CVE-2016-1885 in FreeBSD
Summary
by MITRE
Integer signedness error in the amd64_set_ldt function in sys/amd64/amd64/sys_machdep.c in FreeBSD 9.3 before p39, 10.1 before p31, and 10.2 before p14 allows local users to cause a denial of service (kernel panic) via an i386_set_ldt system call, which triggers a heap-based buffer overflow.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 07/06/2024
The vulnerability identified as CVE-2016-1885 represents a critical integer signedness error within the FreeBSD operating system kernel, specifically affecting versions prior to their respective patch levels. This flaw exists in the amd64_set_ldt function located in sys/amd64/amd64/sys_machdep.c, which handles the loading of descriptor tables for the amd64 architecture. The issue manifests when local users execute an i386_set_ldt system call, which is designed to work with 32-bit compatibility mode on 64-bit systems, creating a scenario where signed integer comparisons fail to properly validate input parameters.
The technical root cause stems from a fundamental flaw in how the system handles integer values during the descriptor table loading process. When the i386_set_ldt system call is invoked, the kernel performs a comparison between signed and unsigned integer values without proper type conversion or validation. This signedness error creates a condition where a maliciously crafted input can cause the kernel to calculate an incorrect buffer size, leading to a heap-based buffer overflow scenario. The flaw specifically affects the amd64_set_ldt function which is responsible for setting up the Local Descriptor Table (LDT) structures, a critical component for memory management and process isolation in x86 architectures.
The operational impact of this vulnerability is severe, as it allows local attackers to trigger a kernel panic, effectively causing a denial of service condition that crashes the entire system. This type of vulnerability is particularly dangerous because it operates at the kernel level, meaning that exploitation does not require elevated privileges beyond standard user access. The heap-based buffer overflow occurs when the kernel attempts to allocate memory for the descriptor table structures, but due to the signed integer error, it calculates an invalid memory address that exceeds the allocated buffer boundaries. This results in memory corruption that the kernel cannot recover from, leading to an immediate system crash.
From a cybersecurity perspective, this vulnerability aligns with CWE-194, which describes the weakness of unexpected signedness in integer operations, and represents a classic example of how improper type handling can lead to memory corruption issues. The ATT&CK framework categorizes this as a privilege escalation technique through kernel exploitation, as local users can leverage this flaw to gain system-wide control. The vulnerability also demonstrates characteristics of a buffer overflow attack pattern, specifically targeting heap memory management in the kernel. The impact extends beyond simple denial of service as it represents a potential pathway for more sophisticated attacks, as kernel crashes can be used to mask other malicious activities or as a stepping stone for further exploitation attempts.
Mitigation strategies for CVE-2016-1885 require immediate patch application to affected FreeBSD versions, as the vulnerability cannot be effectively addressed through configuration changes or workarounds. System administrators should prioritize updating to FreeBSD 9.3-p39, 10.1-p31, or 10.2-p14 and later versions that contain the necessary fixes for the integer signedness error. Additionally, organizations should implement monitoring for unusual system crashes or kernel panics that might indicate exploitation attempts. Network segmentation and privilege separation can help limit the potential impact if exploitation occurs, though these measures do not prevent the vulnerability itself. The fix implemented in the patched versions involves proper type conversion and validation of integer parameters before buffer allocation, ensuring that signed and unsigned values are correctly compared and handled within the kernel's memory management subsystem.