x86/boot: Preserve boot_params.secure_boot from sanitizing

Commit

  a90118c ("x86/boot: Save fields explicitly, zero out everything else")

now zeroes the secure boot setting information (enabled/disabled/...)
passed by the boot loader or by the kernel's EFI handover mechanism.

The problem manifests itself with signed kernels using the EFI handoff
protocol with grub and the kernel loses the information whether secure
boot is enabled in the firmware, i.e., the log message "Secure boot
enabled" becomes "Secure boot could not be determined".

efi_main() arch/x86/boot/compressed/eboot.c sets this field early but it
is subsequently zeroed by the above referenced commit.

Include boot_params.secure_boot in the preserve field list.

 [ bp: restructure commit message and massage. ]

Fixes: a90118c ("x86/boot: Save fields explicitly, zero out everything else")
Signed-off-by: John S. Gruber <JohnSGruber@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: stable <stable@vger.kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/CAPotdmSPExAuQcy9iAHqX3js_fc4mMLQOTr5RBGvizyCOPcTQQ@mail.gmail.com

arch/x86/include/asm/bootparam_utils.h

@@ -70,6 +70,7 @@ static void sanitize_boot_params(struct boot_params *boot_params)
 			BOOT_PARAM_PRESERVE(eddbuf_entries),
 			BOOT_PARAM_PRESERVE(edd_mbr_sig_buf_entries),
 			BOOT_PARAM_PRESERVE(edd_mbr_sig_buffer),
+			BOOT_PARAM_PRESERVE(secure_boot),
 			BOOT_PARAM_PRESERVE(hdr),
 			BOOT_PARAM_PRESERVE(e820_table),
 			BOOT_PARAM_PRESERVE(eddbuf),