firmware: raspberrypi: Remove VLA usage

In the quest to remove all stack VLA usage from the kernel[1], this
removes the VLA in favor of a maximum size and adds a sanity check.
Existing callers of the firmware interface never need more than 24
bytes (struct gpio_set_config). This chooses 32 just to stay ahead
of future growth.

v2: Fix the length passed to rpi_firmware_property_list (by anholt,
    acked by Kees).

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>

drivers/firmware/raspberrypi.c

@@ -21,6 +21,8 @@
 #define MBOX_DATA28(msg)		((msg) & ~0xf)
 #define MBOX_CHAN_PROPERTY		8
 
+#define MAX_RPI_FW_PROP_BUF_SIZE	32
+
 static struct platform_device *rpi_hwmon;
 
 struct rpi_firmware {
@@ -145,18 +147,22 @@ int rpi_firmware_property(struct rpi_firmware *fw,
 	/* Single tags are very small (generally 8 bytes), so the
 	 * stack should be safe.
 	 */
-	u8 data[buf_size + sizeof(struct rpi_firmware_property_tag_header)];
+	u8 data[sizeof(struct rpi_firmware_property_tag_header) +
+		MAX_RPI_FW_PROP_BUF_SIZE];
 	struct rpi_firmware_property_tag_header *header =
 		(struct rpi_firmware_property_tag_header *)data;
 	int ret;
 
+	if (WARN_ON(buf_size > sizeof(data) - sizeof(*header)))
+		return -EINVAL;
+
 	header->tag = tag;
 	header->buf_size = buf_size;
 	header->req_resp_size = 0;
 	memcpy(data + sizeof(struct rpi_firmware_property_tag_header),
 	       tag_data, buf_size);
 
-	ret = rpi_firmware_property_list(fw, &data, sizeof(data));
+	ret = rpi_firmware_property_list(fw, &data, buf_size + sizeof(*header));
 	memcpy(tag_data,
 	       data + sizeof(struct rpi_firmware_property_tag_header),
 	       buf_size);