efi/libstub: arm64: Warn when efi_random_alloc() fails

Randomization of the physical load address of the kernel image relies on
efi_random_alloc() returning successfully, and currently, we ignore any
failures and just carry on, using the ordinary, non-randomized page
allocator routine. This means we never find out if a failure occurs,
which could harm security, so let's at least warn about this condition.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

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

@@ -130,6 +130,8 @@ efi_status_t handle_kernel_image(unsigned long *image_addr,
 		 */
 		status = efi_random_alloc(*reserve_size, min_kimg_align,
 					  reserve_addr, phys_seed);
+		if (status != EFI_SUCCESS)
+			efi_warn("efi_random_alloc() failed: 0x%lx\n", status);
 	} else {
 		status = EFI_OUT_OF_RESOURCES;
 	}