PT-2026-90448 · Linux · Linux
CVE-2026-89732
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Published
2026-09-11
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Updated
2026-09-11
None
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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:
- The configfs teardown calls functionfs unbind(), which queues a FUNCTIONFS UNBIND event.
- The daemon wakes up, consumes the event, and drops the mutex.
- However, if the daemon loops and immediately issues another read() before exiting, it reacquires ffs->mutex and again goes into an interruptible sleep.
- Meanwhile, functionfs unbind() continues execution and attempts to acquire ffs->mutex to tear down ep0req.
- 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. Found an issue in the description? Have something to add? Feel free to write us 👾
Related Identifiers
Affected Products
Linux