IMA官方文档：Linux内核完整性度量与远程证实技术详解

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IMA (Integrity Measurement Architecture) 是 Linux 内核引入的一种完整性度量框架，旨在增强系统的安全性，特别是在处理应用程序加载的模块、动态链接库和程序自身时。该框架是 Linux 安全性体系的一部分，包括内核组件和用户空间工具。在内核层面，IMA 的核心组件有： 1. **IMA**：负责在系统内执行度量任务，确保软件组件的完整性。它通过对内存和代码进行测量来验证数据的一致性和正确性。 2. **IMA-Appraisal**：这是一个关键模块，用于评估被度量对象的有效性和信任状态。它会验证签名和证书，以确认度量值的来源和有效性。 3. **IMA-Appraisal-Directory-Extension**：扩展了目录功能，用于存储和管理度量信息，便于查找和验证度量结果。 4. **IMA-Appraisal-Signature-Extension**：增加对签名的支持，确保度量数据的完整性和来源认证。 5. **EVM (Enclave Virtual Machine)**：提供一个安全环境，可以在其中执行敏感操作，如度量和审计，保护不受恶意攻击。 6. **Trusted and Encrypted Keys**：关键的加密和密钥管理，确保数据的保密性。在应用和支持方面，有： - **Trousers/Utilities**：一组实用工具，帮助用户管理和配置IMA，包括安装、配置和维护度量策略。 - **OpenPTS (Open Platform Trusted Services)**：一套开放的平台可信服务，支持远程证实，即在远程服务器上验证系统启动过程的可信性，通过TPM (Trusted Platform Module) 实现可信启动，并记录度量日志。目标： IMA的主要目标是实现三个关键的安全属性——完整性、真实性以及保密性。完整性是最基础的保障，它确保系统中的程序和数据不被篡改或损坏。只有当系统保持完整性时，才能进一步保证其数据的来源真实可靠以及信息的保密性。这个文档概述了IMA组件的功能、设计初衷、当前状态以及集成后的整体架构，有助于开发者和系统管理员理解如何利用这些工具来提升Linux系统的安全性，特别是在面临远程证实等高级安全需求时。尽管有些组件尚处于开发阶段，但文档的目的是提供一个清晰的路线图，以便最终整合所有组件，形成一个完整且高效的Linux完整性度量框架。

Unfortunately it's not that simple - encryption is not integrity. First, as a simple example

consider a stream cipher such as RC4, which was for a long time used as the standard encryption

for all wireless and internet (https) communication. RC4 by itself provides no integrity protection

whatsoever, and it is trivial for an attacker to "bit-twiddle" any desired bits in the encrypted

stream. Even with block ciphers, with the current standard chaining methods, encrypted blocks

can be cut and pasted and replayed between files and sessions with a good probability of success.

Second, there are significant performance penalties for encrypting system files, where integrity,

not confidentiality is all that is required.

In IPSEC, a deliberate decision was made to allow integrity (AH - authentication headers)

without encryption, but not encryption without integrity.

The bottom line is that encryption by itself does not guarantee integrity, and you really want

both.

One of the hardest problems in confidentiality is key management. It does no good to have

strong encryption with a key system that makes it easy for an attacker to steal the keys.

The trusted key component does two things to help with secure key management on Linux. First,

it provides a kernel key ring service in which the symmetric encryption keys are never visible in

plain text to userspace. The keys are created in the kernel, and sealed by a hardware device such

as a TPM, with userspace seeing only the sealed blobs. Malicious or compromised applications

cannot steal a trusted key, since only the kernel can see the unsealed blobs. Secondly, the trusted

keys can tie key unsealing to the integrity measurements, so that keys cannot be stolen in an

offline attack, such as by booting an unlocked Linux image from CD or USB. As the

measurements will be different, the TPM chip will refuse to unseal the keys, even for the kernel.

In this way, Linux ties confidentiality and integrity together. The integrity of the system is a

prerequisite for decrypting confidential files.

Threat Models:

There are many different types of attacks on file integrity, authenticity, and confidentiality, and

there are significant tradeoffs between security and cost and performance, depending on which of

the threats need to be defended against.

Remote Attacks

The most common attack is the remote software attack, in which the attacker attempts either to

trick a user into running the attacker's malicious code (trojan), or in which the attacker sends

malicious data in an attempt to exploit a vulnerability in the system's software (injection,

overflow).

Local Attacks

Local attacks assume that the attacker has physical access to the system, such as with a malicious

insider (operator), or in the case of theft of the system. Local attacks can be software based, such

as in an offline attack, or can be hardware based, such as with simple JTAG memory probes,

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