nixos/stage2-init: test nosuid and nodev mount options

We can make some evil files in the store within stage1,
and verify that they don't work.

nixos/tests/boot-stage2.nix

@@ -10,7 +10,24 @@ import ./make-test-python.nix (
         lib,
         ...
       }:
+      let
+        # Prints the user's UID. Can't just do a shell script
+        # because setuid is ignored for interpreted programs.
+        uid = pkgs.writeCBin "uid" ''
+          #include <unistd.h>
+          #include <stdio.h>
+          int main(void) {
+            printf("%d\n", geteuid());
+            return 0;
+          }
+        '';
+      in
       {
+        users.users.alice = {
+          isNormalUser = true;
+          uid = 1000;
+        };
+
         virtualisation = {
           emptyDiskImages = [ 256 ];
 
@@ -32,6 +49,10 @@ import ./make-test-python.nix (
           };
         };
 
+        environment.systemPackages = [ pkgs.xxd ];
+
+        system.extraDependencies = [ uid ];
+
         boot = {
           initrd = {
             # Format the upper Nix store.
@@ -50,6 +71,19 @@ import ./make-test-python.nix (
               mount -t overlay overlay \
                 -o lowerdir=/mnt-root/nix/store/ro,upperdir=/mnt-root/nix/store/rw,workdir=/mnt-root/nix/store/work \
                 /mnt-root/nix/store
+
+              # Be very rude and try to put suid files and/or devices into the store.
+              evil=/mnt-root/nix/store/evil
+              mkdir -p $evil/bin $evil/dev
+
+              echo "making evil suid..." >&2
+              cp /mnt-root/${builtins.unsafeDiscardStringContext "${uid}"}/bin/uid $evil/bin/suid
+              chmod 4755 $evil/bin/suid
+              [ -u $evil/bin/suid ] || exit 1
+
+              echo "making evil devzero..." >&2
+              mknod -m 666 $evil/dev/zero c 1 5
+              [ -c $evil/dev/zero ] || exit 1
             '';
 
             kernelModules = [ "overlay" ];
@@ -70,6 +104,28 @@ import ./make-test-python.nix (
       for opt in ["ro", "nosuid", "nodev"]:
         with subtest(f"testing store mount option: {opt}"):
           machine.succeed(f'[[ "$(findmnt --direction backward --first-only --noheadings --output OPTIONS /nix/store)" =~ (^|,){opt}(,|$) ]]')
+
+      # should still be suid
+      machine.succeed('[ -u /nix/store/evil/bin/suid ]')
+      # runs as alice and is not root
+      machine.succeed('[ "$(sudo -u alice /nix/store/evil/bin/suid)" == 1000 ]')
+      # can be remounted and runs as root
+      machine.succeed('mount -o remount,suid,bind /nix/store && mount >&2')
+      machine.succeed('[ "$(sudo -u alice /nix/store/evil/bin/suid)" == 0 ]')
+      # double checking we can undo it
+      machine.succeed('mount -o remount,nosuid,bind /nix/store && mount >&2')
+      machine.succeed('[ "$(sudo -u alice /nix/store/evil/bin/suid)" == 1000 ]')
+
+      # should still be a character device
+      machine.succeed('[ -c /nix/store/evil/dev/zero ]')
+      # should not work
+      machine.fail('[ "$(dd if=/nix/store/evil/dev/zero bs=1 count=1 | xxd -pl1)" == 00 ]')
+      # can be remounted and works
+      machine.succeed('mount -o remount,dev,bind /nix/store && mount >&2')
+      machine.succeed('[ "$(dd if=/nix/store/evil/dev/zero bs=1 count=1 | xxd -pl1)" == 00 ]')
+      # double checking we can undo it
+      machine.succeed('mount -o remount,nodev,bind /nix/store && mount >&2')
+      machine.fail('[ "$(dd if=/nix/store/evil/dev/zero bs=1 count=1 | xxd -pl1)" == 00 ]')
     '';
 
     meta.maintainers = with pkgs.lib.maintainers; [ numinit ];