[Bug 1604499] [grub2-signed/xenial] possible regression found

[Ubuntu Foundations Team Bug Bot]

As a part of the Stable Release Updates quality process a search for
Launchpad bug reports using the version of grub2-signed from xenial-
proposed was performed and bug 1609350 was found.  Please investigate
this bug report to ensure that a regression will not be created by this
SRU. In the event that this is not a regression remove the
"verification-failed" tag from this bug report and add the tag "bot-
stop-nagging" to bug 1609350 (not this bug). Thanks!

** Tags added: verification-failed

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https://bugs.launchpad.net/bugs/1604499

Title:
  include loopback and squash4 modules in EFI binary

Status in grub2 package in Ubuntu:
  Fix Released
Status in grub2-signed package in Ubuntu:
  Fix Released
Status in grub2 source package in Xenial:
  Fix Committed
Status in grub2-signed source package in Xenial:
  Fix Committed

Bug description:
  [SRU Justification]
  Development versions of snappy Ubuntu Core leverage grub's squashfs support to load kernels and initramfs directly from the kernel snap (which is a squashfs-format archive).  This requires the loopback and the squash4 grub modules to be loaded.

  Currently, neither of these modules is included in the signed EFI
  binaries, therefore this boot strategy is not compatible with
  SecureBoot.

  We should verify that the loopback and squash4 modules are suitable
  for inclusion in the signed binary, and include them.

  [Test case]
  1. Grab the snappy image from https://people.canonical.com/~mvo/all-snaps/amd64-all-snap.img.xz and uncompress it.
  2. Install grub-efi-amd64-signed from xenial-updates.
  3. Use kpartx to loop mount /dev/mapper/loopNp2.
  4. Replace boot/efi/BOOT/BOOTX64.EFI in the boot partition with /usr/lib/grub/x86_64-efi-signed/grubx64.efi.signed.
  5. Unmount the boot partition.
  6. Boot the image in a VM using UEFI firmware (not BIOS)
  7. Confirm that the image fails to boot with an error about the loopback command not found.
  8. Shut down the VM.
  9. Install grub-efi-amd64-signed from xenial-proposed.
  10. Mount the boot partition again.
  11. Replace boot/efi/BOOT/BOOTX64.EFI in the boot partition with /usr/lib/grub/x86_64-efi-signed/grubx64.efi.signed.
  12. Unmount the boot partition and remove the kpartx mapping.
  13. Boot the image in a VM again, using UEFI firmware.
  14. Confirm that the image boots successfully.

  [Regression potential]
  Minimal.  This SRU adds two additional modules to the UEFI boot images, which add a new command and a new filesystem driver respectively.  Users who do not have the 'loopback' command in their grub.cfg, and who do not have any squashfs filesystems as raw disks or partitions, should not see any behavior difference.  The added modules slightly increase the size of the grub images, from ~1.1MiB to ~1.2MiB.  This should not affect the usability of these bootloader images.

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