GitLab CI denies access to push using a deploy key with write access - ssh

Question

I added a deploy key with write access to my GitLab repository. My .gitlab-ci.yml file contains:
- git clone git#gitlab.domain:user/repo.git
- git checkout master
- git add myfile.pdf
- git commit -m "Generated PDF file"
- git push origin master
The deploy key works when cloning the repository.
Pushing is not possible, even if the deploy key has write access.
remote: You are not allowed to upload code.
fatal: unable to access 'https://gitlab-ci-token:xxxxxxxxxxxxxxxxxxxx#domain/user/repo.git/': The requested URL returned error: 403

Answer 1

I just encountered the same problem and saw this question without answer, so there is my solution.
Problem
The problem is caused by the fact that the remote url used by git to push the code is in the form http(s)://gitlab-ci-token:xxxxxxxxxxxxxxxxxxxx#git.mydomain.com/group/project.git.
This url is using http(s) protocol so git doesn't use the ssh deploy key that you setup.
Solution
The solution is to change the push url of the remote origin so it matches ssh://git#git.mydomain.com/group/project.git.
The easiest way to do so is to use the predefined variable CI_REPOSITORY_URL.
Here is an example of code doing so by using sed:
# Change url from http(s) to ssh
url_host=$(echo "${CI_REPOSITORY_URL}" | sed -e 's|https\?://gitlab-ci-token:.*#|ssh://git#|g')
echo "${url_host}"
# ssh://git#git.mydomain.com/group/project.git
# Set the origin push url to the new one
git remote set-url --push origin "${url_host}"
Also, those using docker executor may want to verify the SSH host key as suggested by the gitlab documentation on deploy keys for docker executor.
So I give a more complete example for docker executor.
The code is mainly from gitlab documentation on ssh deploy keys.
In this example, the private deploy key is stored inside a variable named SSH_PRIVATE_KEY.
create:push:pdf:
before_script:
- 'which ssh-agent || ( apt-get update -y && apt-get install openssh-client -y )'
- eval $(ssh-agent -s)
- echo "${SSH_PRIVATE_KEY}" | tr -d '\r' | ssh-add - > /dev/null
- mkdir -p ~/.ssh
- chmod 700 ~/.ssh
- git config --global user.email "email#example.com"
- git config --global user.name "User name"
- gitlab_hostname=$(echo "${CI_REPOSITORY_URL}" | sed -e 's|https\?://gitlab-ci-token:.*#||g' | sed -e 's|/.*||g')
- ssh-keyscan "${gitlab_hostname}" >> ~/.ssh/known_hosts
- chmod 644 ~/.ssh/known_hosts
script:
- git checkout master
- git add myfile.pdf
- git commit -m "Generated PDF file"
- url_host=$(echo "${CI_REPOSITORY_URL}" | sed -e 's|https\?://gitlab-ci-token:.*#|ssh://git#|g')
- git remote set-url --push origin "${url_host}"
- git push origin master