CVE-2015-2877 in Linux
Summary
by MITRE
** DISPUTED ** Kernel Samepage Merging (KSM) in the Linux kernel 2.6.32 through 4.x does not prevent use of a write-timing side channel, which allows guest OS users to defeat the ASLR protection mechanism on other guest OS instances via a Cross-VM ASL INtrospection (CAIN) attack. NOTE: the vendor states "Basically if you care about this attack vector, disable deduplication." Share-until-written approaches for memory conservation among mutually untrusting tenants are inherently detectable for information disclosure, and can be classified as potentially misunderstood behaviors rather than vulnerabilities.
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Analysis
by VulDB Data Team • 11/12/2024
The vulnerability described in CVE-2015-2877 relates to Kernel Samepage Merging, a memory optimization feature in Linux systems that allows multiple memory pages to be merged when they contain identical data. This feature operates at the kernel level and is designed to reduce memory consumption by eliminating redundant copies of data. The issue arises from the implementation of KSM in Linux kernel versions ranging from 2.6.32 through 4.x, where the system fails to adequately protect against write-timing side channel attacks that can be exploited by malicious users within virtualized environments.
The technical flaw manifests through a Cross-VM ASLR Infiltration (CAIN) attack vector that specifically targets the memory deduplication capabilities of KSM. When multiple virtual machines share the same memory pages, attackers can exploit timing variations in write operations to infer information about memory layouts and potentially bypass Address Space Layout Randomization protections. This vulnerability operates at the intersection of virtualization security and memory management, where the optimization mechanism intended to conserve resources becomes a vector for information leakage. The attack exploits the fact that different memory locations respond to write operations with measurable timing variations, allowing an attacker to map memory structures and potentially discover sensitive information about other guest operating systems running on the same host.
The operational impact of this vulnerability extends beyond simple information disclosure, as it fundamentally undermines the security isolation that virtualized environments are designed to provide. In cloud computing and multi-tenant environments, where multiple users share the same physical infrastructure, this vulnerability allows malicious users to perform memory introspection attacks against other virtual machines. The implications are particularly severe in scenarios where different tenants operate untrusted software, as the attack can reveal memory layout information that would normally be protected by ASLR mechanisms. This capability significantly reduces the effectiveness of security mitigations and can potentially lead to privilege escalation or further exploitation of other vulnerabilities within the system.
The vendor's response categorically states that the issue stems from the fundamental nature of memory deduplication approaches, suggesting that the solution lies in disabling the deduplication feature rather than implementing complex software patches. This assessment aligns with the broader security principle that certain memory optimization techniques inherently conflict with information flow security controls. The vulnerability demonstrates the classic tension between performance optimization and security isolation in virtualized environments, where the same mechanisms that provide resource efficiency can simultaneously create information leakage channels. From an ATT&CK framework perspective, this vulnerability maps to techniques involving credential access through memory manipulation and information gathering through side channel attacks. The issue also relates to CWE-200, which covers "Exposure of Sensitive Information to an Unauthorized Actor," and CWE-310, which addresses "Cryptographic Issues." Organizations should consider disabling KSM in environments where security isolation is paramount, implementing strict access controls, and ensuring that virtualization platforms properly isolate memory spaces between tenants. The fundamental recommendation reflects the broader security principle that in multi-tenant environments, the risks associated with memory deduplication techniques often outweigh their performance benefits, particularly when dealing with mutually untrusting parties sharing the same infrastructure.