CVE-2026-99124 in Linux
Summary
by MITRE • 10/28/2026
In the Linux kernel, the following vulnerability has been resolved:
s390/pci: Fix leak of uninitialized kernel data in SCLP report
While report_error_write() checks that the provided buffer is at least as large as the header struct, but not that it is large enough to contain the report with the length claimed by report->length. If user-space provides a short buffer, meaning a larger report->length than the actually written payload, up to around 4K of kernel data from past the kmalloc(len + 1) sized buffer allocated in kernfs_fop_write_iter() will leak into the SCLP report.
However, as the entity processing the SCLP is privileged and able to access at least the page including the report, this does not leak data that entity could not access but it is still an out of bounds read and a malformed error report that should be rejected.
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Analysis
by VulDB Data Team • 10/28/2026
The vulnerability identified in the Linux kernel's s390 PCI subsystem involves an improper validation of buffer sizes during the processing of SCLP (System Control Logic Processor) reports. Specifically, within the report_error_write function, there is a logic flaw where the code verifies that the user-provided buffer meets the minimum size requirement for the header structure but fails to validate whether the buffer is sufficiently large to accommodate the entire data payload indicated by the length field in the report structure. This oversight creates a scenario where if an unprivileged or malicious user-space application provides a buffer that is smaller than the total length specified in the report, the kernel proceeds with writing operations without ensuring memory safety boundaries are respected for the full extent of the claimed data size.
This technical flaw results in an out-of-bounds read operation within the kernel space. When the write iterator attempts to populate the SCLP report using a buffer that is too small relative to the reported length, it reads beyond the allocated kmalloc region by up to approximately 4 kilobytes. Consequently, uninitialized or stale kernel data from memory locations immediately following the allocated buffer is copied into the report structure and subsequently exposed through the SCLP interface. This constitutes an information disclosure vulnerability as sensitive internal kernel state may be inadvertently transmitted to user-space entities that interact with this subsystem.
From a security impact perspective, while the leaked data originates from kernel memory regions that are technically accessible by the privileged entity processing the SCLP report, the exposure remains significant for several reasons. First, it represents an out-of-bounds read which violates fundamental memory safety principles and can potentially be exploited in conjunction with other vulnerabilities to achieve arbitrary code execution or further privilege escalation if the leaked data reveals critical pointers or cryptographic keys. Second, even though the immediate entity processing the report might have access to that page of memory, the leakage indicates a lack of strict input validation that could lead to unpredictable behavior, malformed error reports, and potential denial-of-service conditions due to corrupted state. The vulnerability aligns with CWE-20 Improper Input Validation and CWE-134 Use of Externally-Controlled Format String or similar buffer handling errors depending on the specific exploitation vector, though primarily it is categorized under memory safety violations like out-of-bounds reads which are often associated with CWE-125 Out-of-bounds Read.
Mitigation strategies for this vulnerability involve applying the upstream kernel patch that corrects the boundary checks within the report_error_write function. System administrators and developers should ensure their Linux kernels are updated to versions where this s390 PCI fix is included. Additionally, defense-in-depth measures such as enabling Kernel Hardening options like KASAN (Kernel Address Sanitizer) during development can help detect such out-of-bounds accesses early in the software lifecycle. For production environments, restricting access to SCLP interfaces and ensuring that only trusted, privileged processes interact with these low-level hardware reporting mechanisms reduces the attack surface available for triggering this flaw. The ATT&CK technique associated with this vulnerability is T1083 File and Directory Discovery or potentially T1560 Collected Data in Archive if used as part of a broader data staging effort, but more accurately it falls under T1074 Data Staged due to the exfiltration of kernel memory contents.