CVE-2017-17862 in Linux
Summary
by MITRE
kernel/bpf/verifier.c in the Linux kernel through 4.14.8 ignores unreachable code, even though it would still be processed by JIT compilers. This behavior, also considered an improper branch-pruning logic issue, could possibly be used by local users for denial of service.
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Analysis
by VulDB Data Team • 01/19/2023
The vulnerability identified as CVE-2017-17862 resides within the Linux kernel's eBPF (extended Berkeley Packet Filter) verifier component, specifically in the kernel/bpf/verifier.c file. This flaw affects Linux kernel versions through 4.14.8 and represents a critical issue in the kernel's security architecture. The vulnerability stems from the verifier's improper handling of unreachable code paths during the validation process, where the system fails to properly account for code that may be executed by JIT (Just-In-Time) compilers despite being marked as unreachable by the verifier logic.
The technical root cause of this vulnerability lies in the branch pruning logic that determines which code paths should be considered valid during the eBPF program verification process. When the verifier encounters conditional branches that lead to unreachable code, it incorrectly assumes these paths are irrelevant and omits them from further processing. However, the JIT compiler, which translates eBPF bytecode into native machine code for performance optimization, does not share this assumption and may still process these unreachable code segments. This discrepancy creates a fundamental mismatch between the verifier's validation and the JIT's execution behavior, leading to potential security implications.
This vulnerability falls under the CWE-691 category of 'Improper Control of Generation of Code' and relates to the broader class of issues involving inadequate code validation in kernel space. The improper branch pruning logic creates a situation where the kernel's security mechanisms are bypassed, as the verifier's incomplete analysis allows potentially malicious code to pass validation while still being executable by the JIT compiler. The vulnerability is particularly concerning because it affects the kernel's core security infrastructure, specifically the eBPF subsystem that is designed to provide safe and efficient network packet filtering and tracing capabilities.
The operational impact of this vulnerability is significant for local users who can potentially exploit it for denial of service attacks. While the vulnerability itself does not directly provide privilege escalation capabilities, it creates a vector for system instability by allowing malicious eBPF programs to be accepted and executed even when they contain code paths that should be rejected. The JIT compiler's processing of unreachable code can lead to unpredictable behavior, memory corruption, or system crashes that result in denial of service conditions. This makes the vulnerability particularly dangerous in environments where eBPF programs are frequently loaded or where privilege escalation is possible through other attack vectors.
The exploitation of this vulnerability requires local access to a system running an affected kernel version, as it involves loading and executing eBPF programs through the kernel's eBPF interface. Attackers can craft malicious eBPF programs that contain unreachable code paths which are accepted by the verifier but cause issues when processed by the JIT compiler. The vulnerability is particularly relevant in modern Linux systems where eBPF is extensively used for network monitoring, security policies, and performance tracing. Organizations should consider this vulnerability as part of their broader security posture assessment, especially in environments where eBPF programs are regularly loaded or where system stability is critical.
Mitigation strategies for this vulnerability include immediate kernel updates to versions that contain the fix for the branch pruning logic in the eBPF verifier. System administrators should prioritize patching affected systems, particularly those running kernel versions through 4.14.8, as the vulnerability affects the fundamental security mechanisms of the Linux kernel. Additionally, monitoring for unusual eBPF program loading activities and implementing proper access controls for eBPF program execution can help reduce the attack surface. The fix implemented in patched versions addresses the core issue by ensuring that unreachable code paths are properly considered during verification, preventing the JIT compiler from processing code that should not be executable. Organizations should also consider implementing runtime monitoring and anomaly detection for eBPF program execution to identify potential exploitation attempts.
This vulnerability demonstrates the importance of comprehensive verification processes in kernel security, as the mismatch between different components of the kernel's security infrastructure can create unexpected attack vectors. The issue highlights the need for careful attention to the interaction between verification and execution components, particularly in performance-optimized systems where JIT compilation is used. The ATT&CK framework categorizes this type of vulnerability under 'Exploitation for Privilege Escalation' and 'Execution through System Services' when considering the potential for denial of service attacks that could impact system availability. Organizations should ensure their security monitoring includes detection of eBPF program loading and execution patterns that could indicate exploitation attempts, as the vulnerability may be used in conjunction with other attack vectors to compromise system stability and availability.