CVE-2007-4849 in OLPC Linux
Summary
by MITRE
JFFS2, as used on One Laptop Per Child (OLPC) build 542 and possibly other Linux systems, when POSIX ACL support is enabled, does not properly store permissions during (1) inode creation or (2) ACL setting, which might allow local users to access restricted files or directories after a remount of a filesystem, related to "legacy modes" and an inconsistency between dentry permissions and inode permissions.
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Analysis
by VulDB Data Team • 08/01/2019
The vulnerability described in CVE-2007-4849 represents a critical file system permission inconsistency within the JFFS2 file system implementation on the One Laptop Per Child OLPC platform. This issue specifically manifests when POSIX Access Control List support is enabled, creating a scenario where file system permissions become desynchronized between different layers of the file system hierarchy. The vulnerability affects OLPC build 542 and potentially other Linux systems utilizing JFFS2 with POSIX ACL functionality, presenting a significant security risk for embedded systems where file access control is paramount.
The technical flaw occurs during two critical phases of file system operation: inode creation and ACL setting processes. During these operations, the JFFS2 implementation fails to properly synchronize permission information between the inode structure and the dentry cache, which maintains directory entry information. This inconsistency creates what is essentially a permission leak where the file system's internal representation of file permissions becomes disconnected from the actual access controls that should be enforced. The issue is particularly insidious because it only becomes apparent after a filesystem remount operation, making it difficult to detect through normal system monitoring.
The operational impact of this vulnerability extends beyond simple unauthorized file access, creating potential for privilege escalation and data exposure in embedded systems environments. When a filesystem is remounted, the mismatch between dentry permissions and inode permissions can allow local users to access files and directories that should be restricted, effectively bypassing the intended access control mechanisms. This behavior violates fundamental security principles and can compromise the integrity of the entire system, especially in educational environments where multiple users might share the same device. The vulnerability is classified under CWE-284, which addresses improper access control, and aligns with ATT&CK technique T1068, which covers privilege escalation through local system exploits.
Mitigation strategies for this vulnerability require careful system administration and potentially system-level patches. The most effective immediate solution involves disabling POSIX ACL support in JFFS2 when not strictly necessary, which eliminates the problematic code path entirely. System administrators should also implement regular filesystem integrity checks and monitor for unauthorized access patterns that might indicate exploitation attempts. Long-term solutions include updating to patched versions of the Linux kernel that properly address the permission synchronization issue between inode and dentry structures, ensuring that all permission changes are consistently applied across both storage layers. Additionally, implementing proper access control policies and regular security audits can help detect and prevent exploitation of this type of permission inconsistency vulnerability.