CVE-2016-0821 in Android
Summary
by MITRE
The LIST_POISON feature in include/linux/poison.h in the Linux kernel before 4.3, as used in Android 6.0.1 before 2016-03-01, does not properly consider the relationship to the mmap_min_addr value, which makes it easier for attackers to bypass a poison-pointer protection mechanism by triggering the use of an uninitialized list entry, aka Android internal bug 26186802, a different vulnerability than CVE-2015-3636.
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Analysis
by VulDB Data Team • 07/10/2022
The vulnerability described in CVE-2016-0821 resides within the Linux kernel's LIST_POISON feature implementation in the include/linux/poison.h header file, affecting kernel versions prior to 4.3 and specifically impacting Android 6.0.1 systems before the 2016-03-01 security patch. This flaw represents a critical weakness in the kernel's memory safety mechanisms designed to prevent use-after-free vulnerabilities and pointer corruption attacks. The LIST_POISON mechanism is intended to initialize list entries with specific poison values that would cause immediate kernel panics when accessed, thereby detecting memory corruption errors during development and testing phases. However, the vulnerability emerges from the improper handling of the relationship between this poisoning mechanism and the mmap_min_addr kernel parameter that controls minimum virtual memory address boundaries for memory mapping operations.
The technical flaw occurs when the LIST_POISON feature fails to properly account for the mmap_min_addr value during the initialization of list structures, creating a scenario where attackers can manipulate memory layout to bypass the intended protection. This vulnerability specifically enables attackers to trigger the use of uninitialized list entries that would normally be detected and terminated by the poison pointer mechanism. The relationship between these two kernel parameters becomes critical because mmap_min_addr establishes boundaries for where memory mappings can be placed in virtual address space, while LIST_POISON operates on list entry initialization. When these mechanisms interact improperly, it creates a condition where poison values can be overwritten or rendered ineffective due to memory layout considerations, particularly in scenarios involving memory allocation and deallocation patterns that align with the mmap_min_addr constraints.
The operational impact of this vulnerability is significant for Android systems running vulnerable kernel versions, as it allows attackers to circumvent kernel memory protection mechanisms that are essential for maintaining system integrity and preventing privilege escalation attacks. This weakness enables adversaries to potentially exploit uninitialized memory regions that should be protected by the poison pointer mechanism, creating opportunities for information disclosure, privilege escalation, and system compromise. The vulnerability is particularly concerning because it operates at the kernel level and can be leveraged to bypass multiple security controls that depend on proper list entry initialization and memory management. Attackers can manipulate the memory layout to ensure that list entries are placed within address ranges that are not properly protected by the poison mechanism, effectively neutralizing a critical defense-in-depth layer.
The vulnerability manifests as a bypass of memory safety protections that are fundamental to preventing use-after-free attacks and other memory corruption exploits. It is classified under CWE-122, which deals with improper restriction of operations within a recognized security boundary, and relates to the broader category of memory safety issues in kernel space. From an attack perspective, this vulnerability aligns with ATT&CK techniques involving privilege escalation through kernel exploits, specifically targeting the T1068 privilege escalation tactic. The flaw demonstrates poor memory management practices in kernel code where the interaction between different security mechanisms was not properly considered, creating a scenario where one protection mechanism fails to function correctly when combined with another. This vulnerability underscores the importance of comprehensive testing of security mechanisms when they interact with each other, as the combination of memory layout constraints and poisoning mechanisms should have been thoroughly validated to ensure that all protections remain effective.
Mitigation strategies for this vulnerability require immediate kernel updates to version 4.3 or later, where the relationship between LIST_POISON and mmap_min_addr has been properly addressed. System administrators and device manufacturers should prioritize applying the relevant security patches to prevent exploitation of this memory safety bypass. Additionally, implementing additional runtime protections such as stack canaries, kernel address space layout randomization, and enhanced memory management monitoring can provide supplementary defense layers. Organizations should also conduct comprehensive security assessments of their kernel configurations to ensure that mmap_min_addr and related memory management parameters are properly tuned and that no other similar interactions between security mechanisms exist. The vulnerability highlights the need for robust testing procedures that validate the interaction between multiple kernel security features, particularly those that operate at different levels of the memory management hierarchy, and emphasizes the importance of maintaining up-to-date kernel versions to protect against known vulnerabilities.