex, while the userspace daemon is in an interruptible sleep holding the mutex forever because no more events will arrive. To fix this, we drop both the waitqueue spinlock and ffs->mutex before going to sleep, and use wait_event_interruptible_exclusive() instead. Upon waking up, we jump back to the `retry` label to safely reacquire the mutex and re-evaluate the state machine. By not sleeping with ffs->mutex held, we natively decouple gadget teardowns (which require the mutex) from userspace polling.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS