[RFC 0097] Unset read permission bit on /nix/store for other users

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

@@ -0,0 +1,130 @@
+---
+feature: no-read-store-dir
+start-date: 2021-07-04
+author: Las Safin
+co-authors:
+shepherd-team: @kevincox @7c6f434c @edolstra
+shepherd-leader: @edolstra
+related-issues:
+---
+
+# Summary
+[summary]: #summary
+
+Set the permissions for `/nix/store`, `/nix/store/.links` and `/nix/store/trash` to 1771 instead of 1775, disabling reading the directory for other users.
+
+# Motivation
+[motivation]: #motivation
+
+This is a simple change that acts as an extra layer of security by making
+it harder to access store paths that programs don't need to access.
+
+For a directory, being able to execute without being able to read it means
+that you can access paths inside the directory, if and only if you know the path beforehand,
+since you can not list the directory entries.
+
+In our case, with this change, all users that are not root and are not part of nixbld,
+will only be able to directly access paths inside `/nix/store` which they already know,
+i.e. the hash and the name.
+If you run some program that does not have access to the nix daemon under such
+a user, it will not be able to search the store for built system configurations,
+but will instead have to find the path to one through e.g. `/nix/var/nix/profiles/system` (if accessible).
+
+For tight sandboxes under NixOS, where only the store paths needed are mounted,
+this is not very important, since they can already only access the ones they need,
+however, this feature is useful for making sandboxes that have access to the entirety
+of `/nix/store`, where you want to be sure that any store path encountered inside the sandbox
+can be accessed. This will often happen when you want a quick sandbox with bubblewrap where
+it's too much of a pain to manually find all store paths you need.
+This type of sandbox can still be secure, granted that you are careful not to "leak" store paths
+into the sandbox, which is not trivial but not impossible either. Specifics are discussed in [Drawbacks](#drawbacks).
+
+NB: While this change is simple, it is not possible for end-users to do without changing Nix, since at the moment any
+`nix` command will reset the permissions back to 1775.
+
+# Detailed design
+[design]: #detailed-design
+
+Set the 1775
+in [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),
+in [nix/scripts/install-multi-user.sh](https://github.com/NixOS/nix/blob/cf1d4299a8fa8906f62271dcd878018cef84cc30/scripts/install-multi-user.sh#L577),
+in [nix/src/libstore/globals.hh](https://github.com/NixOS/nix/blob/ba8b39c13003c8ddafb6bec308997e09b9851c46/src/libstore/globals.hh#L278),
+in [nix/src/libstore/build/local-derivation-goal.cc](https://github.com/NixOS/nix/blob/6182ae689826554d915b4ed72e07f7978dc1d13c/src/libstore/build/local-derivation-goal.cc#L641), and
+in [nix/src/libstore/local-store.cc](https://github.com/NixOS/nix/blob/0a535dd5ac93576f7152d786464e330ae3d46b50/src/libstore/local-store.cc#L181)
+to 1771.
+
+`/nix/store/trash` and `/nix/store/.links` will also have to have their read bit removed, resulting in 0751.
+
+Various parts of NixOS also ought have their permissions fixed, which is beneficial with and without this change.
+Specifically, the permissions for these things ought to be looked at again:
+- `/proc/cmdline`
+- `/proc/sys`
+- `/proc/config.gz`
+- The other `/proc/*` stuff
+- `dmesg`
+- The nix daemon
+
+Sandboxes should preferably not have access to the above in any case.
+
+Nothing else has to be done likely, since setting the store permissions to `1771` manually doesn't break
+anything other than what is mentioned in this document (though it is undone if you run a `nix` command).
+
+# Examples and Interactions
+[examples-and-interactions]: #examples-and-interactions
+
+Losing the read (r) bit means that you can't list the files inside the store.
+The execute (x) bit allows us to `cd` to it and also access paths inside the store.
+
+E.g. `ls "$(readlink /nix/var/nix/profiles/system)"` will still work, since this is a directory
+inside the store, and not the store itself, but an unprivileged user can't `ls /nix/store` to find the system configuration.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+It might be a slight annoyance since shell completion won't work in the `/nix/store` anymore, e.g.
+if you have some hash 48914, you wouldn't be able to type `/nix/store/48914<tab>` to get the full path anymore.
+This is likely the biggest drawback.
+
+External tooling that does a traversal of the nix store (`find`, `du -s`, `ncdu`) would need `sudo` or explicitly given permissions.
+
+A sandboxed program could still have some idea of how the host machine is used by checking each store path that Hydra has ever built,
+then it could estimate whether the host machine has e.g. Tor on it.
+
+Attempting to have security comparable to bind-mounting each necessary store path inside a sandbox is not easy,
+since you need to be careful not to leak paths to the sandbox through global state, for example `/proc/cmdline`.
+There is no easy way to know whether some path is readable from inside the sandbox, since the problem is
+essentially proving that the sandbox doesn't have access to some secret that in this case is the store path itself.
+Since Unix software has not traditionally treated paths as secrets, this is a bit problematic if you're
+trying to make sure your sandbox can't access some path. We also can not know how future kernel interfaces and such
+will be, and they will likely not have the same considerations for "paths as secrets" as us.
+Essentially, you have to consider the store paths as encodings of their contents wrt. sandboxes.
+
+The above points about sandboxed programs are not negative effects of changing the permissions of /nix/store and such,
+but rather considerations for making use of this change in the context of sandboxes.
+
+# Alternatives
+[alternatives]: #alternatives
+
+If Nix was made to not reset the permissions of /nix/store back to 1775, users who want this change could
+do it themselves by simply putting this into their configuration.nix:
+```nix
+{
+  system.activationScripts.chmod-store.text = ''
+    ${pkgs.util-linux}/bin/unshare -m ${pkgs.bash}/bin/sh -c '${pkgs.util-linux}/bin/mount -o remount,rw /nix/store ; ${pkgs.coreutils}/bin/chmod 1771 /nix/store ; ...'
+  '';
+}
+```
+
+Traditional sandboxing could be seen as an alternative to this, however, they are not mutually exclusive,
+since this change is fundamentally just changing some permissions.
+Improving the permissions for /nix/store will not decrease security for traditional sandboxes.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+There doesn't seem to be any.
+
+# Future work
+[future]: #future-work
+
+Future work includes fixing the permissions of global state in NixOS, notably access to the nix daemon, much of the kernel API, such as `/proc`.