fix: reject symlinks in ConfigMap file entries

[GrosQuildu]

Description

The createConfigMapFromDirOrFiles uses NOFOLLOW_LINKS only to decide whether a path is a directory, then reads non-directory entries with Files.readAllBytes(file). A symlink to a regular file is accepted and the target bytes are embedded into the returned ConfigMap.

The kubernetes-client does follow symlinks in general. The explicit use of NOFOLLOW_LINKS in this single case raises a question of the expected kubernetes-client behavior in regards to symbolic links.

Exploit Scenario

A low privileged user creates a symlink staging/app.properties -> /etc/shadow.

Path staging = new File("./staging").toPath();
ConfigMap configMap =
        KubernetesResourceUtil.createConfigMapFromDirOrFiles("poc-config", staging);
String leaked = configMap.getData().get("app.properties");

The symlink is resolved by kubernetes-client. Unexpected file is leaked.

Threat Model

Should kubernetes-client not follow symlinks? Or should this behavior be configurable?

Note that the scope of the question is huge:

• PodUpload.upload and PodOperationsImpl.copy may resolve symlinks on remote pods
• Java generator follows input CRD symlink dirs and writes symlinked .java files with explicit FOLLOW_LINKS
• CRD generator output may resolve project-controlled symlinks in CI
• cert/key/truststore, OpenShift fromFile, and many other functionalities

Fix

If the client is supposed to follow symbolic links, then removing NOFOLLOW_LINKS from createConfigMapFromDirOrFiles is recommended for consistency, and an entry to threat model is recommended.

If client should not follow symlinks, then harden createConfigMapFromDirOrFiles and plan to harden other functionalities around symbolic links handling.

Note that the proposed fix still have race condition (time-of-check-time-of-use) issues.

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Paweł Płatek from Trail of Bits in collaboration with OpenAI.

[GrosQuildu]

Note that the proposed patch may be wrong, and correct idea is to just remove NOFOLLOW_LINKS. If so then please just close this PR and followup with a new one.