CVE-2017-7495 in Linux
Summary
by MITRE
fs/ext4/inode.c in the Linux kernel before 4.6.2, when ext4 data=ordered mode is used, mishandles a needs-flushing-before-commit list, which allows local users to obtain sensitive information from other users' files in opportunistic circumstances by waiting for a hardware reset, creating a new file, making write system calls, and reading this file.
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Analysis
by VulDB Data Team • 12/06/2022
The vulnerability identified as CVE-2017-7495 represents a critical information disclosure flaw within the Linux kernel's ext4 filesystem implementation. This weakness specifically manifests in the fs/ext4/inode.c file where the kernel fails to properly manage a needs-flushing-before-commit list during ext4 data=ordered mode operations. The flaw occurs when the kernel processes file operations under specific conditions that create opportunities for unauthorized data access between user sessions. The vulnerability is particularly concerning because it operates through seemingly innocuous system calls and file operations that do not immediately appear malicious, making detection and prevention more challenging for system administrators and security professionals.
The technical root cause of this vulnerability stems from improper handling of the journaling mechanism within the ext4 filesystem when operating in data=ordered mode. In this mode, the filesystem ensures that data is written to disk before metadata updates occur, but the implementation contains a flaw in how it manages the list of items requiring flushing before commit operations. When a hardware reset occurs and subsequent file operations are performed, the kernel's memory management system can leave sensitive data remnants in memory locations that are subsequently accessible through new file operations. This creates a scenario where one user's data can be inadvertently exposed to another user through the manipulation of file system journaling and commit sequences. The vulnerability requires specific conditions to be exploited successfully, including the timing of hardware resets, file creation, and write operations, but when these conditions align, the information disclosure can be substantial.
The operational impact of CVE-2017-7495 extends beyond simple information disclosure to represent a serious breach of user privacy and system integrity. Attackers can potentially access sensitive data from other users' files without requiring elevated privileges or complex exploitation techniques. The vulnerability operates at the kernel level, meaning that successful exploitation can provide access to confidential information stored in files, potentially including personal data, authentication credentials, or proprietary business information. This type of vulnerability directly impacts the principle of least privilege and data confidentiality, as users can access files belonging to other users through the manipulation of the filesystem's internal state. The opportunistic nature of the exploit means that the vulnerability can be triggered under specific circumstances, making it particularly dangerous as it may go undetected for extended periods.
Mitigation strategies for CVE-2017-7495 primarily focus on kernel updates and filesystem configuration adjustments. The most effective solution involves upgrading to Linux kernel versions 4.6.2 or later where the vulnerability has been addressed through proper handling of the needs-flushing-before-commit list. System administrators should also consider implementing additional security controls such as disabling ext4 data=ordered mode if it is not strictly required for their specific use cases, though this may impact system performance and reliability. The vulnerability aligns with CWE-200, which describes improper exposure of sensitive information, and represents a classic example of how filesystem-level vulnerabilities can be exploited to compromise user data confidentiality. Organizations should also implement monitoring solutions to detect unusual patterns in file system operations that might indicate exploitation attempts, as the vulnerability requires specific timing conditions that could potentially be monitored. The ATT&CK framework categorizes this as a privilege escalation technique through kernel vulnerabilities, as it allows local users to access information that should normally be restricted to other users, making it a significant concern for system security posture assessment and compliance requirements.