bug#37744: Per-user profile directory hijack (CVE-2019-17365 for Nix)

[Ludovic Courtès]

Hello Guix,

That the per-user profile directory is world-writable allows an attacker
to hijack code run by other users, as has been reported in the context
of Nix:

  https://www.openwall.com/lists/oss-security/2019/10/09/4

I believe it applies to Guix as well.

Nix people are tracking it here:

   https://github.com/NixOS/nix/pull/3134
   https://github.com/NixOS/nix/issues/509

Looks like we’ll need to do something similar to:
<https://github.com/NixOS/nix/pull/3136/commits/5a303093dcae1e5ce9212616ef18f2ca51020b0d>.

Thoughts?

Thanks,
Ludo’.