Nixpkgs: AppArmor fails to load with kernel 5.1.0

[attente]

Issue description

Upgrading to kernel 5.1.0 from pkgs.linuxPackages_latest breaks apparmor, preventing it from loading. Furthermore, this also breaks nixos-rebuild switch, requiring a kernel downgrade to pkgs.linuxPackages_5_0 (kernel 5.0.13) to fix this.

The error is:

```➜ ~ sudo nixos-rebuild switch --upgrade
[sudo] password for william:
unpacking channels...
building Nix...
building the system configuration...
activating the configuration...
setting up /etc...
reloading user units for william...
setting up tmpfiles
the following new units were started: cpu-throttling.service
warning: the following units failed: apparmor.service

● apparmor.service
Loaded: loaded (/nix/store/csxmp1iy3f5ijy8m0ajx6ch5n8milljc-unit-apparmor.service/apparmor.service; enabled; vendor preset: enabled)
Active: failed (Result: exit-code) since Wed 2019-05-08 10:29:31 EDT; 17ms ago
Process: 2996 ExecStart=/nix/store/5v3ysffjypsndwxkh16byzaa3g55lz4g-apparmor-parser-2.13.1/bin/apparmor_parser -rKv -I /nix/store/gj3zvpgbsnx85nbbrpb5s2kjz4y85w80-apparmor-profiles-2.13.1/etc/apparmor.d -I /nix/store/pmgymay0liaz5xy65z9n029v2qn6k79v-lxc-3.1.0/etc/apparmor.d /nix/store/pmgymay0liaz5xy65z9n029v2qn6k79v-lxc-3.1.0/etc/apparmor.d/usr.bin.lxc-start (code=exited, status=1/FAILURE)
Main PID: 2996 (code=exited, status=1/FAILURE)

May 08 10:29:31 helium systemd[1]: Starting apparmor.service...
May 08 10:29:31 helium apparmor_parser[2996]: Warning from stdin (line 1): config file '/etc/apparmor/parser.conf' not found
May 08 10:29:31 helium apparmor_parser[2996]: Cache read/write disabled: interface file missing. (Kernel needs AppArmor 2.4 compatibility patch.)
May 08 10:29:31 helium apparmor_parser[2996]: Warning: unable to find a suitable fs in /proc/mounts, is it mounted?
May 08 10:29:31 helium apparmor_parser[2996]: Use --subdomainfs to override.
May 08 10:29:31 helium systemd[1]: apparmor.service: Main process exited, code=exited, status=1/FAILURE
May 08 10:29:31 helium systemd[1]: apparmor.service: Failed with result 'exit-code'.
May 08 10:29:31 helium systemd[1]: Failed to start apparmor.service.
warning: error(s) occurred while switching to the new configuration

I believe the main issue is from this line here: `apparmor_parser[2996]: Cache read/write disabled: interface file missing. (Kernel needs AppArmor 2.4 compatibility patch.)`

### Steps to reproduce

`boot.kernelPackages = pkgs.linuxPackages_latest;` and `security.apparmor.enable = true;` should be sufficient to reproduce the problem, although apparmor was already implicitly enabled on my system. I believe it's because of lxd.

## Technical details

➜ ~ nix-shell -p nix-info --run "nix-info -m"

• system: "x86_64-linux"
• host os: Linux 5.1.0, NixOS, 19.09pre178846.7defc47944f (Loris)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.2.2
• channels(root): "nixos-19.09pre178846.7defc47944f, nixos-hardware"
• channels(william): ""
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos
  ```

[attente]

It can alternatively be reproduced with boot.kernelPackages = pkgs.linuxPackages_latest; and virtualisation.lxd.enable = true;. I've removed the second part as a workaround.

[DIzFer]

I can reproduce the apparmor issue (on unstable-small 19.09pre179342.0fe5e0279fe), but I can keep rebuilding once apparmor.service fails. Not sure if I'm understanding that part right though...

[joachifm]

Off the top of my head I'd guess that this is related to securityfs not being mounted; is it? (should be at /sys/kernel/security). I recently tweaked the ordering of the apparmor unit, so this could be fallout from that (EDIT: probably not).

[joachifm]

Hm, from my testing it seems that the selinux lsm is selected by the kernel at boot instead of apparmor for whatever reason under 5.1. I do note that linux_latest.configfile no longer explicitly sets the default security= boot parameter to "apparmor".

Can you check dmesg output? When apparmor is selected, you should see something like
"AppArmor: AppArmor Filesystem Enabled"; if that is missing, look for mentions of selinux.

Explicitly setting boot.kernelParams = [ "security=apparmor" ]; fixes the issue on my end; I'll push a patch to that effect.

[joachifm]

I believe this has been fixed. Please reopen if the problem persists.

[aaronjanse]

This is a problem again. Would somebody please re-open this issue?

[aaronjanse]

Hmmm, this seems to be fixed by a reboot. Is there some way to say "reboot required" from nixos-rebuild switch?

[mlvzk]

Hmmm, this seems to be fixed by a reboot. Is there some way to say "reboot required" from nixos-rebuild switch?

I just had the same issue and reboot indeed fixed it, thanks a lot

[ReliefCrew]

Hmmm, this seems to be fixed by a reboot. Is there some way to say "reboot required" from nixos-rebuild switch?

Just reproduced this by adding virtualisation.lxd.enable = true; to /etc/nixos/configuration.nix.

Making a reboot unnecessary would be even better :-) Either way the problem still exists and significantly impacts usability... any plans on addressing it?