Blog: Forensic container analysis

Authors: Adrian Reber (Red Hat)

In my previous article, Forensic container checkpointing in Kubernetes, I introduced checkpointing in Kubernetes and how it has to be setup and how it can be used. The name of the feature is Forensic container checkpointing, but I did not go into any details how to do the actual analysis of the checkpoint created by Kubernetes. In this article I want to provide details how the checkpoint can be analyzed.

Checkpointing is still an alpha feature in Kubernetes and this article wants to provide a preview how the feature might work in the future.

Preparation

Details about how to configure Kubernetes and the underlying CRI implementation to enable checkpointing support can be found in my Forensic container checkpointing in Kubernetes article.

As an example I prepared a container image (quay.io/adrianreber/counter:blog) which I want to checkpoint and then analyze in this article. This container allows me to create files in the container and also store information in memory which I later want to find in the checkpoint.

To run that container I need a pod, and for this example I am using the following Pod manifest:

apiVersion: v1
kind: Pod
metadata:
 name: counters
spec:
 containers:
 - name: counter
 image: quay.io/adrianreber/counter:blog

This results in a container called counter running in a pod called counters.

Once the container is running I am performing following actions with that container:

$ kubectl get pod counters --template '{{.status.podIP}}'
10.88.0.25
$ curl 10.88.0.25:8088/create?test-file
$ curl 10.88.0.25:8088/secret?RANDOM_1432_KEY
$ curl 10.88.0.25:8088

The first access creates a file called test-file with the content test-filein the container and the second access stores my secret information (RANDOM_1432_KEY) somewhere in the container's memory. The last access just adds an additional line to the internal log file.

The last step before I can analyze the checkpoint it to tell Kubernetes to create the checkpoint. As described in the previous article this requires access to the_kubelet_ only checkpoint API endpoint.

For a container named counter in a pod named counters in a namespace named_default_ the kubelet API endpoint is reachable at:

# run this on the node where that Pod is executing
curl -X POST "https://localhost:10250/checkpoint/default/counters/counter"

For completeness the following curl command-line options are necessary to have curl accept the kubelet's self signed certificate and authorize the use of the kubelet checkpoint API:

--insecure --cert /var/run/kubernetes/client-admin.crt --key /var/run/kubernetes/client-admin.key

Once the checkpointing has finished the checkpoint should be available at/var/lib/kubelet/checkpoints/checkpoint-<pod-name>_<namespace-name>-<container-name>-<timestamp>.tar

In the following steps of this article I will use the name checkpoint.tarwhen analyzing the checkpoint archive.

Checkpoint archive analysis using checkpointctl

To get some initial information about the checkpointed container I am using the tool checkpointctl like this:

$ checkpointctl show checkpoint.tar --print-stats
+-----------+----------------------------------+--------------+---------+---------------------+--------+------------+------------+-------------------+
| CONTAINER | IMAGE | ID | RUNTIME | CREATED | ENGINE | IP | CHKPT SIZE | ROOT FS DIFF SIZE |
+-----------+----------------------------------+--------------+---------+---------------------+--------+------------+------------+-------------------+
| counter | quay.io/adrianreber/counter:blog | 059a219a22e5 | runc | 2023-03-02T06:06:49 | CRI-O | 10.88.0.23 | 8.6 MiB | 3.0 KiB |
+-----------+----------------------------------+--------------+---------+---------------------+--------+------------+------------+-------------------+
CRIU dump statistics
+---------------+-------------+--------------+---------------+---------------+---------------+
| FREEZING TIME | FROZEN TIME | MEMDUMP TIME | MEMWRITE TIME | PAGES SCANNED | PAGES WRITTEN |
+---------------+-------------+--------------+---------------+---------------+---------------+
| 100809 us | 119627 us | 11602 us | 7379 us | 7800 | 2198 |
+---------------+-------------+--------------+---------------+---------------+---------------+

This gives me already some information about the checkpoint in that checkpoint archive. I can see the name of the container, information about the container runtime and container engine. It also lists the size of the checkpoint (CHKPT SIZE). This is mainly the size of the memory pages included in the checkpoint, but there is also information about the size of all changed files in the container (ROOT FS DIFF SIZE).

The additional parameter --print-stats decodes information in the checkpoint archive and displays them in the second table (CRIU dump statistics). This information is collected during checkpoint creation and gives an overview how much time CRIU needed to checkpoint the processes in the container and how many memory pages were analyzed and written during checkpoint creation.

Digging deeper

With the help of checkpointctl I am able to get some high level information about the checkpoint archive. To be able to analyze the checkpoint archive further I have to extract it. The checkpoint archive is a tar archive and can be extracted with the help of tar xf checkpoint.tar.

Extracting the checkpoint archive will result in following files and directories:

• bind.mounts - this file contains information about bind mounts and is needed during restore to mount all external files and directories at the right location
• checkpoint/ - this directory contains the actual checkpoint as created by CRIU
• config.dump and spec.dump - these files contain metadata about the container which is needed during restore
• dump.log - this file contains the debug output of CRIU created during checkpointing
• stats-dump - this file contains the data which is used by checkpointctlto display dump statistics (--print-stats)
• rootfs-diff.tar - this file contains all changed files on the container's file-system

File-system changes - rootfs-diff.tar

The first step to analyze the container's checkpoint further is to look at the files that have changed in my container. This can be done by looking at the file rootfs-diff.tar:

$ tar xvf rootfs-diff.tar
home/counter/logfile
home/counter/test-file

Now the files that changed in the container can be studied:

$ cat home/counter/logfile
10.88.0.1 - - [02/Mar/2023 06:07:29] "GET /create?test-file HTTP/1.1" 200 -
10.88.0.1 - - [02/Mar/2023 06:07:40] "GET /secret?RANDOM_1432_KEY HTTP/1.1" 200 -
10.88.0.1 - - [02/Mar/2023 06:07:43] "GET / HTTP/1.1" 200 -
$ cat home/counter/test-file
test-file 

Compared to the container image (quay.io/adrianreber/counter:blog) this container is based on, I can see that the file logfile contains information about all access to the service the container provides and the file test-filewas created just as expected.

With the help of rootfs-diff.tar it is possible to inspect all files that were created or changed compared to the base image of the container.

Analyzing the checkpointed processes - checkpoint/

The directory checkpoint/ contains data created by CRIU while checkpointing the processes in the container. The content in the directory checkpoint/consists of different image files which can be analyzed with the help of the tool CRIT which is distributed as part of CRIU.

First lets get an overview of the processes inside of the container:

$ crit show checkpoint/pstree.img | jq .entries[].pid
1
7
8

This output means that I have three processes inside of the container's PID namespace with the PIDs: 1, 7, 8

This is only the view from the inside of the container's PID namespace. During restore exactly these PIDs will be recreated. From the outside of the container's PID namespace the PIDs will change after restore.

The next step is to get some additional information about these three processes:

$ crit show checkpoint/core-1.img | jq .entries[0].tc.comm
"bash"
$ crit show checkpoint/core-7.img | jq .entries[0].tc.comm
"counter.py"
$ crit show checkpoint/core-8.img | jq .entries[0].tc.comm
"tee"

This means the three processes in my container are bash, counter.py (a Python interpreter) and tee. For details about the parent child relations of these processes there is more data to be analyzed in checkpoint/pstree.img.

Let's compare the so far collected information to the still running container:

$ crictl inspect --output go-template --template "{{(index .info.pid)}}" 059a219a22e56
722520
$ ps auxf | grep -A 2 722520
fedora 722520 \_ bash -c /home/counter/counter.py 2>&1 | tee /home/counter/logfile
fedora 722541 \_ /usr/bin/python3 /home/counter/counter.py
fedora 722542 \_ /usr/bin/coreutils --coreutils-prog-shebang=tee /usr/bin/tee /home/counter/logfile
$ cat /proc/722520/comm
bash
$ cat /proc/722541/comm
counter.py
$ cat /proc/722542/comm
tee

In this output I am first retrieving the PID of the first process in the container and then I am looking for that PID and child processes on the system where the container is running. I am seeing three processes and the first one is "bash" which is PID 1 inside of the containers PID namespace. Then I am looking at /proc/<PID>/comm and I can find the exact same value as in the checkpoint image.

Important to remember is that the checkpoint will contain the view from within the container's PID namespace because that information is important to restore the processes.

One last example of what crit can tell us about the container is the information about the UTS namespace:

$ crit show checkpoint/utsns-12.img
{
 "magic": "UTSNS",
 "entries": [
 {
 "nodename": "counters",
 "domainname": "(none)"
 }
 ]
}

This tells me that the hostname inside of the UTS namespace is counters.

For every resource CRIU collected during checkpointing the checkpoint/directory contains corresponding image files which can be analyzed with the help of crit.

Looking at the memory pages

In addition to the information from CRIU that can be decoded with the help of CRIT, there are also files containing the raw memory pages written by CRIU to disk:

$ ls checkpoint/pages-*
checkpoint/pages-1.img checkpoint/pages-2.img checkpoint/pages-3.img

When I initially used the container I stored a random key (RANDOM_1432_KEY) somewhere in the memory. Let see if I can find it:

$ grep -ao RANDOM_1432_KEY checkpoint/pages-*
checkpoint/pages-2.img:RANDOM_1432_KEY

And indeed, there is my data. This way I can easily look at the content of all memory pages of the processes in the container, but it is also important to remember that anyone that can access the checkpoint archive has access to all information that was stored in the memory of the container's processes.

Using gdb for further analysis

Another possibility to look at the checkpoint images is gdb. The CRIU repository contains the script coredump which can convert a checkpoint into a coredump file:

$ /home/criu/coredump/coredump-python3
$ ls -al core*
core.1 core.7 core.8

Running the coredump-python3 script will convert the checkpoint images into one coredump file for each process in the container. Using gdb I can also look at the details of the processes:

$ echo info registers | gdb --core checkpoint/core.1 -q

[New LWP 1]

Core was generated by `bash -c /home/counter/counter.py 2>&1 | tee /home/counter/logfile'.

#0 0x00007fefba110198 in ?? ()
(gdb)
rax 0x3d 61
rbx 0x8 8
rcx 0x7fefba11019a 140667595587994
rdx 0x0 0
rsi 0x7fffed9c1110 140737179816208
rdi 0xffffffff 4294967295
rbp 0x1 0x1
rsp 0x7fffed9c10e8 0x7fffed9c10e8
r8 0x1 1
r9 0x0 0
r10 0x0 0
r11 0x246 582
r12 0x0 0
r13 0x7fffed9c1170 140737179816304
r14 0x0 0
r15 0x0 0
rip 0x7fefba110198 0x7fefba110198
eflags 0x246 [PF ZF IF]
cs 0x33 51
ss 0x2b 43
ds 0x0 0
es 0x0 0
fs 0x0 0
gs 0x0 0

In this example I can see the value of all registers as they were during checkpointing and I can also see the complete command-line of my container's PID 1 process: bash -c /home/counter/counter.py 2>&1 | tee /home/counter/logfile

Summary

With the help of container checkpointing, it is possible to create a checkpoint of a running container without stopping the container and without the container knowing that it was checkpointed. The result of checkpointing a container in Kubernetes is a checkpoint archive; using different tools likecheckpointctl, tar, crit and gdb the checkpoint can be analyzed. Even with simple tools like grep it is possible to find information in the checkpoint archive.

The different examples I have shown in this article how to analyze a checkpoint are just the starting point. Depending on your requirements it is possible to look at certain things in much more detail, but this article should give you an introduction how to start the analysis of your checkpoint.

How do I get involved?

You can reach SIG Node by several means:

• Slack: #sig-node
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• Mailing list