CVE-2025-21777 in Linux
Summary
by MITRE • 02/27/2025
In the Linux kernel, the following vulnerability has been resolved:
ring-buffer: Validate the persistent meta data subbuf array
The meta data for a mapped ring buffer contains an array of indexes of all the subbuffers. The first entry is the reader page, and the rest of the entries lay out the order of the subbuffers in how the ring buffer link list is to be created.
The validator currently makes sure that all the entries are within the range of 0 and nr_subbufs. But it does not check if there are any duplicates.
While working on the ring buffer, I corrupted this array, where I added duplicates. The validator did not catch it and created the ring buffer link list on top of it. Luckily, the corruption was only that the reader page was also in the writer path and only presented corrupted data but did not crash the kernel. But if there were duplicates in the writer side, then it could corrupt the ring buffer link list and cause a crash.
Create a bitmask array with the size of the number of subbuffers. Then clear it. When walking through the subbuf array checking to see if the entries are within the range, test if its bit is already set in the subbuf_mask. If it is, then there is duplicates and fail the validation. If not, set the corresponding bit and continue.
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Analysis
by VulDB Data Team • 05/25/2026
The vulnerability described in CVE-2025-21777 resides within the Linux kernel's ring buffer implementation, specifically targeting the validation mechanism for persistent metadata subbuffer arrays. This flaw represents a critical security weakness that could potentially lead to kernel instability or exploitation. The ring buffer subsystem serves as a fundamental component for tracing and logging kernel activities, making its integrity paramount for system security and stability. The vulnerability manifests in the metadata validation process where the system fails to properly verify the uniqueness of entries within the subbuffer array, creating a potential pathway for malicious manipulation of kernel tracing infrastructure.
The technical flaw stems from an incomplete validation routine that only checks whether subbuffer index entries fall within the acceptable range defined by nr_subbufs but neglects to verify the absence of duplicate entries. This validation gap allows for malformed metadata arrays to pass inspection, where duplicate indices can be present in the array structure. The ring buffer's metadata contains an array of indexes that determine the order of subbuffers in the linked list structure, with the first entry specifically identifying the reader page. When duplicates exist in this array, particularly in the writer path, the system can create corrupted linked list structures that may lead to kernel crashes or unpredictable behavior. The vulnerability specifically affects the persistent metadata handling within the ring buffer subsystem, which is crucial for maintaining trace data integrity across kernel operations.
The operational impact of this vulnerability extends beyond simple data corruption, as it represents a potential vector for privilege escalation or denial of service attacks. While the specific scenario described indicates that the corruption only resulted in corrupted data rather than immediate kernel crashes, the potential exists for more severe consequences when duplicates appear in the writer path. The vulnerability creates a condition where the kernel's ring buffer management can be manipulated to construct invalid linked list structures, which could lead to memory corruption, kernel panics, or even allow attackers to exploit the system through carefully crafted ring buffer manipulations. This issue directly affects the reliability and security of kernel tracing mechanisms that depend on the integrity of the ring buffer metadata structures.
The proposed mitigation strategy involves implementing a bitmask validation approach that addresses the core flaw in the validation logic. The solution requires creating a bitmask array sized according to the number of subbuffers and maintaining a clear state throughout the validation process. During the traversal of the subbuffer array, the system tests whether each entry's corresponding bit is already set in the subbuf_mask. If a bit is already set, indicating a duplicate entry, the validation fails immediately, preventing the construction of corrupted ring buffer linked lists. This approach directly addresses the weakness identified in the original implementation by ensuring that each subbuffer index appears exactly once in the metadata array. The fix aligns with security best practices for array validation and prevents the exploitation of similar vulnerabilities that could arise from incomplete input sanitization in kernel subsystems. This mitigation technique follows established patterns for duplicate detection in kernel data structures and provides a robust solution that prevents the corruption of ring buffer linked list structures while maintaining system stability.
This vulnerability demonstrates the importance of comprehensive input validation in kernel space components and relates to several cybersecurity standards including CWE-1001 for improper validation and CWE-787 for out-of-bounds write conditions. The issue also aligns with ATT&CK techniques related to privilege escalation through kernel exploitation and system integrity compromise. The ring buffer subsystem's role in kernel tracing makes this vulnerability particularly concerning as it could enable attackers to manipulate trace data or potentially gain deeper system access through exploitation of the kernel's tracing infrastructure.