RDMA/cma: Fix workqueue crash in cma_netevent_work_handler

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

RDMA/cma: Fix workqueue crash in cma_netevent_work_handler

struct rdma_cm_id has member "struct work_struct net_work"
that is reused for enqueuing cma_netevent_work_handler()s
onto cma_wq.

Below crash[1] can occur if more than one call to
cma_netevent_callback() occurs in quick succession,
which further enqueues cma_netevent_work_handler()s for the
same rdma_cm_id, overwriting any previously queued work-item(s)
that was just scheduled to run i.e. there is no guarantee
the queued work item may run between two successive calls
to cma_netevent_callback() and the 2nd INIT_WORK would overwrite
the 1st work item (for the same rdma_cm_id), despite grabbing
id_table_lock during enqueue.

Also drgn analysis [2] indicates the work item was likely overwritten.

Fix this by moving the INIT_WORK() to __rdma_create_id(),
so that it doesn't race with any existing queue_work() or
its worker thread.

[1] Trimmed crash stack:
=============================================
BUG: kernel NULL pointer dereference, address: 0000000000000008
kworker/u256:6 ... 6.12.0-0...
Workqueue: cma_netevent_work_handler [rdma_cm] (rdma_cm)
RIP: 0010:process_one_work+0xba/0x31a
Call Trace:
worker_thread+0x266/0x3a0
kthread+0xcf/0x100
ret_from_fork+0x31/0x50
ret_from_fork_asm+0x1a/0x30
=============================================

[2] drgn crash analysis:

>>> trace = prog.crashed_thread().stack_trace()
>>> trace
(0) crash_setup_regs (./arch/x86/include/asm/kexec.h:111:15)
(1) __crash_kexec (kernel/crash_core.c:122:4)
(2) panic (kernel/panic.c:399:3)
(3) oops_end (arch/x86/kernel/dumpstack.c:382:3)
...
(8) process_one_work (kernel/workqueue.c:3168:2)
(9) process_scheduled_works (kernel/workqueue.c:3310:3)
(10) worker_thread (kernel/workqueue.c:3391:4)
(11) kthread (kernel/kthread.c:389:9)

Line workqueue.c:3168 for this kernel version is in process_one_work():
3168 strscpy(worker->desc, pwq->wq->name, WORKER_DESC_LEN);

>>> trace[8]["work"]
*(struct work_struct *)0xffff92577d0a21d8 = {
.data = (atomic_long_t){
.counter = (s64)536870912, <=== Note
},
.entry = (struct list_head){
.next = (struct list_head *)0xffff924d075924c0,
.prev = (struct list_head *)0xffff924d075924c0,
},
.func = (work_func_t)cma_netevent_work_handler+0x0 = 0xffffffffc2cec280,
}

Suspicion is that pwq is NULL:
>>> trace[8]["pwq"]
(struct pool_workqueue *)<absent>

In process_one_work(), pwq is assigned from:
struct pool_workqueue *pwq = get_work_pwq(work);

and get_work_pwq() is:
static struct pool_workqueue *get_work_pwq(struct work_struct *work)
{
unsigned long data = atomic_long_read(&work->data);

if (data & WORK_STRUCT_PWQ)
return work_struct_pwq(data);
else
return NULL;
}

WORK_STRUCT_PWQ is 0x4:
>>> print(repr(prog['WORK_STRUCT_PWQ']))
Object(prog, 'enum work_flags', value=4)

But work->data is 536870912 which is 0x20000000.
So, get_work_pwq() returns NULL and we crash in process_one_work():
3168 strscpy(worker->desc, pwq->wq->name, WORKER_DESC_LEN);
=============================================