In the Linux kernel, the following vulnerability has been resolved: xprtrdma: Fix ep kref...

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

xprtrdma: Fix ep kref imbalance on ADDR_CHANGE

rpcrdma_cm_event_handler() falls through to the disconnected: label on RDMA_CM_EVENT_ADDR_CHANGE and calls rpcrdma_ep_put() with no matching get when the event arrives before RDMA_CM_EVENT_ESTABLISHED. The kref then underflows during connect teardown and rpcrdma_xprt_disconnect() operates on a freed ep.

Reference counts across a normal connection lifecycle:

rpcrdma_ep_create()             kref_init     ->1
rpcrdma_xprt_connect()          ep_get        ->2  (before post_recvs)
RDMA_CM_EVENT_ESTABLISHED       ep_get        ->3
RDMA_CM_EVENT_DISCONNECTED      ep_put        ->2
rpcrdma_xprt_drain()            ep_put        ->1
rpcrdma_xprt_disconnect() tail  ep_put        ->0  (ep_destroy)

The connect-time get in rpcrdma_xprt_connect(), taken just before rpcrdma_post_recvs() "while there are outstanding Receives," is balanced by rpcrdma_xprt_drain. ADDR_CHANGE before ESTABLISHED has no get to consume, so its put drops the count to 1 and the drain put then frees the ep while rpcrdma_xprt_disconnect() still holds a pointer to it.

Fix by dispatching on the prior re_connect_status via xchg(): for prev == 0 (pre-ESTABLISHED) wake the connect waiter and return with no put; for prev == 1 call rpcrdma_force_disconnect() and return. The case-1 arm relies on the subsequent RDMA_CM_EVENT_DISCONNECTED event -- reliably delivered when rdma_disconnect() is called on a still-connected cm_id -- to balance the ESTABLISHED get; rpcrdma_xprt_drain() continues to balance only that connect-time get. Any other prior value means teardown is already in flight.