UBUNTU-CVE-2026-64584: Use After Free
In the Linux kernel, the following vulnerability has been resolved: usb: gadget: fmidi: cancel pending IN work before freeing the midi object The fmidi driver embeds a work item (midi->work) whose handler, fmidiinwork(), dereferences the enclosing struct fmidi through containerof(). This work is armed from two sites: fmidicomplete(), on a normal IN-endpoint completion, and fmidiintrigger(), on an ALSA rawmidi output-stream start. Neither fmididisable() nor fmidiunbind() cancels midi->work. fmididisable() only disables the endpoints and drains the inreqfifo; it does not synchronize the work item, and the sound card is released asynchronously to the final free of the midi object. The midi object is reference-counted (midi->freeref) and is freed in fmidifree() only once both the usbfunction reference and the rawmidi privatedata reference have been dropped. In fmidiunbind(), fmididisable() runs before the sound card is released, so while the USB endpoints are already disabled the rawmidi device is still usable by an open substream. A concurrent userspace write on such a substream can reach fmidiintrigger() and queue midi->work again after fmididisable() has returned. A work item armed this way may still be pending when the last reference drops and fmidifree() proceeds to kfree(midi), letting fmidiinwork() dereference the struct after it has been freed, a use-after-free. For this reason cancelling midi->work in fmididisable() would not be sufficient: the ALSA trigger path can rearm the work after disable() returns. Cancelling at the refcount-zero free site is the boundary after which neither arming source can survive, because by then both references that keep the midi object alive have been dropped: the USB endpoints are already disabled and the rawmidi device has been released. Fix this by calling cancelworksync(&midi->work) in the refcount-zero block of fmidifree(), before the embedded workstruct is freed along with the rest of the structure. opts->lock is a sleeping mutex, so calling cancelworksync() under it is permitted, and the handler takes midi->transmitlock rather than opts->lock, so no self-deadlock can occur while it waits for a running instance of the work to finish. This issue was found by an in-house static analysis tool.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
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ubuntu/linux-aws-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1117.124~20.04.1 - Upgrade
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ubuntu/linux-azure-5.15to a version that resolves this vulnerability.Fixed in 5.15.0-1123.132~20.04.1 - Upgrade
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ubuntu/linux-awsto a version that resolves this vulnerability.Fixed in 5.15.0-1117.124 - Upgrade
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ubuntu/linux-awsto a version that resolves this vulnerability.Fixed in 6.8.0-1067.70 - Upgrade
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ubuntu/linux-azureto a version that resolves this vulnerability.Fixed in 6.8.0-1071.79 - Upgrade
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ubuntu/linux-aws-fipsto a version that resolves this vulnerability.Fixed in 6.8.0-1067.70+fips1 - Upgrade
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ubuntu/linux-azure-fipsto a version that resolves this vulnerability.Fixed in 6.8.0-1071.79+fips1 - Upgrade
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ubuntu/linux-fipsto a version that resolves this vulnerability.Fixed in 6.8.0-147.147+fips1 - Upgrade
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ubuntu/linuxto a version that resolves this vulnerability.Fixed in 7.0.0-38.38 - Upgrade
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ubuntu/linux-awsto a version that resolves this vulnerability.Fixed in 7.0.0-1014.14 - Upgrade
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ubuntu/linux-azureto a version that resolves this vulnerability.Fixed in 7.0.0-1017.17 - Upgrade
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ubuntu/linux-azure-fdeto a version that resolves this vulnerability.Fixed in 7.0.0-1016.16 - Upgrade
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ubuntu/linux-oracleto a version that resolves this vulnerability.Fixed in 7.0.0-1013.13 - Upgrade
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ubuntu/linux-riscvto a version that resolves this vulnerability.Fixed in 7.0.0-38.38.1 - Compensating control
In the refcount-zero block of f_midi_free(), call cancel_work_sync(&midi->work) before freeing the embedded work_struct and the f_midi object.
Event History
Frequently Asked Questions
Which systems are exposed to this issue?
Systems using the Linux USB gadget f_midi driver are exposed when the MIDI gadget is disabled or unbound while its ALSA rawmidi device remains usable through an open substream. The listed affected Ubuntu kernel packages include linux, linux-aws, linux-azure, linux-gcp, linux-gke, linux-ibm, linux-realtime, and linux-azure-fde.
What is required to trigger the use-after-free condition?
A userspace process must be able to write to an open ALSA rawmidi substream while the f_midi function is being disabled or unbound. That write can invoke f_midi_in_trigger() and queue work after endpoint disablement, leaving work that may later access a freed f_midi object.
How can I determine whether my deployment is at risk?
Check whether the system is configured to use the USB gadget f_midi function and whether local users or processes can keep its ALSA rawmidi substream open and write to it. The relevant race involves concurrent rawmidi activity during USB gadget disablement or unbinding.