nixos/hardened: set mmap_min_addr

This is set in the hardened linux config as well but sysctl is more
flexible & works with any boot.kernelPackages

nixos/modules/profiles/hardened.nix

···
   # Note: mmap_rnd_compat_bits may not exist on 64bit.
   boot.kernel.sysctl."vm.mmap_rnd_bits" = mkDefault 32;
   boot.kernel.sysctl."vm.mmap_rnd_compat_bits" = mkDefault 16;
+
+  # Allowing users to mmap() memory starting at virtual address 0 can turn a
+  # NULL dereference bug in the kernel into code execution with elevated
+  # privilege.  Mitigate by enforcing a minimum base addr beyond the NULL memory
+  # space.  This breaks applications that require mapping the 0 page, such as
+  # dosemu or running 16bit applications under wine.  It also breaks older
+  # versions of qemu.
+  #
+  # The value is taken from the KSPP recommendations (Debian uses 4096).
+  boot.kernel.sysctl."vm.mmap_min_addr" = mkDefault 65536;
 }