CVE-2016-6522 in OpenBSD
Summary
by MITRE
Integer overflow in the uvm_map_isavail function in uvm/uvm_map.c in OpenBSD 5.9 allows local users to cause a denial of service (kernel panic) via a crafted mmap call, which triggers the new mapping to overlap with an existing mapping.
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Analysis
by VulDB Data Team • 09/04/2020
The vulnerability identified as CVE-2016-6522 represents a critical integer overflow flaw within the OpenBSD operating system's memory management subsystem. This issue resides in the uvm_map_isavail function located in the uvm/uvm_map.c file within OpenBSD version 5.9. The vulnerability stems from improper handling of integer arithmetic during memory mapping operations, specifically when determining whether a new memory mapping can be safely allocated without overlapping with existing mappings. The flaw manifests when a local user crafts a specially designed mmap system call that exploits the integer overflow condition, leading to a kernel panic and subsequent system denial of service.
The technical implementation of this vulnerability involves the uvm_map_isavail function's failure to properly validate integer values during the memory mapping process. When a user process attempts to map memory using mmap with carefully constructed parameters, the function calculates the boundaries of the new mapping and compares them against existing mappings. The integer overflow occurs during these boundary calculations, causing the function to incorrectly determine that a new mapping can be placed at an invalid memory location. This miscalculation results in overlapping memory mappings, which the kernel cannot properly handle, ultimately leading to a kernel panic. The vulnerability is classified as a CWE-190 Integer Overflow or Wraparound, which is a well-documented weakness in software systems where integer arithmetic produces results that exceed the maximum value representable by the data type. The flaw demonstrates poor input validation and inadequate boundary checking mechanisms within the kernel's memory management code.
From an operational impact perspective, this vulnerability presents a significant security concern for OpenBSD systems running version 5.9. Local users who can execute processes on the system gain the ability to trigger a denial of service condition that effectively crashes the kernel and renders the system unusable. The kernel panic resulting from this vulnerability cannot be easily recovered from without a system reboot, making it particularly dangerous in production environments where uptime is critical. The exploit requires only local user privileges, meaning that any user with access to the system can potentially cause the denial of service, making it an attractive target for malicious actors seeking to disrupt system availability. The vulnerability also aligns with ATT&CK technique T1499.004, which covers Network Denial of Service attacks, though in this case the attack operates at the kernel level rather than network level. The impact extends beyond simple availability disruption as kernel panics can potentially expose underlying system instability and may provide opportunities for further exploitation if not properly addressed.
The mitigation strategy for CVE-2016-6522 involves immediate system updates to patched versions of OpenBSD where the integer overflow has been corrected. The fix typically involves implementing proper integer overflow checks within the uvm_map_isavail function to ensure that calculated memory boundaries do not exceed valid ranges before proceeding with mapping operations. System administrators should prioritize patching affected systems and monitor for any signs of exploitation attempts. Additionally, implementing proper access controls and privilege separation can help limit the potential impact of local users who might attempt to exploit this vulnerability. The solution addresses the root cause by ensuring that integer calculations in memory mapping operations are properly validated and bounded, preventing the overflow condition that leads to the kernel panic. Organizations should also consider implementing monitoring solutions that can detect unusual mmap activity patterns that might indicate attempts to exploit this vulnerability, providing an additional layer of defense against potential attacks.