1f5b180742
Recent mixed-mode Linux kernels, i.e., v4.0 or newer, can access EFI runtime services at OS runtime even when the OS was not entered via the EFI stub. This is because, instead of reverting back to the firmware's segment selectors, GDTs and IDTs, the 64-bit kernel simply calls 32-bit runtime services using compatibility mode, i.e., the same mode used for 32-bit user space, without taking down all interrupt handling, exception handling, etc. This means that GRUB's legacy x86 boot mode is sufficient to make use of this: 32-bit i686 builds of GRUB can already boot 64-bit kernels in EFI enlightened mode, but without going via the EFI stub, and provide all the metadata that the OS needs to map the EFI runtime regions and call EFI runtime services successfully. It does mean that GRUB should not attempt to invoke the firmware's LoadImage()/StartImage() methods on kernel builds that it knows cannot be started natively. So, add a check for this in the native EFI boot path and fall back to legacy x86 mode in such cases. Note that in the general case, booting non-native images of the same native word size, e.g., x64 EFI apps on arm64 firmware, might be supported by means of emulation. So, let's only disallow images that use a non-native word size. This will also permit booting i686 kernels on x86_64 builds, although without access to runtime services, as this is not supported by Linux. This change on top of 2.12-rc1 is sufficient to boot ordinary Linux mixed mode builds and get full access to the EFI runtime services. Cc: Daniel Kiper <daniel.kiper@oracle.com> Cc: Steve McIntyre <steve@einval.com> Cc: Julian Andres Klode <julian.klode@canonical.com> Signed-off-by: Ard Biesheuvel <ardb@kernel.org> Acked-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com> Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
This is GRUB 2, the second version of the GRand Unified Bootloader. GRUB 2 is rewritten from scratch to make GNU GRUB cleaner, safer, more robust, more powerful, and more portable. See the file NEWS for a description of recent changes to GRUB 2. See the file INSTALL for instructions on how to build and install the GRUB 2 data and program files. See the file MAINTAINERS for information about the GRUB maintainers, etc. If you found a security vulnerability in the GRUB please check the SECURITY file to get more information how to properly report this kind of bugs to the maintainers. Please visit the official web page of GRUB 2, for more information. The URL is <http://www.gnu.org/software/grub/grub.html>. More extensive documentation is available in the Info manual, accessible using 'info grub' after building and installing GRUB 2. There are a number of important user-visible differences from the first version of GRUB, now known as GRUB Legacy. For a summary, please see: info grub Introduction 'Changes from GRUB Legacy'