device_tree: Fix integer overflowing in load_device_tree()

If the value of get_image_size() exceeds INT_MAX / 2 - 10000, the
computation of @dt_size overflows to a negative number, which then
gets converted to a very large size_t for g_malloc0() and
load_image_size().  In the (fortunately improbable) case g_malloc0()
succeeds and load_image_size() survives, we'd assign the negative
number to *sizep.  What that would do to the callers I can't say, but
it's unlikely to be good.

Fix by rejecting images whose size would overflow.

Reported-by: Kurtis Miller <kurtis.miller@nccgroup.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20190409174018.25798-1-armbru@redhat.com>

device_tree.c

@@ -84,6 +84,10 @@ void *load_device_tree(const char *filename_path, int *sizep)
                      filename_path);
         goto fail;
     }
+    if (dt_size > INT_MAX / 2 - 10000) {
+        error_report("Device tree file '%s' is too large", filename_path);
+        goto fail;
+    }
 
     /* Expand to 2x size to give enough room for manipulation.  */
     dt_size += 10000;