efi/libstub: arm64: Double check image alignment at entry

On arm64, the stub only moves the kernel image around in memory if
needed, which is typically only for KASLR, given that relocatable
kernels (which is the default) can run from any 64k aligned address,
which is also the minimum alignment communicated to EFI via the PE/COFF
header.

Unfortunately, some loaders appear to ignore this header, and load the
kernel at some arbitrary offset in memory. We can deal with this, but
let's check for this condition anyway, so non-compliant code can be
spotted and fixed.

Cc: <stable@vger.kernel.org> # v5.10+
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

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

@@ -119,6 +119,10 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
 	if (image->image_base != _text)
 		efi_err("FIRMWARE BUG: efi_loaded_image_t::image_base has bogus value\n");
 
+	if (!IS_ALIGNED((u64)_text, EFI_KIMG_ALIGN))
+		efi_err("FIRMWARE BUG: kernel image not aligned on %ldk boundary\n",
+			EFI_KIMG_ALIGN >> 10);
+
 	kernel_size = _edata - _text;
 	kernel_memsize = kernel_size + (_end - _edata);
 	*reserve_size = kernel_memsize;