CVE-2026-72025 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
s390/monwriter: Reject buffer reuse with different data length
When data buffers are reused, e.g. for interval sample records, the first record determines the data length, and the size of the buffer for user copy. Current monwriter code does not check if the data length was changed for subsequent records, which also would never happen for valid user programs.
However, a malicious user could change the data length, resulting in out of bounds user copy to the kernel buffer, and memory corruption. By default, the monwriter misc device is created with root-only permissions, so practical impact is typically low.
Fix this by checking for changed data length and rejecting such records.
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Analysis
by VulDB Data Team • 08/16/2026
The Linux kernel's s390 architecture includes a monitoring writer subsystem known as monwriter, which facilitates the collection of performance and diagnostic data from hardware counters on IBM System z mainframes. This component operates through a misc device that allows user-space applications to register buffers for receiving sampled records generated by the kernel. The security flaw identified in this subsystem stems from an insufficient validation mechanism when handling buffer reuse scenarios. Specifically, when a single memory buffer is reused across multiple sampling intervals, the initial record written into that buffer establishes the expected data length and determines the size allocated for copying user-space data back to the kernel or vice versa during subsequent operations. The vulnerability arises because the monwriter implementation fails to verify whether the data length specified in later records matches the length established by the first record within that reused buffer context.
This lack of consistency checking creates a critical boundary condition where a malicious actor can exploit the discrepancy between the expected and actual data lengths. By manipulating the metadata associated with subsequent samples, an attacker can cause the kernel to perform out-of-bounds memory copies. Since the kernel trusts the length field without cross-referencing it against the initial buffer allocation parameters defined by the first record in the sequence, this leads to a write or read operation that extends beyond the allocated boundaries of the user-provided buffer. Such an action results in memory corruption within the kernel space, potentially allowing for arbitrary code execution, privilege escalation, or denial of service conditions depending on how the corrupted memory is utilized by other system components.
From a classification perspective, this vulnerability aligns with CWE-120 Buffer Copy without Checking Size of Input and CWE-20 Improper Input Validation. The failure to validate that subsequent inputs conform to previously established constraints represents a classic logic error in input sanitization. In the context of the MITRE ATT&CK framework, this flaw could be leveraged during techniques involving privilege escalation or defense evasion if an attacker successfully gains initial access and exploits the memory corruption to bypass security controls. The root cause is fundamentally a missing check for data integrity across multiple iterations of buffer usage, which violates the principle that stateful operations must maintain consistency in their parameters throughout their lifecycle.
Despite the severity of the underlying technical flaw involving kernel memory corruption, the practical impact on most systems remains limited due to default configuration settings. The monwriter misc device is typically created with root-only permissions by default. This restrictive access control model means that only privileged users can interact with the interface and trigger the vulnerable code paths. Consequently, an unprivileged user cannot directly exploit this vulnerability without first obtaining elevated privileges through a separate attack vector. However, in environments where the service might be exposed to less trusted accounts or if security policies have been modified to allow broader access, the risk profile increases significantly. The potential for kernel-level compromise via memory corruption remains a serious concern regardless of current default configurations.
The resolution involves implementing strict validation logic within the monwriter subsystem to ensure that any reused buffer maintains data consistency across all records written into it. By checking if the data length in subsequent samples matches the length determined by the first record, and rejecting any discrepancies, the kernel prevents the out-of-bounds copy operation from occurring. This fix enforces integrity checks at the point of entry for each sample record, ensuring that the buffer size expectations remain constant throughout its reuse cycle. Administrators should ensure their systems are updated with this patch to mitigate the risk associated with potential future privilege escalation vectors or if access controls on the monwriter device have been loosened in specific deployment scenarios. Regular auditing of kernel module permissions and adherence to least-privilege principles further reduce the attack surface for such vulnerabilities.