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Host System Changes

Monitor Host System changes

Some pods need to change the host system or kernel parameters in order to perform administrative tasks, obvious examples are pods loading a kernel module to extend the operating system functionality, or pods managing the network.

However, there are also other cases where a compromised container may want to load a kernel module to hide its behaviour.

In this aspect, monitoring such host system changes helps to identify pods and containers that affect the host system.

Monitor Linux kernel modules

A kernel module is a code that can be loaded into the kernel image at runtime, without rebooting. These modules, which can be loaded by pods and containers, can modify the host system. The Monitor Linux kernel modules guide will assist you in observing such events.

1 - Monitor Linux Kernel Modules

Monitor Linux Kernel Modules operations

Monitoring kernel modules helps to identify processes that load kernel modules to add features, to the operating system, to alter host system functionality or even hide their behaviour. This can be used to answer the following questions:

Which process or container is changing the kernel?

Which process or container is loading or unloading kernel modules in the cuslter?

Which process or container requested a feature that triggered the kernel to automatically load a module?

Are the loaded kernel modules signed?

Monitor Loading kernel modules

Kubernetes Environments

After deploying Tetragon, use the monitor-kernel-modules tracing policy which generates ProcessKprobe events to trace kernel module operations.

Apply the monitor-kernel-modules tracing policy:

kubectl apply -f https://raw.githubusercontent.com/cilium/tetragon/main/examples/tracingpolicy/host-changes/monitor-kernel-modules.yaml

Then start monitoring for events with tetra CLI:

kubectl exec -it -n kube-system ds/tetragon -c tetragon -- tetra getevents

When loading an out of tree module named kernel_module_hello.ko with the command insmod, tetra CLI will generate the following ProcessKprobe events:

1. Reading the kernel module from the file system 

{
  "process_kprobe": {
    "process": {
      "exec_id": "OjEzMTg4MTQwNDUwODkwOjgyMDIz",
      "pid": 82023,
      "uid": 0,
      "cwd": "/home/tixxdz/tetragon",
      "binary": "/usr/sbin/insmod",
      "arguments": "contrib/tester-progs/kernel_module_hello.ko",
      "flags": "execve clone",
      "start_time": "2023-08-30T11:01:22.846516679Z",
      "auid": 1000,
      "parent_exec_id": "OjEzMTg4MTM4MjY2ODQyOjgyMDIy",
      "refcnt": 1,
      "tid": 82023
    },
    "parent": {
      "exec_id": "OjEzMTg4MTM4MjY2ODQyOjgyMDIy",
      "pid": 82022,
      "uid": 1000,
      "cwd": "/home/tixxdz/tetragon",
      "binary": "/usr/bin/sudo",
      "arguments": "insmod contrib/tester-progs/kernel_module_hello.ko",
      "flags": "execve",
      "start_time": "2023-08-30T11:01:22.844332959Z",
      "auid": 1000,
      "parent_exec_id": "OjEzMTg1NTE3MTgzNDM0OjgyMDIx",
      "refcnt": 1,
      "tid": 0
    },
    "function_name": "security_kernel_read_file",
    "args": [
      {
        "file_arg": {
          "path": "/home/tixxdz/tetragon/contrib/tester-progs/kernel_module_hello.ko"
        }
      },
      {
        "int_arg": 2
      }
    ],
    "return": {
      "int_arg": 0
    },
    "action": "KPROBE_ACTION_POST"
  },
  "time": "2023-08-30T11:01:22.847554295Z"
}

In addition to the process metadata from exec events, ProcessKprobe events contain the arguments of the observed call. In the above case they are:

  • security_kernel_read_file: the kernel security hook when the kernel loads file specified by user space.
  • file_arg: the full path of the kernel module on the file system.

2. Finalize loading of kernel modules 

{
  "process_kprobe": {
    "process": {
      "exec_id": "OjEzMTg4MTQwNDUwODkwOjgyMDIz",
      "pid": 82023,
      "uid": 0,
      "cwd": "/home/tixxdz/tetragon",
      "binary": "/usr/sbin/insmod",
      "arguments": "contrib/tester-progs/kernel_module_hello.ko",
      "flags": "execve clone",
      "start_time": "2023-08-30T11:01:22.846516679Z",
      "auid": 1000,
      "parent_exec_id": "OjEzMTg4MTM4MjY2ODQyOjgyMDIy",
      "refcnt": 1,
      "tid": 82023
    },
    "parent": {
      "exec_id": "OjEzMTg4MTM4MjY2ODQyOjgyMDIy",
      "pid": 82022,
      "uid": 1000,
      "cwd": "/home/tixxdz/tetragon",
      "binary": "/usr/bin/sudo",
      "arguments": "insmod contrib/tester-progs/kernel_module_hello.ko",
      "flags": "execve",
      "start_time": "2023-08-30T11:01:22.844332959Z",
      "auid": 1000,
      "parent_exec_id": "OjEzMTg1NTE3MTgzNDM0OjgyMDIx",
      "refcnt": 1,
      "tid": 0
    },
    "function_name": "do_init_module",
    "args": [
      {
        "module_arg": {
          "name": "kernel_module_hello",
          "tainted": [
            "TAINT_OUT_OF_TREE_MODULE",
            "TAINT_UNSIGNED_MODULE"
          ]
        }
      }
    ],
    "action": "KPROBE_ACTION_POST"
  },
  "time": "2023-08-30T11:01:22.847638990Z"
}

This ProcessKprobe event contains:

  • do_init_module: the function call where the module is finaly loaded.
  • module_arg: the kernel module information, it contains:
    • name: the name of the kernel module as a string.
    • tainted: the module tainted flags that will be applied on the kernel. In the example above, it indicates we are loading an out-of-tree module, that is unsigned module which may compromise the integrity of our system.

Monitor Kernel Modules Signature

Kernels compiled with CONFIG_MODULE_SIG option will check if the modules being loaded were cryptographically signed. This allows to assert that:

  • If the module being loaded is signed, the kernel has its key and the signature verification succeeded.

  • The integrity of the system or the kernel was not compromised.

Kubernetes Environments

After deploying Tetragon, use the monitor-signed-kernel-modules tracing policy which generates ProcessKprobe events to identify if kernel modules are signed or not.

Apply the monitor-signed-kernel-modules tracing policy:

kubectl apply -f https://raw.githubusercontent.com/cilium/tetragon/main/examples/tracingpolicy/host-changes/monitor-signed-kernel-modules.yaml

Before going forward, deploy the test-pod into the demo-app namespace, which has its security context set to privileged. This allows to run the demo by mountig an xfs file system inside the test-pod which requires privileges, but will also trigger an automatic xfs module loading operation.

kubectl create namespace demo-app
kubectl apply -n demo-app -f https://raw.githubusercontent.com/cilium/tetragon/main/testdata/specs/testpod.yaml

Start monitoring for events with tetra CLI:

kubectl exec -it -n kube-system ds/tetragon -c tetragon -- tetra getevents

In another terminal, kubectl exec into the test-pod and run the following commands to create an xfs filesystem:

kubectl exec -it -n demo-app test-pod -- /bin/sh
apk update
dd if=/dev/zero of=loop.xfs bs=1 count=0 seek=32M
ls -lha loop.xfs
apk add xfsprogs
mkfs.xfs -q loop.xfs
mkdir /mnt/xfs.volume
mount -o loop -t xfs loop.xfs /mnt/xfs.volume/
losetup -a | grep xfs

Now the xfs filesystem should be mounted at /mnt/xfs.volume. To unmount it and release the loop device run:

umount /mnt/xfs.volume/

tetra CLI will generate the following events:

1. Automatic loading of kernel modules

First the mount command will trigger an automatic operation to load the xfs kernel module.

{
  "process_kprobe": {
    "process": {
      "exec_id": "a2luZC1jb250cm9sLXBsYW5lOjQxMjc1NTA0OTk5NTcyOjEzMDg3Ng==",
      "pid": 130876,
      "uid": 0,
      "cwd": "/",
      "binary": "/bin/mount",
      "arguments": "-o loop -t xfs loop.xfs /mnt/xfs.volume/",
      "flags": "execve rootcwd clone",
      "start_time": "2023-09-09T23:27:42.732039059Z",
      "auid": 4294967295,
      "pod": {
        "namespace": "demo-app",
        "name": "test-pod",
        "container": {
          "id": "containerd://1e910d5cc8d8d68c894934170b162ef93aea5652867ed6bd7c620c7e3f9a10f1",
          "name": "test-pod",
          "image": {
            "id": "docker.io/cilium/starwars@sha256:f92c8cd25372bac56f55111469fe9862bf682385a4227645f5af155eee7f58d9",
            "name": "docker.io/cilium/starwars:latest"
          },
          "start_time": "2023-09-09T22:46:09Z",
          "pid": 45672
        },
        "workload": "test-pod"
      },
      "docker": "1e910d5cc8d8d68c894934170b162ef",
      "parent_exec_id": "a2luZC1jb250cm9sLXBsYW5lOjQxMjYyOTc1MjI1MDkzOjEzMDgwOQ==",
      "refcnt": 1,
      "tid": 130876
    },
    "parent": {
      "exec_id": "a2luZC1jb250cm9sLXBsYW5lOjQxMjYyOTc1MjI1MDkzOjEzMDgwOQ==",
      "pid": 130809,
      "uid": 0,
      "cwd": "/",
      "binary": "/bin/sh",
      "flags": "execve rootcwd clone",
      "start_time": "2023-09-09T23:27:30.202263472Z",
      "auid": 4294967295,
      "pod": {
        "namespace": "demo-app",
        "name": "test-pod",
        "container": {
          "id": "containerd://1e910d5cc8d8d68c894934170b162ef93aea5652867ed6bd7c620c7e3f9a10f1",
          "name": "test-pod",
          "image": {
            "id": "docker.io/cilium/starwars@sha256:f92c8cd25372bac56f55111469fe9862bf682385a4227645f5af155eee7f58d9",
            "name": "docker.io/cilium/starwars:latest"
          },
          "start_time": "2023-09-09T22:46:09Z",
          "pid": 45612
        },
        "workload": "test-pod"
      },
      "docker": "1e910d5cc8d8d68c894934170b162ef",
      "parent_exec_id": "a2luZC1jb250cm9sLXBsYW5lOjQxMjYyOTEwMjM3OTQ2OjEzMDgwMA==",
      "tid": 130809
    },
    "function_name": "security_kernel_module_request",
    "args": [
      {
        "string_arg": "fs-xfs"
      }
    ],
    "return": {
      "int_arg": 0
    },
    "action": "KPROBE_ACTION_POST"
  },
  "node_name": "kind-control-plane",
  "time": "2023-09-09T23:27:42.751151233Z"
}

In addition to the process metadata from exec events, ProcessKprobe event contains the arguments of the observed call. In the above case they are:

  • security_kernel_module_request: the kernel security hook where modules are loaded on-demand.
  • string_arg: the name of the kernel module. When modules are automatically loaded, for security reasons, the kernel prefixes the module with the name of the subsystem that requested it. In our case, it’s requested by the file system subsystem, hence the name is fs-xfs.

2. Kernel calls modprobe to load the kernel module

The kernel will then call user space modprobe to load the kernel module.

{
  "process_exec": {
    "process": {
      "exec_id": "a2luZC1jb250cm9sLXBsYW5lOjQxMjc1NTI0MjYzMjIxOjEzMDg3Nw==",
      "pid": 130877,
      "uid": 0,
      "cwd": "/",
      "binary": "/sbin/modprobe",
      "arguments": "-q -- fs-xfs",
      "flags": "execve rootcwd clone",
      "start_time": "2023-09-09T23:27:42.751301124Z",
      "auid": 4294967295,
      "parent_exec_id": "a2luZC1jb250cm9sLXBsYW5lOjE6MA==",
      "tid": 130877
    },
    "parent": {
      "exec_id": "a2luZC1jb250cm9sLXBsYW5lOjE6MA==",
      "pid": 0,
      "uid": 0,
      "binary": "<kernel>",
      "flags": "procFS",
      "start_time": "2023-09-09T11:59:47.227037763Z",
      "auid": 0,
      "parent_exec_id": "a2luZC1jb250cm9sLXBsYW5lOjE6MA==",
      "tid": 0
    }
  },
  "node_name": "kind-control-plane",
  "time": "2023-09-09T23:27:42.751300984Z"
}

The ProcessExec event where modprobe tries to load the xfs module.

3. Reading the kernel module from the file system

modprobe will read the passed xfs kernel module from the host file system.

{
  "process_kprobe": {
    "process": {
      "exec_id": "a2luZC1jb250cm9sLXBsYW5lOjQxMjc1NTI0MjYzMjIxOjEzMDg3Nw==",
      "pid": 130877,
      "uid": 0,
      "cwd": "/",
      "binary": "/sbin/modprobe",
      "arguments": "-q -- fs-xfs",
      "flags": "execve rootcwd clone",
      "start_time": "2023-09-09T23:27:42.751301124Z",
      "auid": 4294967295,
      "parent_exec_id": "a2luZC1jb250cm9sLXBsYW5lOjE6MA==",
      "refcnt": 1,
      "tid": 130877
    },
    "parent": {
      "exec_id": "a2luZC1jb250cm9sLXBsYW5lOjE6MA==",
      "pid": 0,
      "uid": 0,
      "binary": "<kernel>",
      "flags": "procFS",
      "start_time": "2023-09-09T11:59:47.227037763Z",
      "auid": 0,
      "parent_exec_id": "a2luZC1jb250cm9sLXBsYW5lOjE6MA==",
      "tid": 0
    },
    "function_name": "security_kernel_read_file",
    "args": [
      {
        "file_arg": {
          "path": "/usr/lib/modules/6.2.0-32-generic/kernel/fs/xfs/xfs.ko"
        }
      },
      {
        "int_arg": 2
      }
    ],
    "return": {
      "int_arg": 0
    },
    "action": "KPROBE_ACTION_POST"
  },
  "node_name": "kind-control-plane",
  "time": "2023-09-09T23:27:42.752425825Z"
}

This ProcessKprobe event contains:

  • security_kernel_read_file: the kernel security hook when the kernel loads file specified by user space.
  • file_arg: the full path of the kernel module on the host file system.

4. Kernel module signature and sections are parsed

The final event is when the kernel is parsing the module sections. If all succeed the module will be loaded.

{
  "process_kprobe": {
    "process": {
      "exec_id": "a2luZC1jb250cm9sLXBsYW5lOjQxMjc1NTI0MjYzMjIxOjEzMDg3Nw==",
      "pid": 130877,
      "uid": 0,
      "cwd": "/",
      "binary": "/sbin/modprobe",
      "arguments": "-q -- fs-xfs",
      "flags": "execve rootcwd clone",
      "start_time": "2023-09-09T23:27:42.751301124Z",
      "auid": 4294967295,
      "parent_exec_id": "a2luZC1jb250cm9sLXBsYW5lOjE6MA==",
      "refcnt": 1,
      "tid": 130877
    },
    "parent": {
      "exec_id": "a2luZC1jb250cm9sLXBsYW5lOjE6MA==",
      "pid": 0,
      "uid": 0,
      "binary": "<kernel>",
      "flags": "procFS",
      "start_time": "2023-09-09T11:59:47.227037763Z",
      "auid": 0,
      "parent_exec_id": "a2luZC1jb250cm9sLXBsYW5lOjE6MA==",
      "tid": 0
    },
    "function_name": "find_module_sections",
    "args": [
      {
        "module_arg": {
          "name": "xfs",
          "signature_ok": true
        }
      }
    ],
    "action": "KPROBE_ACTION_POST"
  },
  "node_name": "kind-control-plane",
  "time": "2023-09-09T23:27:42.760880332Z"
}

This ProcessKprobe event contains the module argument.

  • find_module_sections: the function call where the kernel parses the module sections.
  • module_arg: the kernel module information, it contains:
    • name: the name of the kernel module as a string.
    • signature_ok: a boolean value, if set to true then module signature was successfully verified by the kernel. If it is false or missing then the signature verification was not performed or probably failed. In all cases this means the integrity of the system has been compromised. Depends on kernels compiled with CONFIG_MODULE_SIG option.

Monitor Unloading of kernel modules

Using the same monitor-kernel-modules tracing policy allows to monitor unloading of kernel modules.

The following ProcessKprobe event will be generated:

Removing kernel modules event 

{
  "process_kprobe": {
    "process": {
      "exec_id": "OjMzNzQ4NzY1MDAyNDk5OjI0OTE3NQ==",
      "pid": 249175,
      "uid": 0,
      "cwd": "/home/tixxdz/tetragon",
      "binary": "/usr/sbin/rmmod",
      "arguments": "kernel_module_hello",
      "flags": "execve clone",
      "start_time": "2023-08-30T16:44:03.471068355Z",
      "auid": 1000,
      "parent_exec_id": "OjMzNzQ4NzY0MjQ4MTY5OjI0OTE3NA==",
      "refcnt": 1,
      "tid": 249175
    },
    "parent": {
      "exec_id": "OjMzNzQ4NzY0MjQ4MTY5OjI0OTE3NA==",
      "pid": 249174,
      "uid": 1000,
      "cwd": "/home/tixxdz/tetragon",
      "binary": "/usr/bin/sudo",
      "arguments": "rmmod kernel_module_hello",
      "flags": "execve",
      "start_time": "2023-08-30T16:44:03.470314558Z",
      "auid": 1000,
      "parent_exec_id": "OjMzNzQ2MjA5OTUxODI4OjI0OTE3Mw==",
      "refcnt": 1,
      "tid": 0
    },
    "function_name": "free_module",
    "args": [
      {
        "module_arg": {
          "name": "kernel_module_hello",
          "tainted": [
            "TAINT_OUT_OF_TREE_MODULE",
            "TAINT_UNSIGNED_MODULE"
          ]
        }
      }
    ],
    "action": "KPROBE_ACTION_POST"
  },
  "time": "2023-08-30T16:44:03.471984676Z"
}

To disable the monitor-kernel-modules run:

kubectl delete -f https://raw.githubusercontent.com/cilium/tetragon/main/examples/tracingpolicy/host-changes/monitor-kernel-modules.yaml