Pointer Dereferencing causes Permission Violation in Kernel Module

Ishpuini · 09-16-2022, 06:25 AM

I am trying to allocate memory for a pointer in another function in Linux Kernel module file, and it causes error_code(0x0001) - permissions violation .

Here is a simpler reproduction of what i am trying to do.

Code:

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/slab.h>

int another(char **name){
    *name = kmalloc(16, GFP_KERNEL);
    return 0;
}

static int __init test_init(void){
    char *name;
    another(&name);
    printk(KERN_INFO "Hello!");
    return 0;
}

static void __exit test_exit(void){
    printk(KERN_INFO "Goodbye!");
}

MODULE_LICENSE("GPL");

module_init(test_init);
module_exit(test_exit);

I did this a lot in userspace and it works just fine, but here it crashes every time. Please let me know if dmesg info is needed.

pan64 · 09-19-2022, 08:40 AM

No, it is just wrong. But anyway would be nice to use code tags to make your post readable.
1. char *name; (in test_init)
This will create a variable named "name" which is a pointer to a char. Not initialized, therefore it does not point to a valid/usable memory address.
2. *name = kmalloc(16, GFP_KERNEL);
This first calls kmalloc and the result will be stored at the location pointed by the local variable name, which is not the same as name in test_init, but actually it contains the same value.
3.
The result is that you want to write into a random address (based on an uninitialized variable), which will actually cause that permission denied.
4.
No it does not work in userspace too.

Ishpuini · 09-19-2022, 08:58 AM

Hi and thanx for replying!

Quote:

Originally Posted by pan64

No it does not work in userspace too.

This compiles and works just fine though...

Code:

#include <stdlib.h>

void custom_alloc(char **ptr){
	*ptr = malloc(128);
}

int main(int argc, char **argv){
	char *ptr;
	custom_alloc(&ptr);
	free(ptr);
	return 0;
}

However in Kernel i am getting error even while just passing the pointer without allocating memory, like this`

Code:

int another(char **name){
    return 0;
}

static int __init test_init(void){
    char *name;
    another(&name);
    printk(KERN_INFO "Hello!");
    return 0;
}

Also it works if i make name static.