You can try to use the following solutions to intercept Linux kernel functions

You can try to use the following solutions to intercept Linux kernel functions:

Using Linux Security API

From the perspective of best practices, we feel that using the hook function of the Linux security API is the best choice, because this socket is designed for this purpose. Key points in the kernel code include security function calls, which may cause the installation of security modules to escalate. This module can study the context of a specific operation and decide whether to allow or prohibit it. Unfortunately, the LinuxSecurityAPI has two major limitations:

In fact, kernel developers have different ideas on whether the system can contain multiple security modules, and it is a certain fact that the modules cannot be loaded dynamically. To ensure that a system remains secure from the start, security modules must be part of the kernel. For this reason, in order to use the Linux security API, we need to create a custom Linux kernel.

Change system call table

Because it is mainly used for operations performed by user applications, we can implement it at the system call level. All Linux system call handlers are stored in the sys_call_table table. Modifying the values ​​in this table results in modified system behavior. We can do this by hooking the system call by saving the old handler value and adding our own handler to the table. These methods also have some advantages and disadvantages.

The main advantages are as follows:

However, these methods also have several disadvantages:

Technical implementation is more complex. In fact, it is not difficult to replace the values ​​​​in the table Call linux kernel functions, but there are some additional tasks that require individual conditions and some non-obvious solutions:

Only hook system calls. Because this approach allows you to replace system call handlers, it greatly limits entry points. All additional detection can be performed before or immediately after the system call, we only have the system call parameters and their return values. Therefore, sometimes we may need to carefully check the access permissions of the process and the validity of the system call parameters. It is reported that in some cases, the need to copy the user process memory twice will cause additional expenses. For example, when a parameter is passed through the watch needle, there are two copies: one created by ourselves and one created by the original handler.

Use Kprobes

Kprobes — An API designed for Linux kernel tracing and debugging. Kprobes allows the installation of pre- and post-processors for any kernel instruction as well as function entry and function return handlers. Handlers have access to registers and can modify them. This way, we have the opportunity to monitor the file execution flow and change it.

The main uses of using Kprobes to track Linux kernel functions are as follows:

Kprobes also have shortcomings:

Splicing

A classic way to configure kernel function hooking: replace the instructions at the beginning of the function with an unconditional jump to a custom handler. The original instruction is wired to a different location and executed before jumping back to the intercepted function. For this reason, with two jumps, the code can be spliced ​​into a function. This works the same way as kprobes jump optimization. Using splicing, you can get the same results as using kprobes, but with less overhead and full control over the process.

The advantages of using splicing are very significant:

However, these methods have a major drawback - technical complexity. Replacing the machine code in a function is not easy. In order to use splicing to call the linux kernel function, you need to complete the following operations:

Relatively speaking, splicing may be a very useful way to hook Linux kernel functions. But the implementation of this method is too complicated. Because considering our own capabilities and some other incentives, we are not prepared to adopt these methods for the time being, so we use them as candidates.

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