nixos/acme: Ensure certs are always protected

As per #121293, I ensured the UMask is set correctly
and removed any unnecessary chmod/chown/chgrp commands.
The test suite already partially covered permissions
checking but I added an extra check for the selfsigned
cert permissions.

(cherry picked from commit 083aba4f83b105c30a1386bdb214cb6c85e119e6)

nixos/modules/security/acme.nix

@@ -46,6 +46,7 @@ let
     serviceConfig = commonServiceConfig // {
       StateDirectory = "acme/.minica";
       BindPaths = "/var/lib/acme/.minica:/tmp/ca";
+      UMask = 0077;
     };
 
     # Working directory will be /tmp
@@ -54,8 +55,6 @@ let
         --ca-key ca/key.pem \
         --ca-cert ca/cert.pem \
         --domains selfsigned.local
-
-      chmod 600 ca/*
     '';
   };
 
@@ -196,6 +195,7 @@ let
 
       serviceConfig = commonServiceConfig // {
         Group = data.group;
+        UMask = 0027;
 
         StateDirectory = "acme/${cert}";
 
@@ -220,10 +220,12 @@ let
         cat cert.pem chain.pem > fullchain.pem
         cat key.pem fullchain.pem > full.pem
 
-        chmod 640 *
-
         # Group might change between runs, re-apply it
         chown 'acme:${data.group}' *
+
+        # Default permissions make the files unreadable by group + anon
+        # Need to be readable by group
+        chmod 640 *
       '';
     };
 
@@ -340,8 +342,6 @@ let
         fi
 
         mv domainhash.txt certificates/
-        chmod 640 certificates/*
-        chmod -R u=rwX,g=,o= accounts/*
 
         # Group might change between runs, re-apply it
         chown 'acme:${data.group}' certificates/*
@@ -357,6 +357,10 @@ let
           ln -sf fullchain.pem out/cert.pem
           cat out/key.pem out/fullchain.pem > out/full.pem
         fi
+
+        # By default group will have no access to the cert files.
+        # This chmod will fix that.
+        chmod 640 out/*
       '';
     };
   };