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207
kernel/hardened.nix
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207
kernel/hardened.nix
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{
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lib,
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config,
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pkgs,
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...
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}:
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let
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cfg = config.custom.kernel.hardened;
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in
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{
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options.custom.kernel.hardened = {
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enable = lib.mkEnableOption "hardened kernel options";
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};
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config = lib.mkIf cfg.enable {
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environment.systemPackages = [
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pkgs.kernel-hardening-checker
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pkgs.lynis
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];
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security = {
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auditd.enable = true;
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audit.enable = true;
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audit.rules = [ "-a always,exit -F arch=b64 -S execve" ];
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protectKernelImage = true;
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lockKernelModules = false; # this breaks iptables, wireguard, and virtd
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# force-enable the Page Table Isolation (PTI) Linux kernel feature
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forcePageTableIsolation = true;
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# User namespaces are required for sandboxing.
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# this means you cannot set `"user.max_user_namespaces" = 0;` in sysctl
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allowUserNamespaces = true;
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# Disable unprivileged user namespaces, unless containers are enabled
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unprivilegedUsernsClone = config.virtualisation.containers.enable;
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allowSimultaneousMultithreading = true;
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};
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boot.kernel.sysctl = {
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"fs.suid_dumpable" = 0;
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# prevent pointer leaks
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"kernel.kptr_restrict" = 2;
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# restrict kernel log to CAP_SYSLOG capability
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"kernel.dmesg_restrict" = 1;
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# Note: certian container runtimes or browser sandboxes might rely on the following
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# restrict eBPF to the CAP_BPF capability
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"kernel.unprivileged_bpf_disabled" = 1;
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# should be enabled along with bpf above
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# "net.core.bpf_jit_harden" = 2;
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# restrict loading TTY line disciplines to the CAP_SYS_MODULE
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"dev.tty.ldisk_autoload" = 0;
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# prevent exploit of use-after-free flaws
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"vm.unprivileged_userfaultfd" = 0;
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# kexec is used to boot another kernel during runtime and can be abused
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"kernel.kexec_load_disabled" = 1;
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# Kernel self-protection
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# SysRq exposes a lot of potentially dangerous debugging functionality to unprivileged users
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# 4 makes it so a user can only use the secure attention key. A value of 0 would disable completely
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"kernel.sysrq" = 4;
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# disable unprivileged user namespaces, Note: Docker, NH, and other apps may need this
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"kernel.unprivileged_userns_clone" = 1;
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# restrict all usage of performance events to the CAP_PERFMON capability
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"kernel.perf_event_paranoid" = 3;
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# Network
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# protect against SYN flood attacks (denial of service attack)
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"net.ipv4.tcp_syncookies" = 1;
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# protection against TIME-WAIT assassination
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"net.ipv4.tcp_rfc1337" = 1;
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# enable source validation of packets received (prevents IP spoofing)
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"net.ipv4.conf.default.rp_filter" = 1;
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"net.ipv4.conf.all.rp_filter" = 1;
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"net.ipv4.conf.all.accept_redirects" = 0;
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"net.ipv4.conf.default.accept_redirects" = 0;
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"net.ipv4.conf.all.secure_redirects" = 0;
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"net.ipv4.conf.default.secure_redirects" = 0;
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# Protect against IP spoofing
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"net.ipv6.conf.all.accept_redirects" = 0;
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"net.ipv6.conf.default.accept_redirects" = 0;
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"net.ipv4.conf.all.send_redirects" = 0;
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"net.ipv4.conf.default.send_redirects" = 0;
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# prevent man-in-the-middle attacks
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"net.ipv4.icmp_echo_ignore_all" = 1;
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# ignore ICMP request, helps avoid Smurf attacks
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"net.ipv4.conf.all.forwarding" = 0;
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"net.ipv4.conf.default.accept_source_route" = 0;
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"net.ipv4.conf.all.accept_source_route" = 0;
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"net.ipv6.conf.all.accept_source_route" = 0;
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"net.ipv6.conf.default.accept_source_route" = 0;
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# Reverse path filtering causes the kernel to do source validation of
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"net.ipv6.conf.all.forwarding" = 0;
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"net.ipv6.conf.all.accept_ra" = 0;
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"net.ipv6.conf.default.accept_ra" = 0;
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## TCP hardening
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# Prevent bogus ICMP errors from filling up logs.
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"net.ipv4.icmp_ignore_bogus_error_responses" = 1;
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# Disable TCP SACK
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"net.ipv4.tcp_sack" = 0;
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"net.ipv4.tcp_dsack" = 0;
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"net.ipv4.tcp_fack" = 0;
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# Userspace
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# restrict usage of ptrace
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"kernel.yama.ptrace_scope" = 2;
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# ASLR memory protection (64-bit systems)
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"vm.mmap_rnd_bits" = 32;
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"vm.mmap_rnd_compat_bits" = 16;
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# only permit symlinks to be followed when outside of a world-writable sticky directory
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"fs.protected_symlinks" = 1;
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"fs.protected_hardlinks" = 1;
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# Prevent creating files in potentially attacker-controlled environments
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"fs.protected_fifos" = 2;
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"fs.protected_regular" = 2;
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# Randomize memory
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"kernel.randomize_va_space" = 2;
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# Exec Shield (Stack protection)
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"kernel.exec-shield" = 1;
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## TCP optimization
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# TCP Fast Open is a TCP extension that reduces network latency by packing
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# data in the sender’s initial TCP SYN. Setting 3 = enable TCP Fast Open for
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# both incoming and outgoing connections:
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"net.ipv4.tcp_fastopen" = 3;
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# Bufferbloat mitigations + slight improvement in throughput & latency
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"net.ipv4.tcp_congestion_control" = "bbr";
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"net.core.default_qdisc" = "cake";
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};
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boot.kernelParams = [
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"audit=1"
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# make it harder to influence slab cache layout
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"slab_nomerge"
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# enables zeroing of memory during allocation and free time
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# helps mitigate use-after-free vulnerabilaties
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"init_on_alloc=1"
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"init_on_free=1"
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# randomizes page allocator freelist, improving security by
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# making page allocations less predictable
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"page_alloc.shuffel=1"
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# enables Kernel Page Table Isolation, which mitigates Meltdown and
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# prevents some KASLR bypasses
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"pti=on"
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# randomizes the kernel stack offset on each syscall
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# making attacks that rely on a deterministic stack layout difficult
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"randomize_kstack_offset=on"
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# disables vsyscalls, they've been replaced with vDSO
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"vsyscall=none"
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# disables debugfs, which exposes sensitive info about the kernel
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"debugfs=off"
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# certain exploits cause an "oops", this makes the kernel panic if an "oops" occurs
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"oops=panic"
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# only alows kernel modules that have been signed with a valid key to be loaded
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# making it harder to load malicious kernel modules
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# can make VirtualBox or Nvidia drivers unusable
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#"module.sig_enforce=1"
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# prevents user space code excalation
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"lockdown=confidentiality"
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# "rd.udev.log_level=3"
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# "udev.log_priority=3"
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];
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boot.blacklistedKernelModules = [
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# Obscure networking protocols
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"dccp" # Datagram Congestion Control Protocol
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"sctp" # Stream Control Transmission Protocol
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"rds" # Reliable Datagram Sockets
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"tipc" # Transparent Inter-Process Communication
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"n-hdlc" # High-level Data Link Control
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"ax25" # Amateur X.25
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"netrom" # NetRom
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"x25" # X.25
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"rose"
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"decnet"
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"econet"
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"af_802154" # IEEE 802.15.4
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"ipx" # Internetwork Packet Exchange
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"appletalk"
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"psnap" # SubnetworkAccess Protocol
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"p8023" # Novell raw IEE 802.3
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"p8022" # IEE 802.3
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"can" # Controller Area Network
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"atm"
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# Various rare filesystems
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"cramfs"
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"freevxfs"
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"jffs2"
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"hfs"
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"hfsplus"
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# "udf"
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# "nfs" # Network File System
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# "nfsv3"
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# "nfsv4"
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"gfs2" # Global File System 2
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# vivid driver is only useful for testing purposes and has been the
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# cause of privilege escalation vulnerabilities
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"vivid"
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];
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};
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}
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