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Linux process credentials

Monitor Linux process credentials

On Linux each process has various associated user, group IDs, capabilities, secure management flags, keyring, LSM security that are used part of the security checks upon acting on other objects. These are called the task privileges or process credentials.

Changing the process credentials is a standard operation to perform privileged actions or to execute commands as another user. The obvious example is sudo that allows to gain high privileges and run commands as root or another user. An other example is services or containers that can gain high privileges during execution to perform restricted operations.

Composition of Linux process credentials

Traditional UNIX credentials

  • Real User ID
  • Real Group ID
  • Effective, Saved and FS User ID
  • Effective, Saved and FS Group ID
  • Supplementary groups

Linux Capabilities

  • Set of permitted capabilities: a limiting superset for the effective capabilities.
  • Set of inheritable capabilities: the set that may get passed across execve(2).
  • Set of effective capabilities: the set of capabilities a task is actually allowed to make use of itself.
  • Set of bounding capabilities: limits the capabilities that may be inherited across execve(2), especially when a binary is executed that will execute as UID 0.

Secure management flags (securebits).

These govern the way the UIDs/GIDs and capabilities are manipulated and inherited over certain operations such as execve(2).

Linux Security Module (LSM)

The LSM framework provides a mechanism for various security checks to be hooked by new kernel extensions. Tasks can have extra controls part of LSM on what operations they are allowed to perform.

Tetragon Process Credentials monitoring

Monitoring Linux process credentials is a good practice to idenfity programs running with high privileges. Tetragon allows retrieving Linux process credentials as a process_credentials object.

Changes to credentials can be monitored either in system calls or in internal kernel functions.

Generally it is better to monitor in internal kernel functions. For further details please read Advantages and disadvantages of kernel layer monitoring compared to the system call layer section.

1 - Monitor Process Credentials changes at the System Call layer

Monitor system calls that change Process Credentials

Tetragon can hook at the system calls that directly manipulate the credentials. This allows us to determine which process is trying to change its credentials and the new credentials that could be applied by the kernel.

This answers the questions:

Which process or container is trying to change its UIDs/GIDs in my cluster?

Which process or container is trying to change its capabilities in my cluster?

Before going forward, verify that all pods are up and running, ensure you deploy our Demo Application to explore the Security Observability Events:

kubectl create -f https://raw.githubusercontent.com/cilium/cilium/v1.15.0-pre.1/examples/minikube/http-sw-app.yaml

It might take several seconds for some pods until they satisfy all the dependencies:

kubectl get pods -A
NAMESPACE            NAME                                         READY   STATUS    RESTARTS   AGE
default              deathstar-54bb8475cc-6c6lc                   1/1     Running   0          2m54s
default              deathstar-54bb8475cc-zmfkr                   1/1     Running   0          2m54s
default              tiefighter                                   1/1     Running   0          2m54s
default              xwing                                        1/1     Running   0          2m54s
kube-system          tetragon-sdwv6                               2/2     Running   0          27m

Monitor UIDs/GIDs credential changes

We use the process.credentials.changes.at.syscalls Tracing Policy that hooks the setuid system calls family:

  • setuid
  • setgid
  • setfsuid
  • setfsgid
  • setreuid
  • setregid
  • setresuid
  • setresgid

Let’s apply the process.credentials.changes.at.syscalls Tracing Policy.

kubectl apply -f https://raw.githubusercontent.com/cilium/tetragon/main/examples/tracingpolicy/process-credentials/process.credentials.changes.at.syscalls.yaml

Then start monitoring events with the tetra CLI:

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

In another terminal, kubectl exec into the xwing Pod:

kubectl exec -it xwing -- /bin/bash

And execute su as this will call the related setuid system calls:

su root

The tetra CLI will generate the following ProcessKprobe events:

{
  "process_kprobe": {
    "process": {
      "exec_id": "a2luZC1jb250cm9sLXBsYW5lOjQwNzk4ODc2MDI2NTk4OjEyNTc5OA==",
      "pid": 125798,
      "uid": 0,
      "cwd": "/",
      "binary": "/bin/su",
      "arguments": "root",
      "flags": "execve rootcwd clone",
      "start_time": "2023-07-05T19:14:30.918693157Z",
      "auid": 4294967295,
      "pod": {
        "namespace": "default",
        "name": "xwing",
        "container": {
          "id": "containerd://55936e548de63f77ceb595d64966dd8e267b391ff0ef63b26c17eb8c2f6510be",
          "name": "spaceship",
          "image": {
            "id": "docker.io/tgraf/netperf@sha256:8e86f744bfea165fd4ce68caa05abc96500f40130b857773186401926af7e9e6",
            "name": "docker.io/tgraf/netperf:latest"
          },
          "start_time": "2023-07-05T18:45:16Z",
          "pid": 19
        },
        "pod_labels": {
          "app.kubernetes.io/name": "xwing",
          "class": "xwing",
          "org": "alliance"
        }
      },
      "docker": "55936e548de63f77ceb595d64966dd8",
      "parent_exec_id": "a2luZC1jb250cm9sLXBsYW5lOjQwNzk1NjYyMDM3MzMyOjEyNTc5Mg==",
      "refcnt": 1,
      "tid": 125798
    },
    "parent": {
      "exec_id": "a2luZC1jb250cm9sLXBsYW5lOjQwNzk1NjYyMDM3MzMyOjEyNTc5Mg==",
      "pid": 125792,
      "uid": 0,
      "cwd": "/",
      "binary": "/bin/bash",
      "flags": "execve rootcwd clone",
      "start_time": "2023-07-05T19:14:27.704703805Z",
      "auid": 4294967295,
      "pod": {
        "namespace": "default",
        "name": "xwing",
        "container": {
          "id": "containerd://55936e548de63f77ceb595d64966dd8e267b391ff0ef63b26c17eb8c2f6510be",
          "name": "spaceship",
          "image": {
            "id": "docker.io/tgraf/netperf@sha256:8e86f744bfea165fd4ce68caa05abc96500f40130b857773186401926af7e9e6",
            "name": "docker.io/tgraf/netperf:latest"
          },
          "start_time": "2023-07-05T18:45:16Z",
          "pid": 13
        },
        "pod_labels": {
          "app.kubernetes.io/name": "xwing",
          "class": "xwing",
          "org": "alliance"
        }
      },
      "docker": "55936e548de63f77ceb595d64966dd8",
      "parent_exec_id": "a2luZC1jb250cm9sLXBsYW5lOjQwNzk1NjE2MTU0NzA2OjEyNTc4Mw==",
      "refcnt": 2,
      "tid": 125792
    },
    "function_name": "__x64_sys_setgid",
    "args": [
      {
        "int_arg": 0
      }
    ],
    "action": "KPROBE_ACTION_POST"
  },
  "node_name": "kind-control-plane",
  "time": "2023-07-05T19:14:30.918977160Z"
}
{
  "process_kprobe": {
    "process": {
      "exec_id": "a2luZC1jb250cm9sLXBsYW5lOjQwNzk4ODc2MDI2NTk4OjEyNTc5OA==",
      "pid": 125798,
      "uid": 0,
      "cwd": "/",
      "binary": "/bin/su",
      "arguments": "root",
      "flags": "execve rootcwd clone",
      "start_time": "2023-07-05T19:14:30.918693157Z",
      "auid": 4294967295,
      "pod": {
        "namespace": "default",
        "name": "xwing",
        "container": {
          "id": "containerd://55936e548de63f77ceb595d64966dd8e267b391ff0ef63b26c17eb8c2f6510be",
          "name": "spaceship",
          "image": {
            "id": "docker.io/tgraf/netperf@sha256:8e86f744bfea165fd4ce68caa05abc96500f40130b857773186401926af7e9e6",
            "name": "docker.io/tgraf/netperf:latest"
          },
          "start_time": "2023-07-05T18:45:16Z",
          "pid": 19
        },
        "pod_labels": {
          "app.kubernetes.io/name": "xwing",
          "class": "xwing",
          "org": "alliance"
        }
      },
      "docker": "55936e548de63f77ceb595d64966dd8",
      "parent_exec_id": "a2luZC1jb250cm9sLXBsYW5lOjQwNzk1NjYyMDM3MzMyOjEyNTc5Mg==",
      "refcnt": 1,
      "tid": 125798
    },
    "parent": {
      "exec_id": "a2luZC1jb250cm9sLXBsYW5lOjQwNzk1NjYyMDM3MzMyOjEyNTc5Mg==",
      "pid": 125792,
      "uid": 0,
      "cwd": "/",
      "binary": "/bin/bash",
      "flags": "execve rootcwd clone",
      "start_time": "2023-07-05T19:14:27.704703805Z",
      "auid": 4294967295,
      "pod": {
        "namespace": "default",
        "name": "xwing",
        "container": {
          "id": "containerd://55936e548de63f77ceb595d64966dd8e267b391ff0ef63b26c17eb8c2f6510be",
          "name": "spaceship",
          "image": {
            "id": "docker.io/tgraf/netperf@sha256:8e86f744bfea165fd4ce68caa05abc96500f40130b857773186401926af7e9e6",
            "name": "docker.io/tgraf/netperf:latest"
          },
          "start_time": "2023-07-05T18:45:16Z",
          "pid": 13
        },
        "pod_labels": {
          "app.kubernetes.io/name": "xwing",
          "class": "xwing",
          "org": "alliance"
        }
      },
      "docker": "55936e548de63f77ceb595d64966dd8",
      "parent_exec_id": "a2luZC1jb250cm9sLXBsYW5lOjQwNzk1NjE2MTU0NzA2OjEyNTc4Mw==",
      "refcnt": 2,
      "tid": 125792
    },
    "function_name": "__x64_sys_setuid",
    "args": [
      {
        "int_arg": 0
      }
    ],
    "action": "KPROBE_ACTION_POST"
  },
  "node_name": "kind-control-plane",
  "time": "2023-07-05T19:14:30.918990583Z"
}

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

  • function_name: the system call, __x64_sys_setuid or __x64_sys_setgid
  • int_arg: the uid or gid to use, in our case it’s 0 which corresponds to the root user.

To disable the process.credentials.changes.at.syscalls Tracing Policy run:

kubectl delete -f https://raw.githubusercontent.com/cilium/tetragon/main/examples/tracingpolicy/process-credentials/process.credentials.changes.at.syscalls.yaml

2 - Monitor Process Credentials changes at the Kernel layer

Monitor Process Credentials changes at the kernel layer

Monitoring Process Credentials changes at the kernel layer is also possible. This allows to capture the new process_credentials that should be applied.

This process-creds-installed tracing policy can be used to answer the following questions:

Which process or container is trying to change its own UIDs/GIDs in the cluster?

Which process or container is trying to change its own capabilities in the cluster?

In which user namespace the credentials are being changed?

How to monitor process_credentials changes?

Advantages and disadvantages of kernel layer monitoring compared to the system call layer

The main advantages of monitoring at the kernel layer compared to the system call layer:

  • Not vulnerable to user space arguments tampering.

  • Ability to display the full new credentials to be applied.

  • It is more reliable since it has full context on where and how the new credentials should be applied including the user namespace.

  • A catch all layer for all system calls, and every normal kernel path that manipulate credentials.

  • One potential disadvantage is that this approach may generate a lot of events, so appropriate filtering must be applied to reduce the noise.

Kubernetes Environments

First, verify that your k8s environment is all setup and that all pods are up and running, and deploy the Demo Application:

kubectl create -f https://raw.githubusercontent.com/cilium/cilium/v1.15.0-pre.1/examples/minikube/http-sw-app.yaml

It might take several seconds until all pods are Running:

kubectl get pods -A
NAMESPACE            NAME                                         READY   STATUS    RESTARTS   AGE
default              deathstar-54bb8475cc-6c6lc                   1/1     Running   0          2m54s
default              deathstar-54bb8475cc-zmfkr                   1/1     Running   0          2m54s
default              tiefighter                                   1/1     Running   0          2m54s
default              xwing                                        1/1     Running   0          2m54s
kube-system          tetragon-sdwv6                               2/2     Running   0          27m

Monitor Process Credentials installation

We use the process-creds-installed Tracing Policy that hooks the kernel layer when credentials are being installed.

So let’s apply the process-creds-installed Tracing Policy.

kubectl apply -f https://raw.githubusercontent.com/cilium/tetragon/main/examples/tracingpolicy/process-credentials/process-creds-installed.yaml

Then we start monitoring for events with tetra cli:

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

In another terminal, inside a pod and as a non root user we will execute a setuid binary (suid):

/tmp/su -

The tetra cli will generate the following ProcessKprobe events:

{
 "process_kprobe": {
    "process": {
      "exec_id": "a2luZC1jb250cm9sLXBsYW5lOjE0MDczMDQyODk3MTc6MjIzODY=",
      "pid": 22386,
      "uid": 11,
      "cwd": "/",
      "binary": "/tmp/su",
      "arguments": "-",
      "flags": "execve rootcwd clone",
      "start_time": "2023-07-25T12:04:59.359333454Z",
      "auid": 4294967295,
      "pod": {
        "namespace": "default",
        "name": "xwing",
        "container": {
          "id": "containerd://2e58c8357465961fd96f758e87d0269dfb5f97c536847485de9d7ec62be34a64",
          "name": "spaceship",
          "image": {
            "id": "docker.io/tgraf/netperf@sha256:8e86f744bfea165fd4ce68caa05abc96500f40130b857773186401926af7e9e6",
            "name": "docker.io/tgraf/netperf:latest"
          },
          "start_time": "2023-07-25T11:44:48Z",
          "pid": 43
        },
        "pod_labels": {
          "app.kubernetes.io/name": "xwing",
          "class": "xwing",
          "org": "alliance"
        }
      },
      "docker": "2e58c8357465961fd96f758e87d0269",
      "parent_exec_id": "a2luZC1jb250cm9sLXBsYW5lOjEzOTU0MDY5OTA1ODc6MjIzNTI=",
      "refcnt": 1,
      "tid": 22386
    },
    "parent": {
      "exec_id": "a2luZC1jb250cm9sLXBsYW5lOjEzOTU0MDY5OTA1ODc6MjIzNTI=",
      "pid": 22352,
      "uid": 11,
      "cwd": "/",
      "binary": "/bin/sh",
      "flags": "execve rootcwd",
      "start_time": "2023-07-25T12:04:47.462035587Z",
      "auid": 4294967295,
      "pod": {
        "namespace": "default",
                "name": "xwing",
        "container": {
          "id": "containerd://2e58c8357465961fd96f758e87d0269dfb5f97c536847485de9d7ec62be34a64",
          "name": "spaceship",
          "image": {
            "id": "docker.io/tgraf/netperf@sha256:8e86f744bfea165fd4ce68caa05abc96500f40130b857773186401926af7e9e6",
            "name": "docker.io/tgraf/netperf:latest"
          },
          "start_time": "2023-07-25T11:44:48Z",
          "pid": 41
        },
        "pod_labels": {
          "app.kubernetes.io/name": "xwing",
          "class": "xwing",
          "org": "alliance"
        }
      },
      "docker": "2e58c8357465961fd96f758e87d0269",
      "parent_exec_id": "a2luZC1jb250cm9sLXBsYW5lOjEzOTU0MDQ3NzY5NzI6MjIzNTI=",
      "refcnt": 2,
      "tid": 22352
    },
    "function_name": "commit_creds",
    "args": [
      {
        "process_credentials_arg": {
          "uid": 0,
          "gid": 0,
          "euid": 0,
          "egid": 0,
          "suid": 0,
          "sgid": 0,
          "fsuid": 0,
          "fsgid": 0,
          "caps": {
            "permitted": [
              "CAP_CHOWN",
              "DAC_OVERRIDE",
              "CAP_FOWNER",
              "CAP_FSETID",
              "CAP_KILL",
              "CAP_SETGID",
              "CAP_SETUID",
              "CAP_SETPCAP",
              "CAP_NET_BIND_SERVICE",
              "CAP_NET_RAW",
              "CAP_SYS_CHROOT",
              "CAP_MKNOD",
              "CAP_AUDIT_WRITE",
              "CAP_SETFCAP"
            ],
            "effective": [
              "CAP_CHOWN",
              "DAC_OVERRIDE",
              "CAP_FOWNER",
              "CAP_FSETID",
              "CAP_KILL",
              "CAP_SETGID",
              "CAP_SETUID",
              "CAP_SETPCAP",
              "CAP_NET_BIND_SERVICE",
              "CAP_NET_RAW",
              "CAP_SYS_CHROOT",
              "CAP_MKNOD",
              "CAP_AUDIT_WRITE",
              "CAP_SETFCAP"
            ]
          },
          "user_ns": {
            "level": 0,
            "uid": 0,
            "gid": 0,
            "ns": {
              "inum": 4026531837,
              "is_host": true
            }
          }
        }
      }
    ],
    "action": "KPROBE_ACTION_POST"
  },
  "node_name": "kind-control-plane",
  "time": "2023-07-25T12:05:01.410834172Z"
}

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

  • function_name: the kernel commit_creds() function to install new credentials.
  • process_credentials_arg: the new process_credentials to be installed on the current process. It includes the UIDs/GIDs, the capabilities and the target user namespace.

Here we can clearly see that the suid binary is being executed by a user ID 11 in order to elevate its privileges to user ID 0 including capabilities.

To disable the process-creds-installed Tracing Policy run:

kubectl delete -f https://raw.githubusercontent.com/cilium/tetragon/main/examples/tracingpolicy/process-credentials/process-creds-installed.yaml