Secure download of packages for cabal-install.

For more information, see https://www.stackage.org/package/stackage-install

stackage-install

stackage-install provides a wrapper around the cabal install command, which will download packages more securely. Initially, this means downloading over an HTTPS connection from FP Complete's Amazon S3 mirror of Hackage, though more hardening is planned for the future (see future improvements below).

To install, simply run cabal update && cabal install stackage-install. Usage is intended to overlap well with cabal install. Whenever you would have run cabal install foo, you can now run stackage-install foo (or stk install foo with stackage-cli installed), which will perform the following steps:

1. Run cabal install --dry-run ... to get cabal's build plan
2. Download the relevant packages from S3, and place them in the locations that cabal-install expects
3. Run cabal install ...

Security

In addition to using HTTPS for download, stackage-install can use features of the all-cabal-hashes repository for extra guarantees. In particular, if you have appropriate data in your package index (see next paragraph), stackage-install will automatically verify both the hash of the package contents and the download size. If the server attempts to send more data than expected, the download will be terminated.

The easiest way to get this extra data into your package index is to use stackage-update with the --hashes option. See hash downloads for more information.

Caveats

If you have a modified remote-repo in your ~/.cabal/config file, this tool will not provide proper hardening. Most users do not modify their remote-repo, so this shouldn't be an issue most of the time.

There are some combinations of cabal install arguments which may not translate well to this tool. One known issue is that passing --dry-run is not supported, but others may apply as well.

This tool necessarily has to call cabal-install twice, once to calculate the dependencies, and then to install them. It's theoretically possible that cabal-install could come up with different build plans between the two calls, in which case the second call may download some packages insecurely. I've opened cabal issue #2566 about disabling downloading in cabal.

The output from cabal install --dry-run doesn't actually give us information on which packages need to be downloaded, only the packages to be installed. This will be different in the case of local packages. Unfortunately, cabal fetch won't work for us either, since it accepts different arguments see #2. The compromise we have now is to just continue working in the presence of errors during download, though a more robust solution would be to check if one of the arguments refers to a local package.

Why not fix cabal?

Hopefully cabal will get fixed soon, the discussion has already started. It's unfortunately unclear how long that discussion will take, and I received a specific request to write this tool. Since it's a small amount of code, I went ahead with this as an interim solution.

That said, some of the future enhancements discussed below are not planned for cabal, in which case this tool will continue to remain relevant for people looking for additional security beyond transport security.

Why Stackage?

See the same question and its answer on stackage-update.

Future enhancements

• Check hashes of all packages downloaded against a collection of package hashes
• Verify signatures from authors against the signature archive.