fix: restrict cmd template function for remote dependency configs

[markoyoungson]

Summary

The cmd template function allows arbitrary command execution during config field expansion. Currently, this function is available to all template contexts, including remote/untrusted config dependencies fetched via git, GCS, or Google Cloud Build repos.

This change restricts the cmd function from being available when processing remote dependency configs to prevent unintended command execution from external config sources.

Changes

• Added CmdAllowed flag in pkg/skaffold/util/env_template.go that controls whether the cmd function is permitted
• runCmdFunc now checks CmdAllowed before executing and returns an error if not allowed
• pkg/skaffold/parser/config.go sets CmdAllowed = false before processing remote dependency configs via processEachDependency
• Added tests for the blocked/allowed behavior

Test plan

• Existing tests pass — cmd still works for local configs
• New test verifies cmd is blocked when CmdAllowed = false
• New test verifies cmd works when CmdAllowed = true
• Manual: local skaffold.yaml with {{cmd "echo" "hello"}} still works
• Manual: remote git dependency with {{cmd ...}} returns an error

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the security posture of the system by preventing arbitrary command execution from untrusted remote configuration sources. It introduces a mechanism to disable the cmd template function when processing external dependencies, thereby mitigating risks associated with malicious or compromised remote configurations. The change ensures that dynamic command execution is only permitted for local, trusted configurations, maintaining functionality where intended while bolstering overall system integrity.

Highlights

• Security Enhancement: The cmd template function, which allows arbitrary command execution, is now restricted from being used within remote/untrusted configuration dependencies to prevent potential security vulnerabilities.
• New CmdAllowed Flag: A new global boolean flag, CmdAllowed, was introduced in pkg/skaffold/util/env_template.go to control the availability of the cmd function.
• Conditional Command Execution: The runCmdFunc now checks the CmdAllowed flag and returns an error if the cmd function is invoked when not permitted.
• Remote Dependency Handling: The pkg/skaffold/parser/config.go file was updated to set CmdAllowed = false specifically when processing remote dependency configurations, ensuring the restriction is applied correctly.
• Comprehensive Testing: New tests were added to verify that the cmd function is correctly blocked for remote contexts and continues to function as expected for local configurations.

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[gemini-code-assist]

Code Review

This pull request implements a security enhancement to prevent arbitrary command execution from remote or untrusted Skaffold configurations. It introduces a CmdAllowed flag that disables the cmd template function when processing such dependencies, and includes corresponding tests. A review comment suggests updating the placeholder issue link in the CmdAllowed variable's documentation for better traceability.

[gemini-code-assist]

The issue link XXXXX in the comment for CmdAllowed is a placeholder. Please update it with the actual issue number for better traceability and context.