I understand that it's considered a bad security practice to run Docker images as root, but I have a specific situation that I wanted to pass by the community to see if anyone can help.
We are currently using a pipeline on an Amazon Linux 2 instance with a single user called ec2-user. Unfortunately, a lot of the scripts we're using for our pipeline have hard-coded paths baked in (notably /home/ec2-user/) ... which may or may not reference the $HOME variable.
I've been talking to one of the engineers that is building a Docker image for our pipeline and suggested that he creates a new user entirely so root user isn't running our pipeline.
For example:
# add clip user
RUN groupadd-r clip && useradd -r -g clip clip
# disable root
RUN chsh -s /usr/sbin/nologin root
# set environment variables
ENV HOME /home/clip
ENV DEBIAN FRONTEND-noninteractive
However, the engineer mentioned that the clip user inside the container will have some uid that may or may not exist in the host machine. For example, if the clip user had uid 1001 in the container, but 1001 was john in the host, all the files created as the clip user inside the container would be owned by john on the outside.
Further, he is more concerned about the situation where the clip user has a uid in the container that doesn’t exist in the host’s passwd. In that case files created by the clip user in the container would be owned by a bare unassociated uid on the host.
If we decided to pass in ids from the host as the user/group to run the image. The kernel will be ok with it (same kernel as the host), and when all is said and done files created inside the container will then be owned by the user/group you pass in. However, the container wouldn’t know who that user/group are, so it’ll just use the raw ids, and stuff like $HOME or whoami won’t work.
With that said, we're curious if anyone else has experienced these problems and if anyone has found solutions?
Everything you say is totally normal. The container has its own /etc/passwd file, and so a given numeric user ID might map to different user names (or to not at all) in the host and in the container. Beyond some cosmetic issues around debug shells, it shouldn't usually matter if the current numeric uid is actually present in the container /etc/passwd, and there's no reason a container uid would need to be mapped in the host /etc/passwd.
Note that there are a couple of ways to directly assume another user ID in Docker, either using the docker run -u option or the Dockerfile USER directive. The RUN chsh command you propose doesn't really do anything and doesn't prevent becoming root inside a container.
clip user inside the container will have some uid that may or may not exist in the host machine.
True, totally normal.
For example, if the clip user had uid 1001 in the container, but 1001 was john in the host, all the files created as the clip user inside the container would be owned by john on the outside.
This is partially true, but only in the case where you've explicitly mapped a host directory into the container with a docker run -v option. Otherwise, the host user with uid 1001 won't be able to navigate to the /var/lib/docker/... directory that actually contains the container files, so it doesn't matter that they could hypothetically write them.
The more usual case around this is to explicitly supply a host uid so that the container process can save its state in a mapped host directory. Pass a numeric uid to the docker run -u option; there's no particular need for that uid to exist in the container's /etc/passwd.
docker run \
-u $(id -u) \
-v "$PWD/data:/data" \
...
the container wouldn’t know who that user/group are, so it’ll just use the raw ids, and stuff like $HOME or whoami won’t work.
Unless your application explicitly calls these things, they won't usually matter. "Home directory" is a pretty poorly defined concept in a Docker container since it's usually a wrapper around a single process.
Related
Problem
For a docker image (alpine based) that is supposed to run as non-root I have two requirements:
I have to mount a FUSE filesystem inside the docker container
The users of the docker image are able to set the UID/GID of the docker
user with docker run --user {uid}:{gid}
FUSE's fusermount command requires a valid entry for the user in /etc/passwd, otherwise it won't mount the filesystem. Given that I don't know the the UID/GID of the user at build time I can't call adduser at build time. And I can't do it at runtime either, as the user then doesn't have the appropriate privileges.
Solutions found
So far I have found two solutions that both feel not appropriate/secure
1. Make /etc/passwd writable
When adding chmod 555 /etc/passwd to the Dockerfile I can then do at runtime
echo "someuser:x:${my_uid}:$(id -g)::/tmp:/sbin/nologin" >> /etc/passwd
This does the job for fusermount. Unfortunately I did not find a way to make change the passwd file back to read-only at runtime and without that I have security concerns that someone might be able to misuse this to gain root rights back. While I could not find a simple way to use the open passwd file for some exploit (while I was able to add/modify password & configurations directly in /etc/passwd for all users and then change users via login, alpine did not allow this for user root (neither via login nor via su). But I guess there are folk out there more clever than me, and somehow the whole solution feels like a quite dirty hack. Does anyone have specific ideas how a writeable passwd file inside a container could be used for getting inappropriate rights inside the container?
2. Replace requirement #2 with two additional environment variables
By introducing DUID and DGID as environment variables and set USER to some newly added non-root user inside the Dockerfile I found a solution with the help of sudo & /etc/sudoers: In a launch script that I use as entrypoint I can call sudo adduser/addgroup for the given DUID/DGID and then launch the actual program with the user specified via sudo -u someuser someprog.
Except for the fact that the whole setup became quite ugly, I disliked the fact the user's of my docker image could no longer use the regular docker run --user option, as this would break the sudo configuration.
Actually, I am trying to run the below following command
docker run -it --rm -v $(pwd):/var/www/html --user node node:12.13.1-alpine ash.
Expected result
The files inside the container (i.e /var/www/html ) should have user as node.
Actual result
But, the files inside the containers are showing the same user as of the host.
Also, can't create a directory inside the container.
It is working for my other colleagues. So, any help in this would be much appreciated.
Many thanks,
Alwin
Note:
Docker version 19.03.7, build 7141c199a2
Have added necessary permission to docker command so that it doesn't
need sudo for running it
Running docker run with --user does not change the permission of the original existing files. From Docker reference:
The developer can set a default user to run the first process with the Dockerfile USER instruction. When starting a container, the operator can override the USER instruction by passing the -u option.
It only overrides the user running Node.js inside the container. During mount, the original permission and owner of /var/www/html is unchanged. Verify this by ls -n and see if the UID of the owner of the folder is the same when mounted inside Docker. Make sure the UID is the same as node user you specified.
I don't know how it works in your colleagues computers though. That's why it's important to use UID/GID instead just using username. The same username in the container can have different UID with the same username in the host.
EDIT: I checked that node image that you use contains node user with UID 1000. The first user created in Linux usually also has UID 1000. So if the /var/www/html is owned by UID 1000, it will run. But UID 1000 could possibly belong to different usernames in Docker and in the host. Because you specified --user node, which is translated into UID 1000 inside the container as username node itself exists, it won't work if /var/www/html is owned by different UID in your host, which probably is your case.
You have to add USER into Dockerfile before building
# App is running normal user mode
USER node
so now when you run docker image it will run with normal node user mode
Even after going through lot of materials and SO answers still I'm not clear on docker uid/user usage or implementation.
I understand the below points:
An instance of an image is called a container.
uid/gid is maintained by the underlying kernel, not by Container.
Kernel understand uid/gid number not username/groupname and name is an alias and just for human readable.
All containers are processes maintained by docker daemon and will be visible as process in host machine (ps -ef)
root (id = 0) is the default user within a container and this can be changed either by USER instruction in Dockerfile or by passing -u flag in docker run
With the above all said, when I have the below command in my Dockerfile, I presume that a new user (my-user) will be created with incremented uid.
RUN addgroup my-group && adduser -D my-user -G my-group
What happens if I run the same image multiple times i.e multiple containers? Will the same uid be assigned to all processes?
What happens if I add same above command in another image and run that image as container? - will I get new uid or same uid as the previous one?
How the uid increment happens in Container in relation with the host machine.
Any pointers would be helpful.
Absent user namespace remapping, there are only two things that matter:
What the numeric user ID is; and
What's in the /etc/passwd file.
Remember that each container and the host have separate filesystems, so each of these things could have separate /etc/passwd files.
What happens if I run the same image multiple times i.e multiple containers? Will the same uid be assigned to all processes?
Yes, because each container gets a copy of the same /etc/passwd file from the image.
What happens if I add same above command in another image and run that image as container? - will I get new uid or same uid as the previous one?
It depends on what adduser actually does; it could be the same or different.
How the uid increment happens in Container in relation with the host machine.
They're completely and totally independent.
Also remember that you can docker push/docker pull a built image to run it on a different host. That will bring the image's /etc/passwd file along with it, but the host environment could be totally different. Correspondingly, it's not a best practice to try to match some specific host's uid mapping in a Dockerfile, because it will be wrong if you try to run the same image anywhere else.
When you try to add users in the RUN statement, it does not create a user on the host. If you do not specify an user with the USER statement in your Dockerfile or the -u flag while starting container (Assuming the parent Dockerfiles also do not include the USER statement), the container process on host will simple run as root user if you have started the docker daemon as root.
So if you create a user using RUN addgroup my-group && adduser -D my-user -G my-group it will simply create an user in the container i.e. the user is local to the container. So each instance (container) of that image you run will have the same uid of the user inside the container. Note: That user will not exist on the host.
If you want to run the container process on host as another user (which exists on host) then you have 3 options:
Add a USER statement in the Dockerfile
Use the -u flag while running the container
You can use docker's user namespace feature
I highly recommend understanding the user namespace and mappings by reading this documentation: Isolate containers with a user namespace
Due to reasons, we have two networks. On network A, the USER who should execute the process in the container might be usera. On network B, the User might be userb. The uid/gid of the users must match ldap definitions, and these are well defined. There are persistent files written by the process to bind mounted directories on the SAN (and clearly a different SAN on each network), so the process owner is important.
If there were only a single USER, I would do the following:
FROM <base image>
RUN groupadd -g 999 usera && useradd -u 999 -g 999 usera
USER usera
CMD ["process", "'params"]
Then the running process would be owned by usera, and all would be well.
However, it would be nice if it were possible to build a single container, but at the point of container startup have the user be set via some parameter.
I suspect it might be possible by having an ENTRYPOINT added to the Dockerfile, and then perhaps sending values via the docker run -e USER=[usera|userb], but I am just coming up to speed with Docker, so I'm not sure exactly how that would work.
I've looked at processes in containers should not run as root, which gave some suggestions. Also, we absolutely cannot have the container run as root. I also looked at Docker Replicate UID/GID in container, which provided a hint on possibly sending values via -e, but the admonishment about the id mismatch on the build system and running system doesn't apply.
How may I achieve this different user owning a process, possibly by passing in a value (though, if I can have a sophisticated enough script, I can detect what network the container is running on, and I could potentially set some variable automatically)?
Edit: due to auditing and review requirements, it would be cleaner if it were possible to ensure the user setting (or fail to start if one were not provided), rather than using, e.g., the --user parameter to the docker run. Nonetheless, if the only/best approach is the --user, then so be it.
You have two options:
Run a script as root (this will be the entrypoint), pass the UID/GID as environment variables, use usermod|groupmod to change the user/group id then exec the real process using the new user. Check the gogs/gogs image for an example of a container where the UID/GID can be customized.
Use the --user switch on the docker run command so the process starts with the correct UID/GID. You don't need to create a user on the Dockerfile with this option as the UID will be overriden with the one from the command line.
The problem with the second approach is that you must prepare the filesystem permissions beforehand, as you cannot chown/chmod once the process is started.
I currently run the official Tensorflow Docker Container (GPU) with Nvidia-Docker:
https://hub.docker.com/r/tensorflow/tensorflow/
https://gcr.io/tensorflow/tensorflow/
However, I can't find a way to set a default user for the container. The default user for this container is "root", which is dangerous in term of security and problematic because it gives root access to the shared folders.
Let's say my host machine run with the user "CNNareCute", is there any way to launch my containers with the same user ?
Docker containers by default run as root. You can override the user by passing --user <user> to docker run command. Note however this might be problematic in case the container process needs root access inside the container.
The security concern you mention is handled in docker using User Namespaces. Usernamespaces basically map users in the container to a different pool of users on the host. Thus you can map the root user inside the container to a normal user on the host and the security concern should be mitigated.
AFAIK, docker images run by default as root. This means that any Dockerfile using the image as a base, doesn't have to jump through hoops to modify it. You could carry out user modification in a Dockerfile - same way you would on any other linux box which would give you the configuration you need.
You won't be able to use users (dynamically) from your host in the containers without creating them in the container first - and they will be in effect separate users of the same name.
You can run commands and ssh into containers as a specific user provided it exists on the container. For example, a PHP application needing commands run that retain www-data privileges, would be run as follows:
docker exec --user www-data application_container_1 sh -c "php something"
So in short, you can set up whatever users you like and use them to run scripts but the default will be root and it will exist unless you remove it which may also have repercussions...