I’d like to allocate a user space buffer in a kernel module (linux kernel v5.4.0). According to this post, this should be doable by using do_mmap().

I’ve written the following code:

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/slab.h>
#include <linux/uaccess.h>
#include <linux/err.h>
#include <linux/fcntl.h>
#include <linux/uio.h>
#include <linux/mm.h>
#include <linux/kallsyms.h>
#include <linux/mman.h>
#include <linux/string.h>

unsigned long (*orig_do_mmap)(struct file *file, unsigned long addr,
                              unsigned long len, unsigned long prot,
                              unsigned long flags, vm_flags_t vm_flags,
                              unsigned long pgoff, unsigned long *populate,
                              struct list_head *uf);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Chibi Gwen");
MODULE_DESCRIPTION("a simple module");
MODULE_VERSION("0.1");

static int __init start(void)
{
    printk(KERN_INFO, "module installedn");

    unsigned long populate;

    orig_do_mmap = kallsyms_lookup_name("do_mmap");
    if (orig_do_mmap == NULL)
    {
        printk("cannot find do_mmap() via kallsyms_lookup_name()n");
        return -EINVAL;
    }
        
    struct mm_struct *mm = current->mm;

    down_write(&mm->mmap_sem);
    unsigned char *buf= (unsigned char *)orig_do_mmap(NULL, 0, 0x2000, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE | MAP_LOCKED, 0, 0, &populate, NULL);
    up_write(&mm->mmap_sem);

    memset(buf, 0x0, 0x2000);

    return 0;
}

static void __exit end(void)
{
    printk(KERN_INFO, "module uninstalledn");
}

module_init(start);
module_exit(end);

However, when executing memset(), the kernel complains:

[   74.674862] BUG: unable to handle page fault for address: 00007f8945966000
[   74.674864] #PF: supervisor write access in kernel mode
[   74.674865] #PF: error_code(0x0002) - not-present page

Indeed do_mmap() successfully returns, and 00007f8945966000 happens to be the address of buf. My guess is that my code hasn’t initialized the corresponding page table entry.

What is the correct way to allocate a buffer in user space from a kernel module?
Any ideas?