bug#60782: Channels and dependency confusion

["Ludovic Courtès" <ludovic.courtes@inria.fr>]

In the light of the “dependency confusion” attack on PyTorch¹, one might
wonder how such a thing could affect Guix.  The threat model is quite
different though because the ‘guix’ channel is peer-reviewed and curated
whereas PyPI isn’t.

Yet, one way to “translate” the attack to Guix is by looking at module
name clashes, as was suggested on Mastodon².

For example, I’m the author of a channel; my packages refer to (@ (gnu
packages guile) guile-3.0), which I expect to be the “genuine” Guile
provided by the ‘guix’ channel.  What happens if the user pulls in an
additional channel that also provides (gnu packages guile) with that
‘guile-3.0’ variable?

Nothing, because the ‘guix’ channel always comes first in the module
search path (see ‘%package-module-path’ in (gnu packages)).  Good.

Now same scenario, but with references to another channel, for example
(@ (past packages boost) boost-1.68) provided by Guix-Past.

This time, if the user pulls in an additional channel that also provides
(@ (past packages boost) boost-1.68), we do not know which one is going
to take precedence.  It may go unnoticed though, because
‘channel-instances->derivation’ calls ‘profile-derivation’, which uses
‘build-profile’, which calls ‘union-build’ with the default file
collision policy, which is to warn (the warning only appears in the
build log).

I think it would be best to error out if multiple channels provide
same-named files.

Thoughts?

Ludo’.

¹ https://pytorch.org/blog/compromised-nightly-dependency/
² https://toot.aquilenet.fr/@Parnikkapore@mastodon.social/109636000975651971