CVE-2004-2536 in Linux
Summary
by MITRE
The exit_thread function (process.c) in Linux kernel 2.6 through 2.6.5 does not invalidate the per-TSS io_bitmap pointers if a process obtains IO access permissions from the ioperm function but does not drop those permissions when it exits, which allows other processes to access the per-TSS pointers, access restricted memory locations, and possibly gain privileges.
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Analysis
by VulDB Data Team • 04/13/2019
The vulnerability described in CVE-2004-2536 represents a critical security flaw in the Linux kernel's thread management subsystem that stems from improper handling of IO permission boundaries during process termination. This issue specifically affects Linux kernel versions 2.6 through 2.6.5 where the exit_thread function fails to properly invalidate per-TSS (Task State Segment) io_bitmap pointers when processes terminate without explicitly dropping IO access permissions obtained through the ioperm system call. The root cause of this vulnerability lies in the kernel's failure to maintain proper memory isolation between processes that have been terminated but whose IO permission state has not been fully cleaned up, creating a persistent security boundary violation that can be exploited by malicious processes.
The technical implementation of this vulnerability exploits the kernel's handling of Task State Segments which are used to maintain per-process IO permission mappings and memory access controls. When a process calls ioperm to gain access to specific IO ports or memory regions, the kernel creates corresponding entries in the TSS io_bitmap that define which ports the process can access. However, upon process termination, the exit_thread function does not properly clear these io_bitmap pointers, leaving stale references that can be accessed by subsequent processes. This creates a scenario where a terminated process's IO permission mappings persist in memory and can be leveraged by newly created processes, effectively allowing them to access restricted memory locations and IO ports that they should not have access to, as defined by the original process's permission set.
The operational impact of this vulnerability extends beyond simple information disclosure to potentially enable privilege escalation attacks, as demonstrated by the ATT&CK framework's privilege escalation techniques that exploit kernel memory corruption and access control bypasses. The vulnerability creates a persistent access control violation that can be exploited by processes to access restricted memory locations and potentially execute code with elevated privileges. This aligns with CWE-284 which describes improper access control mechanisms, and represents a direct violation of the principle of least privilege in kernel security design. Attackers can leverage this flaw to gain unauthorized access to hardware resources and memory regions that should be restricted to specific processes, potentially enabling them to extract sensitive information or manipulate system state through direct memory access.
Mitigation strategies for this vulnerability require immediate kernel updates to versions that properly implement the io_bitmap pointer invalidation during process termination. System administrators should prioritize patching affected systems and implementing monitoring for unauthorized IO access patterns that may indicate exploitation attempts. The fix involves ensuring that when exit_thread is called, all per-TSS io_bitmap pointers are properly invalidated and memory is cleared to prevent subsequent processes from accessing stale permission mappings. Additionally, organizations should implement process monitoring to detect suspicious IO permission usage patterns and establish proper baseline configurations that minimize the risk of unauthorized IO access. This vulnerability highlights the critical importance of proper resource cleanup in kernel space and demonstrates how seemingly isolated process termination issues can create persistent security vulnerabilities that affect system integrity and confidentiality.