下面的简单内核模块在加载后设置 cr4 寄存器的第 13 位 ( CR4.VMXE
),并在退出时清除该位。
vmx.c
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
MODULE_LICENSE("GPL");
static inline uint64_t getcr4(void) {
register uint64_t ret = 0;
asm volatile (
"movq %%cr4, %0\n"
:"=r"(ret)
);
return ret;
}
static inline void setcr4(register uint64_t val) {
asm volatile (
"movq %0, %%cr4\n"
:
:"r"(val)
);
}
static int __init init_routine(void) {
uint64_t cr4 = getcr4();
printk(KERN_INFO "VTX Test loaded: %llu (%u).\n", cr4, (unsigned char)((cr4 >> 13) & 1));
cr4 |= (1 << 13);
setcr4(cr4);
cr4 = getcr4();
printk(KERN_INFO "cr4: %llu (%u).\n", cr4, (unsigned char)((cr4 >> 13) & 1));
return 0;
}
static void __exit exit_routine(void) {
uint64_t cr4 = getcr4();
printk(KERN_INFO "cr4: %llu (%u).\n", cr4, (unsigned char)((cr4 >> 13) & 1));
cr4 &= ~(1 << 13);
setcr4(cr4);
cr4 = getcr4();
printk(KERN_INFO "VTX Test exited: %llu (%u).\n", cr4, (unsigned char)((cr4 >> 13) & 1));
}
module_init(init_routine);
module_exit(exit_routine);
生成文件
obj-m += vmx.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
为了运行我使用的模块 make clean && make && sudo insmod vmx.ko && sudo rmmod vmx && sudo dmesg -c
。这有时会给我以下(预期)输出
[ 2295.121537] VTX Test loaded: 1312736 (0).
[ 2295.121540] cr4: 1320928 (1).
[ 2295.123975] cr4: 1320928 (1).
[ 2295.123977] VTX Test exited: 1312736 (0).
有时还有以下内容:
[ 2296.256982] VTX Test loaded: 1320928 (1).
[ 2296.256984] cr4: 1320928 (1).
[ 2296.259481] cr4: 1312736 (0).
[ 2296.259483] VTX Test exited: 1312736 (0).
第二个输出中的第二行和第三行对我来说似乎很奇怪,因为似乎修改后的控制寄存器 cr4 在离开 init_routine
后已被重置。另外,奇怪的是,在第一行中似乎设置了 VMXE 位,这实际上没有任何意义。这种行为正常吗?怎么解释呢?是否有另一个正在运行的内核模块修改了 CR4?这看起来很奇怪,因为我见过几个 VTX 实现,它们都在初始化例程中设置了 VMXE 位,并以与本模块中相同的方式在退出例程中清除该位。
最佳答案
事实证明,问题在于寄存器并未在所有 CPU 内核上进行修改。为了确保修改发生在所有内核上,调用 on_each_cpu
似乎就足够了。修复了下面的代码,Makefile 不变。
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
MODULE_LICENSE("GPL");
static inline uint64_t getcr4(void) {
register uint64_t ret = 0;
asm volatile (
"movq %%cr4, %0\n"
:"=r"(ret)
);
return ret;
}
static inline void setcr4(register uint64_t val) {
asm volatile (
"movq %0, %%cr4\n"
:
:"r"(val)
);
}
static void setvmxe(void* info) {
uint64_t cr4 = getcr4();
cr4 |= (1 << 13);
setcr4(cr4);
}
static void clearvmxe(void* info) {
uint64_t cr4 = getcr4();
cr4 &= ~(1 << 13);
setcr4(cr4);
}
static int __init init_routine(void) {
uint64_t cr4 = getcr4();
printk(KERN_INFO "VTX Test loaded: %llu (%u).\n", cr4, (unsigned char)((cr4 >> 13) & 1));
on_each_cpu(setvmxe, NULL, 0);
cr4 = getcr4();
printk(KERN_INFO "cr4: %llu (%u).\n", cr4, (unsigned char)((cr4 >> 13) & 1));
return 0;
}
static void __exit exit_routine(void) {
uint64_t cr4 = getcr4();
printk(KERN_INFO "cr4: %llu (%u).\n", cr4, (unsigned char)((cr4 >> 13) & 1));
on_each_cpu(clearvmxe, NULL, 0);
cr4 = getcr4();
printk(KERN_INFO "VTX Test exited: %llu (%u).\n", cr4, (unsigned char)((cr4 >> 13) & 1));
}
module_init(init_routine);
module_exit(exit_routine);
关于c - 修改内核模块中的控制寄存器,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/27954917/