kwbimage: Fix out of bounds access

The kwbimage format is reading beyond its header structure if it
misdetects a Xilinx Zynq image and tries to read it. Fix it by
sanity checking that the header we want to read fits inside our
file size.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

tools/kwbimage.c

@@ -1615,6 +1615,10 @@ static int kwbimage_verify_header(unsigned char *ptr, int image_size,
 				  struct image_tool_params *params)
 {
 	uint8_t checksum;
+	size_t header_size = kwbimage_header_size(ptr);
+
+	if (header_size > image_size)
+		return -FDT_ERR_BADSTRUCTURE;
 
 	if (!main_hdr_checksum_ok(ptr))
 		return -FDT_ERR_BADSTRUCTURE;