Contain handler_folder within the build context

[welteki]

Description

Validate handler-folder paths to reject absolute paths and traversal outside
the function's build context. Require function names to be single directory
components before clearing the context.

Also fix extra-path validation accepting similarly named sibling directories,
and derive copy destinations from validated relative paths to prevent writes
outside the context.

Motivation and context

handler_folder from a language template's template.yml was joined onto the
build context without a containment check, so a value containing .. segments
could create and copy the function handler tree outside of the build context
with the build user's privileges. This allows a malicious or compromised
template to write or overwrite arbitrary files on the machine running the
build. The existing pathInScope check covered copyExtraPaths, but not
handler_folder.

How has this been tested

• Regression tests cover traversal, outside-file preservation, extra-path
  containment and scope-root rejection.
• Reproduced the escape with faas-cli build --shrinkwrap against a crafted
  template and verified the build now fails safely with an error and no files
  are written outside of the build context.

[derek]

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[reviewfn]

AI Pull Request Overview

Disclaimer: This review was generated by automated AI and may contain errors. Do not trust its outputs without human verification.

Summary

• Adds validation for per-function build context paths before RemoveAll runs.
• Constrains non-dockerfile handler overlays to local paths under the function build context.
• Reworks extra path copying to use the validated scope-relative destination path.
• Fixes prefix-sibling path checks by using filepath.Rel and filepath.IsLocal instead of string-prefix matching.
• Adds unit coverage for traversal, absolute paths, sibling-prefix paths, scope-root rejection, and destination overwrite prevention.

Approval rating (1-10)

9/10. The changes address the scoped path traversal risks with focused validation and relevant regression coverage.

Summary per file

Summary per file

File path	Summary
builder/builder.go	Validates function names, handler overlays, and extra path destinations before copying.
builder/builder_test.go	Adds regression tests for traversal rejection and safe copy destinations.

Overall Assessment

The patch is narrowly targeted and improves the build-context containment model in the areas touched by the PR. The move from prefix checks to filepath.Rel plus filepath.IsLocal closes the sibling-prefix bypass for extra paths, and validating the function build path before os.RemoveAll removes a destructive traversal hazard. The handler overlay check is also applied before directory creation, so rejected overlay paths do not create or overwrite out-of-context destinations. I did not find blocking issues in the scoped diff.

Detailed Review

Detailed Review

No blocking findings.

The changed CreateBuildContext flow validates functionName before constructing and clearing the build directory, which is the right ordering for avoiding traversal-driven deletion outside the configured build directory.

The handlerFolderWithinScope helper uses local-path validation before joining with the build root, so absolute paths and .. escapes are rejected before MkdirAll or handler copying can affect out-of-scope destinations.

The updated pathInScope returns both the absolute source and scope-relative path, which avoids reusing raw extra-path input containing .. segments for destination construction. This closes the destination escape covered by the new regression test.

The added tests exercise the important edge cases introduced by this patch, including sibling-prefix paths, build-context deletion order, rejected handler overlays, and destination placement for accepted extra paths.

AI agent details.

Agent processing time: 1m40.085s
Environment preparation time: 3.246s
Total time from webhook: 1m45.974s