linux: drop `*_latest_hardened`-attributes in favor of versioned attributes

The problem behind this is that the hardened patchset[1]. Quite recently
this led to a weird problem when Linux 5.12 was dropped (and thus had to
be removed from `nixpkgs`), there were no patches for 5.13, so
`linuxPackages_hardened_latest` had to be downgraded to 5.10 as base[2]
which may be rather unintuitive and unexpected.

To avoid these kind of "silent downgrades" in the future, it makes sense
to drop the attribute entirely. If somebody wants to use a hardened
kernel, it's better to explicitly pin it using the newly introduced
versioned attributes, e.g. `linuxPackages_4_14_hardened`.

[1] https://github.com/anthraxx/linux-hardened/
[2] https://github.com/NixOS/nixpkgs/pull/133587

nixos/doc/manual/from_md/release-notes/rl-2111.section.xml

···
           option was removed and HTTPS is always used.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          The attribute <literal>linuxPackages_latest_hardened</literal>
+          was dropped because the hardened patches lag behind the
+          upstream kernel which made version bumps harder. If you want
+          to use a hardened kernel, please pin it explicitly with a
+          versioned attribute such as
+          <literal>linuxPackages_5_10_hardened</literal>.
+        </para>
+      </listitem>
     </itemizedlist>
   </section>
   <section xml:id="sec-release-21.11-notable-changes">

nixos/doc/manual/release-notes/rl-2111.section.md

···
 
 - The `kubernetes` package was upgraded to 1.22.  The `kubernetes.apiserver.kubeletHttps` option was removed and HTTPS is always used.
 
+- The attribute `linuxPackages_latest_hardened` was dropped because the hardened patches
+  lag behind the upstream kernel which made version bumps harder. If you want to use
+  a hardened kernel, please pin it explicitly with a versioned attribute such as
+  `linuxPackages_5_10_hardened`.
+
 ## Other Notable Changes {#sec-release-21.11-notable-changes}
 
 - The setting [`services.openssh.logLevel`](options.html#opt-services.openssh.logLevel) `"VERBOSE"` `"INFO"`. This brings NixOS in line with upstream and other Linux distributions, and reduces log spam on servers due to bruteforcing botnets.

nixos/tests/kernel-generic.nix

···
   linux_5_10 = makeKernelTest "5.10" linuxPackages_5_10;
   linux_5_13 = makeKernelTest "5.13" linuxPackages_5_13;
 
+  linux_hardened_4_14 = makeKernelTest "4.14" linuxPackages_4_14_hardened;
+  linux_hardened_4_19 = makeKernelTest "4.19" linuxPackages_4_19_hardened;
+  linux_hardened_5_4 = makeKernelTest "5.4" linuxPackages_5_4_hardened;
+  linux_hardened_5_10 = makeKernelTest "5.10" linuxPackages_5_10_hardened;
+
   linux_testing = makeKernelTest "testing" linuxPackages_testing;
 }

pkgs/top-level/aliases.nix

···
   linuxPackages_xen_dom0_hardened = linuxPackages_hardened;
   linuxPackages_latest_xen_dom0_hardened = linuxPackages_latest_hardened;
 
+  # added 2021-08-16
+  linuxPackages_latest_hardened = throw ''
+    The attribute `linuxPackages_hardened_latest' was dropped because the hardened patches
+    frequently lag behind the upstream kernel. In some cases this meant that this attribute
+    had to refer to an older kernel[1] because the latest hardened kernel was EOL and
+    the latest supported kernel didn't have patches.
+
+    If you want to use a hardened kernel, please check which kernel minors are supported
+    and use a versioned attribute, e.g. `linuxPackages_5_10_hardened'.
+
+    [1] for more context: https://github.com/NixOS/nixpkgs/pull/133587
+  '';
+  linux_latest_hardened = linuxPackages_latest_hardened;
+
   linux-steam-integration = throw "linux-steam-integration has been removed, as the upstream project has been abandoned"; # added 2020-05-22
   loadcaffe = throw "loadcaffe has been removed, as the upstream project has been abandoned"; # added 2020-03-28
   lobster-two = google-fonts; # added 2021-07-22

pkgs/top-level/all-packages.nix

···
   linuxPackages_testing_bcachefs = recurseIntoAttrs (linuxPackagesFor pkgs.linux_testing_bcachefs);
 
   # Hardened Linux
-  hardenedLinuxPackagesFor = kernel': overrides:
-    let # Note: We use this hack since the hardened patches can lag behind and we don't want to delay updates:
-      linux_latest_for_hardened = pkgs.linux_5_10;
-      kernel = (if kernel' == pkgs.linux_latest then linux_latest_for_hardened else kernel').override overrides;
-    in linuxPackagesFor (kernel.override {
+  hardenedLinuxPackagesFor = kernel: overrides:
+    linuxPackagesFor (kernel.override {
       structuredExtraConfig = import ../os-specific/linux/kernel/hardened/config.nix {
         inherit lib;
         inherit (kernel) version;
···
   linuxPackages_hardened = recurseIntoAttrs (hardenedLinuxPackagesFor pkgs.linux { });
   linux_hardened = linuxPackages_hardened.kernel;
 
-  linuxPackages_latest_hardened = recurseIntoAttrs (hardenedLinuxPackagesFor pkgs.linux_latest { });
-  linux_latest_hardened = linuxPackages_latest_hardened.kernel;
+  linuxPackages_4_14_hardened = recurseIntoAttrs (hardenedLinuxPackagesFor pkgs.linux_4_14 { });
+  linux_4_14_hardened = linuxPackages_4_14_hardened.kernel;
+
+  linuxPackages_4_19_hardened = recurseIntoAttrs (hardenedLinuxPackagesFor pkgs.linux_4_19 { });
+  linux_4_19_hardened = linuxPackages_4_19_hardened.kernel;
+
+  linuxPackages_5_4_hardened = recurseIntoAttrs (hardenedLinuxPackagesFor pkgs.linux_5_4 { });
+  linux_5_4_hardened = linuxPackages_5_4_hardened.kernel;
+
+  linuxPackages_5_10_hardened = recurseIntoAttrs (hardenedLinuxPackagesFor pkgs.linux_5_10 { });
+  linux_5_10_hardened = linuxPackages_5_10_hardened.kernel;
 
   # Hardkernel (Odroid) kernels.
   linuxPackages_hardkernel_4_14 = recurseIntoAttrs (linuxPackagesFor pkgs.linux_hardkernel_4_14);