Oh sad! Dang I actually really liked the feature, it's super convenient for keeping developer environments in sync. I left a comment in that thread asking for clarification.
Here is a slightly contrived, but realistic example of why it is a bad idea:
1) Attacker discovers vulnerability in an older version of the Rust toolchain
2) Attacker creates useful crate and helps it to get widely adopted or becomes trusted contributor to a crate that is already popular
3) Attacker creates and publishes crate changes with exploit code and rust-toolchain.toml to trigger use of older, vulnerable Rust toolchain
4) Unsuspecting developers build the trapped crate or something that depends on it and get owned
Perhaps they could make it a configuration setting that developers could opt-in? That would let developers who want automatic toolchain installs to have it and others who do not want it (or whose employers will not allow it) to not have it.
In Go case though the version can only go higher, not lower (e.g.: it will not download a toolchain if the Go version is set to lower than your current one, only higher). So I can't see the same attack being executed here.
That is good to know, assuming that the newer hypothetical toolchain is not vulnerable (e.g. a zero-day in the newer toolchain).
My opinion is still that toolchains (newer or older) should not be implicitly installed without the developer's explicit permission. This could be a configuration setting that the developer has opted-in or a "This package requires toolchain version X. Install it? (y/n)" prompt.
This would still be really difficult to explore:
0. An undiscovered zero-day in the newest version of Go
1. The only way to force users to use the newest toolchain is if your project has dependencies where the attacker has control, so e.g.: they can change their go.mod and set a `go` directive that upgrades to the later version
2. You need to update to this new version, because this will make Go complain that your `go` directive inside your `go.mod` is out-of-date
3. After all this, yes, Go will download the newest version of Go (that is vulnerable)
Again, not saying that this is an impossible scenario, just find it highly unlikely.
> This could be a configuration setting that the developer has opted-in or a "This package requires toolchain version X. Install it? (y/n)" prompt.
I concur at this part though, however keep in mind that this `toolchain` feature is more akin to adding a module dependency, since it reuses the whole module system.
I think there is already lots of trust that is implicit when you are managing modules, and this new feature doesn't make it worse. If anything, considering that the binaries can only come from golang.org/toolchain and I assume Go already check the checksum of every module that it downloads (and also that Go is perfectly reproducible: https://go.dev/blog/rebuild), if anything this is probably more trusted than a random module that you add to your project.
As for configuration: to me, having it be opt-in negates the entire benefit. My point is that automatically installing the correct toolchain makes it far easier to collaborate with others who aren't nearly as obsessive about Rust as I am.
Possibly, which is why the example is a bit contrived. In most cases, the toolchains will likely be more trusted and be on approved lists whereas binaries created by third-party crates are not.
For more secure environments, explicitness is valued and automatic installation of anything is frowned upon because it can introduce unvetted changes which could include vulnerabilities.
It depends on what work is being done and how much toolchains and ecosystem can be trusted.
If you mean build.rs build scripts, yes, those do run, but it is not arbitrary code. You can view and inspect them before building. If you need more security, you can download all the dependencies and build inside an isolated container.
uhh ya it is. There's also https://github.com/eleijonmarck/do-not-compile-this-code
https://github.com/eleijonmarck/do-not-compile-this-code/blo...
This is true for all third-party libraries. If you blindly download and execute code from the Internet, this is a risk you are assuming.
As I stated above, if you need more security, you can download all the dependencies and build inside an isolated container.