test: spl: check load_simple_fit() rejects an oversized data-size

Add a regression test that builds a FIT with external data, inflates
the data-size property far beyond the image and any plausible load
region, and confirms that spl_load_simple_fit() returns -EFBIG instead
of reading the declared size off the device. Without the bounds check
in load_simple_fit() this test overruns memory and crashes; with it the
load is rejected cleanly.

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

test/image/spl_load.c

@@ -367,6 +367,55 @@ SPL_IMG_TEST(spl_test_image, IMX8, 0);
 SPL_IMG_TEST(spl_test_image, FIT_INTERNAL, 0);
 SPL_IMG_TEST(spl_test_image, FIT_EXTERNAL, 0);
 
+/*
+ * A FIT image's data-size property is not covered by the configuration
+ * signature, so it is untrusted input. load_simple_fit() must reject a
+ * data-size larger than the destination rather than overrun it, because the
+ * device read happens before the image hash is verified.
+ */
+static int spl_test_fit_external_oversize(struct unit_test_state *uts)
+{
+	size_t img_size, img_data, data_size = SPL_TEST_DATA_SIZE;
+	struct spl_image_info info_write = {
+		.name = "oversize",
+		.size = data_size,
+	}, info_read = { };
+	struct spl_load_info load;
+	void *img;
+	int node;
+
+	if (!image_supported(FIT_EXTERNAL))
+		return -EAGAIN;
+
+	img_size = create_image(NULL, FIT_EXTERNAL, &info_write, &img_data);
+	ut_assert(img_size);
+	img = calloc(img_size, 1);
+	ut_assertnonnull(img);
+
+	generate_data(img + img_data, data_size, "oversize");
+	ut_asserteq(img_size, create_image(img, FIT_EXTERNAL, &info_write,
+					   NULL));
+
+	/*
+	 * Inflate data-size far beyond the image buffer and any plausible
+	 * load region. Without a bounds check, load_simple_fit() reads this
+	 * many bytes off the "device" before the hash is checked.
+	 */
+	node = fdt_path_offset(img, FIT_IMAGES_PATH);
+	ut_assert(node >= 0);
+	node = fdt_first_subnode(img, node);
+	ut_assert(node >= 0);
+	ut_assertok(fdt_setprop_inplace_u32(img, node, FIT_DATA_SIZE_PROP,
+					    0x40000000));
+
+	spl_load_init(&load, spl_test_read, img, 1);
+	ut_asserteq(-EFBIG, spl_load_simple_fit(&info_read, &load, 0, img));
+
+	free(img);
+	return 0;
+}
+SPL_TEST(spl_test_fit_external_oversize, 0);
+
 /*
  * LZMA is too complex to generate on the fly, so let's use some data I put in
  * the oven^H^H^H^H compressed earlier