CVE-2008-3890 in FreeBSD
Summary
by MITRE
The kernel in FreeBSD 6.3 through 7.0 on amd64 platforms can make an extra swapgs call after a General Protection Fault (GPF), which allows local users to gain privileges by triggering a GPF during the kernel s return from (1) an interrupt, (2) a trap, or (3) a system call.
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Analysis
by VulDB Data Team • 05/01/2017
The vulnerability identified as CVE-2008-3890 represents a critical privilege escalation flaw within the FreeBSD kernel implementation on amd64 architectures. This issue manifests during the kernel's handling of exception contexts, specifically when transitioning back from interrupt, trap, or system call execution paths following a General Protection Fault condition. The flaw stems from an improper sequence in the kernel's exception handling mechanism that results in an additional swapgs instruction being executed beyond the normal execution flow. The swapgs instruction is a crucial component in modern x86 processors that switches between kernel and user privilege levels by modifying the global descriptor table pointer. When this instruction is executed inappropriately, it creates a scenario where the kernel's privilege context becomes corrupted, potentially allowing malicious code to manipulate the processor state in ways that should be restricted to the kernel itself.
The technical exploitation of this vulnerability requires local user access and the ability to trigger a General Protection Fault during specific kernel return paths. Attackers can leverage this condition by crafting malicious code that deliberately causes a GPF while the kernel is in the process of returning from an interrupt, trap, or system call. The extra swapgs instruction execution creates a window where the kernel's privilege level is not properly maintained, potentially enabling attackers to execute code with elevated privileges. This type of vulnerability falls under the category of kernel-level privilege escalation flaws that are particularly dangerous because they can allow local users to gain root access to the system. The vulnerability is specifically present in FreeBSD versions 6.3 through 7.0, making these releases particularly susceptible to exploitation in environments where local access is possible.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it represents a fundamental flaw in the kernel's exception handling architecture that can be exploited to bypass security mechanisms designed to protect system integrity. When successfully exploited, attackers can gain full administrative control over affected systems, potentially leading to complete system compromise, data exfiltration, and persistent access. The vulnerability's exploitation requires specific timing and conditions related to the kernel's execution context, but once achieved, it provides a direct path to privilege elevation that is difficult to detect and prevent through standard security measures. This type of flaw can be classified under CWE-248, which addresses "Exception Handling" issues where an exception is not properly handled, and aligns with ATT&CK technique T1068, which involves "Exploitation for Privilege Escalation" through kernel exploits. The nature of the vulnerability also demonstrates the complexity involved in maintaining correct privilege contexts in modern operating systems, particularly when dealing with advanced processor features like swapgs that are essential for security boundary enforcement.
Mitigation strategies for this vulnerability primarily involve applying the appropriate security patches provided by FreeBSD, which correct the improper swapgs instruction execution during kernel return paths. System administrators should prioritize updating affected systems to versions that contain the fix, as this vulnerability does not have any known workarounds that can be implemented without modifying the kernel code. Organizations should also implement monitoring for suspicious kernel behavior patterns that might indicate exploitation attempts, particularly around interrupt and system call return operations. The vulnerability highlights the importance of thorough testing of kernel code modifications and proper handling of exception contexts, especially in systems that rely heavily on processor-level security features. Additionally, implementing proper access controls and limiting local user privileges can reduce the potential impact of such vulnerabilities, though they do not prevent exploitation by users who already have local access to the system.