一、问题来源
测试内核漏洞利用
二、解决步骤
1.编写内核模块 hello_world_mod.c
#include <linux/init.h>
#include <linux/module.h>
MODULE_LICENSE("Dual BSD/GPL");
static int hello_init(void)
{
printk(KERN_ALERT "Hello, world\n");
return 0;
}
static void hello_exit(void)
{
printk(KERN_ALERT "Goodbye, world\n");
}
module_init(hello_init);
module_exit(hello_exit);
2.编译
编写Makefile
obj-m += hello_world_mod.o
all:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
编译
invincible@ubuntu:~/Desktop/my_mods$ ls
hello_world_mod.c Makefile
invincible@ubuntu:~/Desktop/my_mods$ make
make -C /lib/modules/4.4.0-112-generic/build M=/home/invincible/Desktop/my_mods modules
make[1]: Entering directory '/usr/src/linux-headers-4.4.0-112-generic'
CC [M] /home/invincible/Desktop/my_mods/hello_world_mod.o
Building modules, stage 2.
MODPOST 1 modules
CC /home/invincible/Desktop/my_mods/hello_world_mod.mod.o
LD [M] /home/invincible/Desktop/my_mods/hello_world_mod.ko
make[1]: Leaving directory '/usr/src/linux-headers-4.4.0-112-generic'
invincible@ubuntu:~/Desktop/my_mods$ ls
hello_world_mod.c hello_world_mod.mod.c hello_world_mod.o modules.order
hello_world_mod.ko hello_world_mod.mod.o Makefile Module.symvers
3.加载和卸载
invincible@ubuntu:~/Desktop/my_mods$ sudo insmod hello_world_mod.ko
[sudo] password for invincible:
invincible@ubuntu:~/Desktop/my_mods$ dmesg | tail -1
[23470.393026] Hello, world
invincible@ubuntu:~/Desktop/my_mods$ sudo rmmod hello_world_mod
invincible@ubuntu:~/Desktop/my_mods$ dmesg |tail -1
[23500.854576] Goodbye, world
三、知识点
什么是内核模块？
Modules are an efficient way of adding device drivers, filesystems and other components dynamically into the Linux kernel without having to build a new kernel or reboot the system.
模块是一种有效的向Linux Kernel中动态添加device drivers, filesystems 等内核组件的方法，这种方式不需要重新编译内核也不需要重启内核。
Externally, modules are just normal relocatable object files, as a file call will quickly confirm:
wolfgang@meitner> file vfat.ko
vfat.ko: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped
They are, of course, neither executable files nor program libraries as normally found in system program- ming; however, the basic structure of the binary module file is based on the same scheme also used for the above purposes.
模块也是一种ELF格式的文件。
内核模块代码含义？
hello_init和hello_exit
This module defines two functions, one to be invoked when the module is loaded into the kernel(hello_init) and one for when the module is removed(hello_exit).
自定义的两个函数，分别作为module_init和module_exit宏的参数，当模块被加载和卸载时调用。
module_init和module_exit宏
The module_init and module_exit lines use special kernel macros to indicate the role of these two functions.
用于声明模块加载和卸载的时候调用的函数。
MODULE_LICENSE
Another special macro(MODULE_LICENSE) is used to tell the kernel that this module bears a free license; without such a declaration, the kernel complains when the module is loaded.
用于声明License, 带着就行了
invincible$ grep "MODULE_LICENSE" -nr ./
.//kvm_main.c:72:MODULE_LICENSE("GPL");
kvm 用的GPL license
Makefile代码含义？
Linux/Documentation/kbuild/makefiles.txt
3.1 Goal definitions
Goal definitions are the main part (heart) of the kbuild Makefile.
...
The most simple kbuild makefile contains one line:
Example:
obj-y += foo.o
This tells kbuild that there is one object in that directory, named foo.o. foo.o will be built from foo.c or foo.S.
If foo.o shall be built as a module, the variable obj-m is used.
Linux/Documentation/kbuild/modules.txt
=== 2. How to Build External Modules
--- 2.1 Command Syntax
The command to build an external module is:
$ make -C <path_to_kernel_src> M=$PWD
The kbuild system knows that an external module is being built
due to the "M=<dir>" option given in the command.
To build against the running kernel use:
$ make -C /lib/modules/`uname -r`/build M=$PWD
...
--- 2.2 Options
($KDIR refers to the path of the kernel source directory.)
make -C $KDIR M=$PWD
-C $KDIR
The directory where the kernel source is located.
"make" will actually change to the specified directory
when executing and will change back when finished.
M=$PWD
Informs kbuild that an external module is being built.
The value given to "M" is the absolute path of the
directory where the external module (kbuild file) is
located.
--- 2.3 Targets
When building an external module, only a subset of the "make"
targets are available.
make -C $KDIR M=$PWD [target]
The default will build the module(s) located in the current
directory, so a target does not need to be specified. All
output files will also be generated in this directory. No
attempts are made to update the kernel source, and it is a
precondition that a successful "make" has been executed for the
kernel.
modules
The default target for external modules. It has the
same functionality as if no target was specified. See
description above.
modules_install
...
clean
Remove all generated files in the module directory only.
help
...
参考资料：
《Linux Kernel Architecture》Chapter 7
《Linux Device Driver 3》Chapter 2
《Linux Kernel Makefiles》
《Building External Modules》
