net/rds: size a connection's path set by the transport it ends up with

In the Linux kernel, the following vulnerability has been resolved:

net/rds: size a connection's path set by the transport it ends up with

__rds_conn_create() computes npaths from the caller's transport before
it decides whether a connection to one of the host's own addresses is
to be handled by the loopback transport instead. That substitution is
what an RDS/TCP socket sending to a local address gets, and after it
the path init loop still runs for the TCP transport's RDS_MPATH_WORKERS
paths and allocates an ordered workqueue for each, while
rds_loop_conn_alloc() only ever provides transport data for path 0.

rds_conn_destroy() sizes its teardown from c_trans, by then the
loopback transport, so it visits path 0 only - and
rds_conn_path_destroy() would skip the other paths anyway, since it
returns before destroy_workqueue() for a path without transport data.
kfree(c_path) then drops the last pointers to seven workqueues. That
repeats for every such connection, on every netns teardown or module
unload, and every distinct local destination address is a separate
connection.

Recompute npaths once the transport is final, so that creation and
destruction agree on the set of paths. The c_path array stays sized
for the caller's transport; the unused entries are freed with it.