[RFC 0097] Unset read permission bit on /nix/store for other users (#97)

rfcs/0097-no-read-store-dir.md

@@ -0,0 +1,80 @@
+---
+feature: nix-store-perms
+start-date: 2021-07-04
+author: Las Safin
+co-authors:
+shepherd-team: @kevincox @7c6f434c @edolstra
+shepherd-leader: @edolstra
+related-issues:
+---
+
+# Summary
+[summary]: #summary
+
+- NixOS should have a module for configuring the permissions set for `/nix/store` on boot.
+- Nix should not enforce the permissions used for `/nix/store`.
+- The default permissions if the store doesn't exist should be 1735 when the store is made by Nix or the NixOS installer.
+  This means that the nixbld group can't `ls` the directory.
+
+# Motivation
+[motivation]: #motivation
+
+Right now you can't set the permissions for `/nix/store`, since they'll be overwritten
+by Nix anytime you use `nix`.
+
+`chmod g-r /nix/store` is beneficial because the `nixbld` group doesn't actually
+need to read the directory. It only needs to be able to write and "execute" it.
+This, however, should be optional, since the user should be able to configure
+the permissions however they want.
+
+Some users might also want to do things like `chmod o-r /nix/store`, which
+gives you the interesting property that you can not access paths you do not
+already know of.
+Do note that given that all processes can by default read `/proc/cmdline`,
+`/run/current-system`, and many other places which reveal your
+system's closure, making this permission change an insufficient solution for
+security in many cases. This, however, is also entirely optional and is not
+the default in any way.
+
+# Detailed design
+[design]: #detailed-design
+
+Where we previously would enforce the permissions, we now need to
+only set them if there is no directory in the first place.
+The same applies for `/nix/store/trash` and `/nix/store/.links`.
+
+Specifically, we need to modify the following places (not exhaustive):
+- [nixpkgs/nixos/modules/system/boot/stage-2-init.sh](https://github.com/NixOS/nixpkgs/blob/8284fc30c84ea47e63209d1a892aca1dfcd6bdf3/nixos/modules/system/boot/stage-2-init.sh#L62)
+- [nix/scripts/install-multi-user.sh](https://github.com/NixOS/nix/blob/cf1d4299a8fa8906f62271dcd878018cef84cc30/scripts/install-multi-user.sh#L577)
+- [nix/src/libstore/globals.hh](https://github.com/NixOS/nix/blob/ba8b39c13003c8ddafb6bec308997e09b9851c46/src/libstore/globals.hh#L278)
+- [nix/src/libstore/build/local-derivation-goal.cc](https://github.com/NixOS/nix/blob/6182ae689826554d915b4ed72e07f7978dc1d13c/src/libstore/build/local-derivation-goal.cc#L641)
+- [nix/src/libstore/local-store.cc](https://github.com/NixOS/nix/blob/0a535dd5ac93576f7152d786464e330ae3d46b50/src/libstore/local-store.cc#L181)
+
+# Examples and Interactions
+[examples-and-interactions]: #examples-and-interactions
+
+You should be able to do something like the following:
+```nix
+nix.store-perms = "xxxx";
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+If a user on a non-NixOS platform mistakenly sets the permissions for `/nix/store` to
+something undesirable, it won't be reverted by Nix automatically.
+
+# Alternatives
+[alternatives]: #alternatives
+
+You could not do this and keep it as it is.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+There doesn't seem to be any.
+
+# Future work
+[future]: #future-work
+
+In the future we likely want to reduce the default permissions for `/nix/store` as much as possible.