CVE-2026-90310 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
xen/xenbus: check otherend_id only after it has been initialized
When device just got initialized (for example on module load), the otherend_id field is initialized only after xenbus_read_otherend_details() gets called. If xenstore watch triggers xenbus_dev_changed() before that, it might consider still zeroed otherend_id field (not matching actual xenstore content) as a sign of device state reset. It can happen because xenstore watch are handled in another thread (xenwatch), which can run in parallel to the initial device probe running at module load. In that case, it would call device_unregister(), which would deadlock against device probe from module init.
Fix this by considering dev->otherend_id change only after dev->otherend is set (which happen after otherend_id is initialized).
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Analysis
by VulDB Data Team • 09/18/2026
The vulnerability identified in the Linux kernel's Xen bus subsystem represents a critical race condition arising from improper synchronization during device initialization and state monitoring. Specifically, the issue centers on the handling of the otherend_id field within the xenbus_dev structure. This identifier serves as a crucial indicator for determining whether a connected virtual machine or backend domain has undergone a reset or reconnection event. The root cause lies in the asynchronous nature of kernel operations where device probing and watch notification handlers execute concurrently without adequate mutual exclusion regarding specific initialization states. When a Xen device is initialized, such as during module loading, the otherend_id field remains uninitialized or zeroed until xenbus_read_otherend_details() completes its execution to fetch current state information from the hypervisor's store.
The operational flaw occurs when an external event triggers a xenstore watch notification before this initialization sequence concludes. These notifications are processed by the dedicated xenwatch thread, which operates independently of the main device probe routine running in the context of module initialization. If the xenwatch handler invokes xenbus_dev_changed() while otherend_id is still zeroed, it misinterprets this uninitialized state as a significant change or reset event because the value does not match the expected persistent content from xenstore. This false positive triggers a cascade of cleanup operations intended for handling device resets, most notably calling device_unregister().
The severity of this vulnerability escalates due to the resulting deadlock condition. The call to device_unregister() attempts to remove and destroy the device structure while the original module init probe is still holding locks or references related to that same device. Since both threads are attempting to manipulate the lifecycle of the same kernel object without proper serialization, a circular dependency in locking occurs. This leads to a system hang where neither thread can proceed, effectively causing a denial of service for the affected Xen guest instance and potentially impacting host stability depending on the criticality of the driver involved.
From a classification perspective, this flaw aligns with CWE-362, which describes concurrent execution using shared resources with improper synchronization. The lack of checks to ensure that initialization is complete before acting on state changes allows for unsafe access patterns typical of race conditions in kernel space. Furthermore, within the MITRE ATT&CK framework, this vulnerability facilitates Local Privilege Escalation or Denial of Service by allowing a local user or compromised process with access to xenstore watches to trigger system instability. The attack vector relies on timing precision and interaction with virtualization interfaces, making it relevant to threats targeting cloud infrastructure integrity.
To mitigate this risk, the kernel developers implemented a fix that introduces explicit state verification before processing device changes. The updated logic ensures that dev->otherend_id is only considered for change detection after dev->other has been set, which serves as a reliable indicator that otherend_id has also been properly initialized by xenbus_read_otherend_details(). This simple but effective check prevents the race condition from manifesting by ensuring that state comparisons occur only when all prerequisite data structures are fully populated and stable. System administrators should apply kernel updates containing this patch to restore safe operation of Xen virtualization components. Regular auditing of driver initialization sequences for similar synchronization gaps is recommended to prevent analogous vulnerabilities in other subsystems handling asynchronous hardware or hypervisor events.