It was discovered that the cls_route filter implementation in the Linux kernel would not remove an old filter from the hashtable before freeing it if its handle had the value 0.
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
The product calls free() twice on the same memory address.