efi/libstub/arm64: Avoid image_base value from efi_loaded_image

Commit:

  9f922377 ("efi/libstub/arm: Make efi_entry() an ordinary PE/COFF entrypoint")

did some code refactoring to get rid of the EFI entry point assembler
code, and in the process, it got rid of the assignment of image_addr
to the value of _text. Instead, it switched to using the image_base
field of the efi_loaded_image struct provided by UEFI, which should
contain the same value.

However, Michael reports that this is not the case: older GRUB builds
corrupt this value in some way, and since we can easily switch back to
referring to _text to discover this value, let's simply do that.

While at it, fix another issue in commit 9f922377, which may result
in the unassigned image_addr to be misidentified as the preferred load
offset of the kernel, which is unlikely but will cause a boot crash if
it does occur.

Finally, let's add a warning if the _text vs. image_base discrepancy is
detected, so we can tell more easily how widespread this issue actually
is.
Reported-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-efi@vger.kernel.org

drivers/firmware/efi/libstub/arm64-stub.c

@@ -116,6 +116,7 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
 		 * Mustang), we can still place the kernel at the address
 		 * 'dram_base + TEXT_OFFSET'.
 		 */
+		*image_addr = (unsigned long)_text;
 		if (*image_addr == preferred_offset)
 			return EFI_SUCCESS;
 
@@ -140,7 +141,11 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
 		}
 		*image_addr = *reserve_addr + TEXT_OFFSET;
 	}
-	memcpy((void *)*image_addr, image->image_base, kernel_size);
+
+	if (image->image_base != _text)
+		pr_efi_err("FIRMWARE BUG: efi_loaded_image_t::image_base has bogus value\n");
+
+	memcpy((void *)*image_addr, _text, kernel_size);
 
 	return EFI_SUCCESS;
 }