Subject: [PATCH v2 2/2] gnu: bootloader: grub: Add support for loading an additional initrd

From: Tomas Volf <[email-masked]>
Date: Wed, 2 Aug 2023 15:02:45 +0200

In order to be able to provide decryption keys for the LUKS device, they need
to be available in the initial ram disk.  However they cannot be stored inside
the usual initrd, since it is stored in the store and being a
world-readable (as files in the store are) is not a desired property for a
initrd containing decryption keys.  This commit adds an option to load
additional initrd during the boot, one that is not stored inside the store and
therefore can contain secrets.

Since only grub supports encrypted /boot, only grub is modified to use the
extra-initrd.  There is no use case for the other bootloaders.

* doc/guix.texi (Bootloader Configuration): Describe the new extra-initrd
field.
* gnu/bootloader.scm: Add extra-initrd field to bootloader-configuration
* gnu/bootloader/grub.scm: Use the new extra-initrd field
---
 doc/guix.texi           | 20 ++++++++++++++++++++
 gnu/bootloader.scm      |  6 +++++-
 gnu/bootloader/grub.scm |  6 ++++--
 3 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index a857654191..c63f28786e 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -40078,6 +40078,26 @@ Bootloader Configuration
 @code{u-boot} bootloader, where the device tree has already been loaded
 in RAM, it can be handy to disable the option by setting it to
 @code{#f}.
+
+@item @code{extra-initrd} (default: @code{#f})
+Path to an additional initrd to load.  Should not point to a file in the
+store.  Typical use case is making keys to unlock LUKS device available
+during the boot process.  For any use case not involving secrets, you
+should use regular initrd (@pxref{operating-system Reference,
+@code{initrd}}) instead.
+
+Suitable image can be created for example like this:
+
+@example
+echo /key-file.bin | cpio -oH newc >/key-file.cpio
+chmod 0000 /key-file.cpio
+@end example
+
+Be careful when using this option, since pointing to a file that is not
+readable by the grub while booting will cause the boot to fail and
+require a manual edit of the initrd line in the grub menu.
+
+Currently only supported by grub.
 @end table
 
 @end deftp
diff --git a/gnu/bootloader.scm b/gnu/bootloader.scm
index 2c36d8c6cf..8cebcf8965 100644
--- a/gnu/bootloader.scm
+++ b/gnu/bootloader.scm
@@ -77,6 +77,7 @@ (define-module (gnu bootloader)
             bootloader-configuration-serial-unit
             bootloader-configuration-serial-speed
             bootloader-configuration-device-tree-support?
+            bootloader-configuration-extra-initrd
 
             %bootloaders
             lookup-bootloader-by-name
@@ -279,7 +280,10 @@ (define-record-type* <bootloader-configuration>
   (serial-speed          bootloader-configuration-serial-speed
                          (default #f))    ;integer | #f
   (device-tree-support?  bootloader-configuration-device-tree-support?
-                         (default #t)))   ;boolean
+                         (default #t))    ;boolean
+  (extra-initrd          bootloader-configuration-extra-initrd
+                         (default #f))    ;string | #f
+  )
 
 (define-deprecated (bootloader-configuration-target config)
   bootloader-configuration-targets
diff --git a/gnu/bootloader/grub.scm b/gnu/bootloader/grub.scm
index 5f3fcd7074..49cb3f7725 100644
--- a/gnu/bootloader/grub.scm
+++ b/gnu/bootloader/grub.scm
@@ -386,7 +386,8 @@ (define* (make-grub-configuration grub config entries
                                      store-directory-prefix))
               (initrd (normalize-file (menu-entry-initrd entry)
                                       device-mount-point
-                                      store-directory-prefix)))
+                                      store-directory-prefix))
+              (extra-initrd (bootloader-configuration-extra-initrd config)))
           ;; Here DEVICE is the store and DEVICE-MOUNT-POINT is its mount point.
           ;; Use the right file names for LINUX and INITRD in case
           ;; DEVICE-MOUNT-POINT is not "/", meaning that the store is on a
@@ -397,11 +398,12 @@ (define* (make-grub-configuration grub config entries
           #~(format port "menuentry ~s {
   ~a
   linux ~a ~a
-  initrd ~a
+  initrd ~a ~a
 }~%"
                     #$label
                     #$(grub-root-search device linux)
                     #$linux (string-join (list #$@arguments))
+                    (or #$extra-initrd "")
                     #$initrd)))
        (multiboot-kernel
         (let* ((kernel (menu-entry-multiboot-kernel entry))
-- 
2.41.0