CVE-2026-90209 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
s390/debug: Fix deadlock during unregister
Unregistering an s390dbf debug area while one of the associated debugfs files is being written to can cause a deadlock:
$ echo >.../vmur/level $ rmmod vmur =================================================== debugfs write debugfs_file_get() debug_unregister() mutex_lock(debug_mutex) debugfs_remove() wait for debugfs_file_put() debug_file_ops.write() debug_input() mutex_lock(debug_mutex) ==> DEADLOCK
Fix this by splitting debug_unregister() into an s390dbf and debugfs part, and running only the s390dbf part with debug_mutex locked.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel vulnerability identified in the s390 architecture debugging subsystem involves a classic concurrency issue manifesting as a deadlock during module unregistration operations. This flaw specifically affects the interaction between the s390debug framework, which provides low-level diagnostic capabilities for IBM mainframe systems, and the debugfs virtual file system used to expose these diagnostics to user space. The root cause lies in improper synchronization when tearing down resources while they are still actively accessed by concurrent processes. Specifically, the vulnerability arises because the unregister routine attempts to remove debugfs entries without ensuring that all active references from userspace writers have been properly released and synchronized with the internal locking mechanisms of the debugging subsystem.
The technical flaw is characterized by a circular dependency in lock acquisition order during race conditions. When an administrator executes commands such as writing to a debugfs file like vmur/level while simultaneously removing the associated kernel module via rmmod, two distinct execution paths contend for resources. The first path involves the write operation on the debugfs interface, which acquires the debug_mutex after initiating input processing. Simultaneously, the second path triggered by module removal invokes debug_unregister, which also attempts to acquire the same debug_mutex while holding references that require waiting for file handle releases via debugfs_remove and subsequent wait conditions. This creates a scenario where one thread holds a lock needed by another, while simultaneously waiting for a condition dependent on releasing resources held by the first thread, resulting in an indefinite block or deadlock state that can freeze system operations related to debugging and potentially impact overall kernel stability if not isolated properly.
From an operational perspective, this vulnerability poses significant risks regarding system availability and reliability. In production environments utilizing s390 hardware for critical workloads, such as banking or government services where mainframes are prevalent, the ability to dynamically load and unload diagnostic modules is essential for maintenance and troubleshooting without full reboots. A deadlock during this process can render debugging tools unusable precisely when they are most needed, complicating incident response efforts. Furthermore, if the kernel enters an unrecoverable state due to such deadlocks, it may necessitate a hard reset of the system, leading to service interruptions and potential data loss depending on the criticality of ongoing transactions. The vulnerability essentially undermines the reliability of dynamic debugging capabilities in high-availability mainframe environments.
This issue aligns with CWE-833, which describes Deadlock conditions resulting from improper handling of concurrent access to shared resources without correct synchronization protocols. Additionally, it relates to ATT&CK technique T1059, Command and Scripting Interpreter, as the vulnerability is triggered through standard shell commands used by administrators or potentially malicious actors attempting to disrupt system availability. The exploitation vector relies on local privilege escalation capabilities since interacting with debugfs typically requires root access or specific group memberships, limiting broad remote exploitability but increasing risk for insider threats or compromised privileged accounts aiming to cause denial of service against the management infrastructure.
To mitigate this vulnerability, kernel maintainers have implemented a structural fix that decouples the cleanup processes within the unregister routine. By splitting debug_unregister into distinct phases handling s390dbf internal state and debugfs interface removal, the code ensures that only the necessary parts are protected by the debug_mutex during critical sections. This approach prevents the circular wait condition by allowing the debugfs removal to proceed independently after ensuring no active writers remain, thereby eliminating the race window where locks were contended incorrectly. System administrators should ensure their kernels are updated with patches addressing this specific s390/debug deadlock issue. Regular auditing of module loading and unloading procedures in production environments can also help identify potential misuse patterns that might trigger such concurrency issues before they lead to system instability.