CVE-2026-89983 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
i2c: core: fix debugfs UAF on adapter removal
i2c_del_adapter() frees the adapter's debugfs directory before it unregisters the adapter device, but the new_device sysfs attribute stays writable until device_del(). A write racing with removal still reaches i2c_device_probe(), which passes the freed adap->debugfs to debugfs_create_dir() as the new client's parent:
BUG: KASAN: slab-use-after-free in lookup_noperm_common+0x407/0x430 Read of size 4 at addr ffff88803ef87810 by task syz.0.61/6090 lookup_noperm_common+0x407/0x430 simple_start_creating+0x9c/0x110 debugfs_start_creating+0xdb/0x1a0 debugfs_create_dir+0x24/0x350 i2c_device_probe+0x814/0xbf0
It's technically possible to create a client after i2c_deregister_clients has run. That client will never be unregistered and make wait_for_completion hang.
Close the window by removing the new_device attribute at the start of i2c_del_adapter(). device_remove_file() will drain any clients left.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel's I2C subsystem contains a critical use-after-free vulnerability within its debugfs handling logic during adapter removal operations. This flaw arises from an incorrect ordering of cleanup steps in the i2c_del_adapter function, where the adapter’s debugfs directory is freed before the associated device is fully unregistered. Specifically, while the core infrastructure begins tearing down resources by releasing the debugfs entries, the sysfs attribute new_device remains writable and active until the subsequent call to device_del completes this process. This temporal gap creates a race condition window that can be exploited or triggered unintentionally through concurrent system interactions.
When an attacker or automated tool such as syzkuzzer writes to the new_device sysfs attribute during this narrow interval, it triggers i2c_device_probe for a newly requested client. The probe function attempts to create debugfs entries for this new client using the parent directory provided by the adapter structure. However, because the adapter’s debugfs directory has already been freed in the earlier step of i2c_del_adapter, the kernel passes a pointer to memory that is no longer valid or allocated. This results in a slab-use-after-free error as detected by Kernel Address Sanitizer (KASAN), specifically manifesting within lookup_noperm_common and subsequent debugfs creation routines like simple_start_creating and debugfs_create_dir.
The operational impact of this vulnerability extends beyond immediate kernel crashes to potential security implications including privilege escalation or denial of service through system instability. The use-after-free condition allows arbitrary code execution possibilities if the freed memory is reallocated for malicious purposes before being overwritten. Furthermore, there exists a secondary issue where creating a client after i2c_deregister_clients has executed leads to an orphaned device that never gets unregistered. This results in wait_for_completion hanging indefinitely, causing resource leaks and potential system hangs due to blocked completion waits that are never signaled by the missing unregister routine.
This vulnerability aligns with CWE-416, Use After Free, as it involves accessing memory after it has been freed without proper validation of its validity state. In terms of MITRE ATT&CK mapping, this scenario reflects techniques associated with Tactic TA0005 Defense Evasion and potentially TA0004 Privilege Escalation if the use-after-free is leveraged to gain higher privileges within the kernel space. The specific mechanism involves exploiting race conditions in system interface handling which falls under exploitation of non-atomic operations or timing dependencies common in complex subsystems like I2C management.
To mitigate this vulnerability, the fix implements a strict ordering correction by removing the new_device sysfs attribute at the very beginning of i2c_del_adapter rather than leaving it active until device removal completes. This change ensures that no new clients can be instantiated once the adapter deletion process has commenced, effectively closing the race condition window. Additionally, calling device_remove_file drains any remaining client references safely before proceeding with further teardown steps. This approach guarantees that all dynamic resources are properly cleaned up in a deterministic order preventing both use-after-free conditions and orphaned completion waits.
System administrators should ensure their kernels are updated to include this patch which addresses the root cause by enforcing proper resource lifecycle management within the I2C core subsystem. Regular security audits focusing on race conditions in device driver interfaces can help identify similar vulnerabilities across other kernel modules. Developers working with sysfs or debugfs interactions must adhere strictly to atomicity principles and ensure that all user-accessible attributes are disabled before any underlying data structures are released to prevent exploitation of timing windows inherent in asynchronous hardware management tasks.