kernel: dns_resolver upcall security issue

CIFS has the ability to chase MS-DFS referrals. In order to do this it has to be able to resolve hostnames into IP addresses. For this, it uses the keys API to upcall to the cifs.upcall userspace helper. It then resolves the name and hands the address back to the kernel.

The dns_resolver upcall currently used by CIFS is susceptible to cache stuffing. It's possible for a malicious user to stuff the keyring with the results of a lookup, and then trick the server into mounting a server of his choosing.