CVE-2026-80824 in Linux
Summary
by MITRE • 09/04/2026
In the Linux kernel, the following vulnerability has been resolved:
usb: usbfs: fix use-after-free of usb_device in usbdev_release()
usbdev_release() drops its reference to the struct usb_device before draining the list of completed async URBs, but that drain path reads back through the same object: free_async() calls dec_usb_memory_use_count() for any URB whose buffer came from the usbfs mmap() region, and its first statement is bus_to_hcd(ps->dev->bus).
After a disconnect the usbfs reference can be the last one, in which case usb_put_dev() frees the device and the subsequent loop reads offset 80 of freed memory and uses the result as a struct usb_hcd *, which hcd_buffer_free_pages() then dereferences.
This is reachable by an unprivileged process that has read/write access to a /dev/bus/usb node: mmap() the fd, submit one URB with a buffer inside the mapping, wait for the device to be unplugged, then munmap() and close(). It reproduces on every attempt rather than being a race, because a live MAP_SHARED vma holds a reference on the struct file, so usbdev_release() cannot run until the last vma is gone and the freeing branch of dec_usb_memory_use_count() is always taken.
BUG: KASAN: slab-use-after-free in dec_usb_memory_use_count+0x3ae/0x410 Read of size 8 at addr ffff8880122ee050 by task poc/769 CPU: 1 UID: 1000 PID: 769 Comm: poc Tainted: G B 6.12.94 #3
Call Trace: dec_usb_memory_use_count+0x3ae/0x410 free_async+0x2aa/0x4f0 usbdev_release+0x375/0x460 __fput+0x3ea/0xb50 __x64_sys_close+0x86/0x100
Allocated by task 11: usb_alloc_dev+0x55/0xd90 hub_event+0x2524/0x43d0
Freed by task 769: kfree+0x121/0x360 device_release+0xd2/0x280 usb_put_dev+0x23/0x30 usbdev_release+0x2d8/0x460
Release the device reference after the drain loop instead. Nothing between the two points requires it to have been dropped.
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Analysis
by VulDB Data Team • 09/05/2026
The Linux kernel contains a critical use-after-free vulnerability within the USB subsystem, specifically in the usbfs implementation during the release of file descriptors associated with USB devices. This flaw arises from an incorrect ordering of operations in the usbdev_release function, which is responsible for cleaning up resources when a user-space process closes its handle to a /dev/bus/usb node. The core issue lies in the fact that the function drops its reference count on the struct usb_device object before completing the necessary cleanup of asynchronous USB Request Blocks (URBs). This premature release allows the kernel memory allocator to free the device structure while it is still being accessed by subsequent code paths, leading to a classic use-after-free condition.
The technical mechanism involves the interaction between user-space memory mapping and kernel-side resource management. When an unprivileged process maps the file descriptor using mmap(), it creates a shared virtual memory area that holds a reference to the underlying struct file object. This ensures that usbdev_release cannot execute until all mapped regions are unmapped, effectively serializing the release process relative to active mappings. However, once the mapping is removed and the close syscall triggers usbdev_release, the function immediately calls usb_put_dev(), which decrements the device's reference count. If this was the last reference held by the kernel due to a prior disconnect event, the struct usb_device object is freed via kfree(). Immediately following this deallocation, the code proceeds to drain the list of completed async URBs through free_async. This function iterates over pending operations and calls dec_usb_memory_use_count for any URB whose buffer originated from the usbfs mmap region. The first operation within dec_usb_memory_use_count is a call to bus_to_hcd, which dereferences ps->dev->bus. Since ps points to the now-freed struct usb_device, this access reads stale memory at offset 80 of the freed object and interprets it as a pointer to a struct usb_hcd structure. This invalid pointer is then passed to hcd_buffer_free_pages for further processing, resulting in undefined behavior or kernel panic.
From an operational impact perspective, this vulnerability allows any local user with read-write access to USB device nodes to trigger arbitrary code execution or cause a denial of service by crashing the system. The exploitability is high because it does not rely on complex timing races; rather, it exploits a deterministic logic error in resource management that occurs reliably after a device disconnect and subsequent file descriptor closure. An attacker can reproduce this flaw consistently by mapping the USB node, submitting an URB with a buffer inside the mapped region, waiting for physical disconnection of the device, and then unmapping and closing the handle. The resulting kernel memory corruption is detected via KASAN as a slab-use-after-free read, confirming that the system state has been compromised through invalid memory access patterns typical of heap-based vulnerabilities.
This vulnerability maps to CWE-416, Use After Free, which describes situations where software continues to use a pointer after it has been freed, leading to unpredictable behavior and potential security breaches. In terms of attack vectors, this aligns with MITRE ATT&CK techniques related to privilege escalation or system compromise through local exploitation of kernel vulnerabilities. The lack of proper reference counting discipline in the release path highlights a fundamental flaw in how shared resources are managed during teardown sequences. To mitigate this risk, developers must ensure that all references to critical objects like struct usb_device are held until all dependent operations, such as draining async URB lists and freeing associated buffers, have completed. The fix involves reordering the code within usbdev_release so that the device reference is released only after the drain loop has finished processing all pending asynchronous requests. This ensures that no dangling pointers remain active during cleanup phases, thereby preventing access to freed memory regions and maintaining kernel integrity against local exploitation attempts.