Incus Allocation of Resources Without Limits allows firewall rule bypass on managed bridge networks

Summary

When using an ACL on a device connected to a bridge, Incus generates nftables rules for local services (DHCP, DNS...) that partially bypass security options security.mac_filtering, security.ipv4_filtering and security.ipv6_filtering. This can lead to DHCP pool exhaustion and opens the door for other attacks.

Details

In commit a7c33301738aede3c035063e973b1d885d9bac7c, the following rules are added at the top of the bridge input chain:

iifname "{{.hostName}}" ether type ip ip saddr 0.0.0.0 ip daddr 255.255.255.255 udp dport 67 accept
iifname "{{.hostName}}" ether type ip6 ip6 saddr fe80::/10 ip6 daddr ff02::1:2 udp dport 547 accept
iifname "{{.hostName}}" ether type ip6 ip6 saddr fe80::/10 ip6 daddr ff02::2 icmpv6 type 133 accept

However, these rules accept packets that should be filtered and maybe dropped by later rules in the "MAC filtering" snippet:

iifname "{{.hostName}}" ether type arp arp saddr ether != {{.hwAddr}} drop
iifname "{{.hostName}}" ether type ip6 icmpv6 type 136 @nh,528,48 != {{.hwAddrHex}} drop

Therefore, the MAC filtering is ineffective on those new rules. This allows an attacker to request as many IP as they want by sending a lot of DHCP requests with different MAC addresses. Doing so, they can exhaust the DHCP pool, resulting in a DoS of the bridge's network.

Additionaly, the commit adds non-restricted access to the local dnsmasq DNS server:

{{ if .dnsIPv4 }}
{{ range .dnsIPv4 }}
iifname "{{$.hostName}}" ip daddr "{{.}}" tcp dport 53 accept
iifname "{{$.hostName}}" ip daddr "{{.}}" udp dport 53 accept
{{ end }}
{{ end }}

{{ if .dnsIPv6 }}
{{ range .dnsIPv6 }}
iifname "{{$.hostName}}" ip6 daddr "{{.}}" tcp dport 53 accept
iifname "{{$.hostName}}" ip6 daddr "{{.}}" udp dport 53 accept
{{ end }}
{{ end }}

An attacker can send DNS requests with arbitrary MAC and IP addresses as well. These rules should also be after the MAC/IPv4/IPv6 filtering.

PoC

With this terraform infrastructure:

resource "incus_network_acl" "acl_allow_out" {
  name    = "acl-allow-out"
  egress = [
    {
      action           = "allow"
      destination      = "0.0.0.0-9.255.255.255,11.0.0.0-172.15.255.255,172.32.0.0-192.167.255.255,192.169.0.0-255.255.255.254"
      state            = "enabled"
    },
  ]
}
resource "incus_network_acl" "acl_allow_in" {
  name    = "acl-allow-in"
  ingress = [
    {
      action           = "allow"
      state            = "enabled"
    },
  ]
}

resource "incus_network" "br0" {
  name = "br0"
  config = {
    "ipv4.address"          = "10.0.0.1/24"
    "ipv4.nat"              = "true"
  }
}

resource "incus_instance" "machine1" {
  name  = "machine1"
  image = "images:archlinux/cloud"
  type = "virtual-machine"
  config = {
    "limits.memory" = "2GiB"
    "security.secureboot" = false
    "boot.autostart" = false
    "cloud-init.vendor-data" = <<-EOF
      #cloud-config
      package_update: true
      packages:
        - dhclient
        - tcpdump
      runcmd:
        - systemctl disable --now systemd.networkd.service
        - systemctl disable --now systemd.networkd.socket
    EOF
  }
  device {
    type = "disk"
    name = "root"
    properties = {
      pool = "default"
      path = "/"
      size = "64GiB"
    }
  }
  device {
    type = "nic"
    name = "eth0"
    properties = {
      network = incus_network.br0.name
      "security.ipv4_filtering" = true
      "security.acls" = join(",",
        [
          incus_network_acl.acl_allow_out.name,
          incus_network_acl.acl_allow_in.name,
        ])
    }
  }
}

resource "incus_instance" "machine2" {
  name  = "machine2"
  image = "images:archlinux/cloud"
  type = "virtual-machine"
  config = {
    "limits.memory" = "2GiB"
    "security.secureboot" = false
    "boot.autostart" = false
  }
  device {
    type = "disk"
    name = "root"
    properties = {
      pool = "default"
      path = "/"
      size = "64GiB"
    }
  }
  device {
    type = "nic"
    name = "eth0"
    properties = {
      network = incus_network.br0.name
    }
  }
}