CVE-2018-18445 in Linux
Summary
by MITRE • 01/25/2023
In the Linux kernel 4.14.x, 4.15.x, 4.16.x, 4.17.x, and 4.18.x before 4.18.13, faulty computation of numeric bounds in the BPF verifier permits out-of-bounds memory accesses because adjust_scalar_min_max_vals in kernel/bpf/verifier.c mishandles 32-bit right shifts.
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Analysis
by VulDB Data Team • 05/30/2023
The vulnerability identified as CVE-2018-18445 represents a critical flaw in the Linux kernel's eBPF (extended Berkeley Packet Filter) verifier component that affects multiple kernel versions including 4.14.x through 4.18.x prior to 4.18.13. This issue stems from improper handling of numeric bounds computation within the BPF verifier, specifically in the adjust_scalar_min_max_vals function located in kernel/bpf/verifier.c. The flaw manifests when the verifier processes 32-bit right shift operations, leading to incorrect assumptions about memory access bounds that can be exploited by malicious actors.
The technical root cause of this vulnerability lies in the BPF verifier's inability to correctly compute the minimum and maximum values of scalar variables during program analysis. When processing 32-bit right shift operations, the adjust_scalar_min_max_vals function fails to properly account for all possible value ranges, resulting in underestimation of potential memory access offsets. This miscalculation creates a scenario where the verifier believes a memory access is within safe bounds when it actually extends beyond the allocated memory region, enabling out-of-bounds memory access patterns. The vulnerability is particularly concerning because it affects the kernel's security boundary, allowing potentially malicious BPF programs to access memory locations they should not be permitted to reach.
The operational impact of this vulnerability extends beyond simple memory corruption, as it provides attackers with a pathway to bypass kernel security mechanisms and potentially execute arbitrary code with kernel privileges. The BPF subsystem is designed to provide safe sandboxed execution environments for network filtering, tracing, and monitoring operations, but this flaw undermines that security model. Attackers could leverage this vulnerability to perform memory reads and writes beyond intended boundaries, potentially accessing sensitive kernel data structures, modifying critical system variables, or even escalating privileges to root level access. The vulnerability affects systems that utilize BPF programs for network packet filtering, system call tracing, or performance monitoring, making it particularly dangerous in production environments.
The flaw aligns with CWE-129, which addresses improper validation of array indices, and demonstrates characteristics consistent with ATT&CK technique T1059.006 for kernel-mode rootkits and privilege escalation. This vulnerability represents a classic example of a bounds checking failure in kernel space, where the security model's assumptions about memory access patterns are violated. Organizations running affected kernel versions face significant risk as this vulnerability can be exploited through legitimate BPF program loading mechanisms, making detection difficult and exploitation relatively straightforward for attackers with knowledge of the kernel internals.
Mitigation strategies for CVE-2018-18445 require immediate kernel updates to versions 4.18.13 or later where the fix has been implemented. System administrators should also consider implementing additional security controls such as disabling unnecessary BPF program loading capabilities, monitoring for suspicious BPF program usage patterns, and applying kernel lockdown mechanisms where appropriate. The fix in the patched versions addresses the specific numeric bounds computation issue in the adjust_scalar_min_max_vals function by properly handling 32-bit right shift operations and ensuring accurate minimum and maximum value calculations. Organizations should also conduct comprehensive security assessments of their BPF program usage and implement principle of least privilege controls to minimize the potential impact of any remaining vulnerabilities in their systems.