Metasploit Module: Linux BPF Sign Extension Local Privilege Escalation
Type: exploit
Path: modules/exploits/linux/local/bpf_sign_extension_priv_esc.rb
Disclosure date: 2017-11-12
References: BID-102288, CVE-2017-16995, EDB-44298, EDB-45010, URL-https://github.com/rlarabee/exploits/blob/master/cve-2017-16995/cve-2017-16995.c, URL-https://github.com/brl/grlh/blob/master/get-rekt-linux-hardened.c, URL-https://cyseclabs.com/exploits/upstream44.c, URL-https://blog.aquasec.com/ebpf-vulnerability-cve-2017-16995-when-the-doorman-becomes-the-backdoor, URL-https://ricklarabee.blogspot.com/2018/07/ebpf-and-analysis-of-get-rekt-linux.html, URL-https://www.debian.org/security/2017/dsa-4073

Description:
Linux kernel prior to 4.14.8 contains a vulnerability in the Berkeley
          Packet Filter (BPF) verifier. The `check_alu_op` function performs
          incorrect sign extension which allows the verifier to be bypassed,
          leading to arbitrary kernel read/write.

          The target system must be compiled with BPF support and permit
          unprivileged access to BPF with `kernel.unprivileged_bpf_disabled`
          not set to 1.

          This module has been tested successfully on:

          Debian 9.0 kernel 4.9.0-3-amd64;
          Deepin 15.5 kernel 4.9.0-deepin13-amd64;
          ElementaryOS 0.4.1 kernel 4.8.0-52-generic;
          Fedora 24 kernel 4.5.5-300.fc24.x86_64;
          Fedora 25 kernel 4.8.6-300.fc25.x86_64;
          Fedora 26 kernel 4.11.8-300.fc26.x86_64;
          Fedora 27 kernel 4.13.9-300.fc27.x86_64;
          Gentoo 2.2 kernel 4.5.2-aufs-r;
          Linux Mint 17.3 kernel 4.4.0-89-generic;
          Linux Mint 18.0 kernel 4.8.0-58-generic;
          Linux Mint 18.3 kernel 4.13.0-16-generic;
          Mageia 6 kernel 4.9.35-desktop-1.mga6;
          Manjero 16.10 kernel 4.4.28-2-MANJARO;
          Solus 3 kernel 4.12.7-11.current;
          Ubuntu 14.04.1 kernel 4.4.0-89-generic;
          Ubuntu 16.04.2 kernel 4.8.0-45-generic;
          Ubuntu 16.04.3 kernel 4.10.0-28-generic;
          Ubuntu 17.04 kernel 4.10.0-19-generic;
          ZorinOS 12.1 kernel 4.8.0-39-generic.

Check method (fingerprinting):
arch = kernel_hardware

    unless arch.include?('x86_64')
      return CheckCode::Safe("System architecture #{arch} is not supported")
    end

    vprint_good("System architecture #{arch} is supported")

    release = kernel_release
    if Rex::Version.new(release.split('-').first) > Rex::Version.new('4.14.11') ||
       Rex::Version.new(release.split('-').first) < Rex::Version.new('4.0')
      return CheckCode::Safe("Kernel version #{release} is not vulnerable")
    end

    vprint_good("Kernel version #{release} appears to be vulnerable")

    if unprivileged_bpf_disabled?
      return CheckCode::Safe('Unprivileged BPF loading is not permitted')
    end

    vprint_good('Unprivileged BPF loading is permitted')

    if lkrg_installed?
      return CheckCode::Safe('LKRG is installed')
    end

    vprint_good('LKRG is not installed')

    if grsec_installed?
      return CheckCode::Safe('grsecurity is in use')
    end

    vprint_good('grsecurity is not in use')

    config = kernel_config

    if config.nil?
      return CheckCode::Detected('Could not retrieve kernel config')
    end

    unless config.include?('CONFIG_BPF_SYSCALL=y')
      return CheckCode::Safe('Kernel config does not include CONFIG_BPF_SYSCALL')
    end

    vprint_good('Kernel config has CONFIG_BPF_SYSCALL enabled')

    CheckCode::Appears("Kernel version #{release} appears to be vulnerable")
  end