CVE-2007-3732 in Linux
Summary
by MITRE
In Linux 2.6 before 2.6.23, the TRACE_IRQS_ON function in iret_exc calls a C function without ensuring that the segments are set properly. The kernel's %fs needs to be restored before the call in TRACE_IRQS_ON and before enabling interrupts, so that "current" references work. Without this, "current" used in the window between iret_exc and the middle of error_code where %fs is reset, would crash.
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Analysis
by VulDB Data Team • 02/05/2024
The vulnerability described in CVE-2007-3732 represents a critical flaw in the Linux kernel's interrupt handling mechanism that existed in versions prior to 2.6.23. This issue specifically affects the TRACE_IRQS_ON function within the iret_exc context, which is responsible for managing interrupt state transitions during exception handling. The flaw stems from improper segment register management during kernel execution, creating a window where critical kernel data structures become inaccessible due to incorrect processor state setup.
The technical root cause of this vulnerability lies in the failure to properly restore the kernel's %fs segment register before invoking the TRACE_IRQS_ON function. In x86 architecture, the %fs register serves as a segment selector that points to the kernel's per-CPU data structures, including the current process context. When the kernel executes the iret_exc function during interrupt return operations, it must ensure that all segment registers are properly configured before calling C functions that reference kernel data structures. The vulnerability occurs because the %fs register is not restored to its correct value before the TRACE_IRQS_ON call, creating a temporal window where the "current" pointer becomes invalid.
This flaw directly impacts the kernel's ability to maintain proper process context during interrupt handling operations. The "current" pointer is essential for accessing per-process kernel data structures and maintaining system stability. When the %fs register is not properly restored, any reference to "current" within the vulnerable window between iret_exc and the error_code handling section results in invalid memory access patterns. This condition can lead to immediate kernel crashes, system instability, or potentially exploitable conditions that could allow privilege escalation attacks.
The operational impact of this vulnerability extends beyond simple system crashes, as it represents a fundamental flaw in the kernel's interrupt management subsystem that could be exploited by malicious actors. The vulnerability is particularly concerning because it occurs during the critical interrupt return path, which is frequently executed during normal system operation. Attackers could potentially leverage this condition to cause denial of service attacks or, in more sophisticated scenarios, to gain elevated privileges by manipulating the kernel's execution flow during interrupt handling.
From a security standards perspective, this vulnerability maps directly to CWE-248, which describes "Uncaught Exception" in operating systems, and CWE-121, which addresses "Stack-based Buffer Overflow". The flaw also aligns with ATT&CK technique T1068, "Exploitation for Privilege Escalation," as it represents a kernel-level vulnerability that could be exploited to gain elevated privileges. The vulnerability demonstrates the critical importance of proper processor state management in kernel code and highlights the necessity of maintaining architectural consistency in interrupt handling operations. The fix implemented in kernel version 2.6.23 involved ensuring that the %fs segment register is properly restored before calling TRACE_IRQS_ON, thereby maintaining the integrity of kernel data structures during interrupt transitions.
This vulnerability underscores the complexity of kernel-level programming and the critical nature of segment register management in protected mode operating systems. The issue demonstrates how seemingly minor oversights in processor state management can lead to catastrophic system failures, emphasizing the need for rigorous code review processes and comprehensive testing of kernel interrupt handling mechanisms. The remediation approach required careful attention to the processor's segment register state during critical execution paths, ensuring that all kernel data structures remain accessible throughout the interrupt handling lifecycle.