RDMA/ucma: Lock the handler in ucma_write_cm_event()

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

RDMA/ucma: Lock the handler in ucma_write_cm_event()

ctx->file may only be changed under the handler lock and the xa_lock, which
is what stops uevents being queued for a ctx while ucma_migrate_id() moves
it to another file. The CM core takes that lock before invoking
ucma_event_handler(), but the write() paths that queue uevents themselves
do not.

ucma_write_cm_event() re-reads ctx->file for each of its four dereferences,
so ucma_migrate_id() can swap it mid-sequence:

mutex_lock(&ctx->file->mut); /* file A */
list_add_tail(&uevent->list, &ctx->file->event_list); /* file B */
mutex_unlock(&ctx->file->mut); /* file B */
wake_up_interruptible(&ctx->file->poll_wait); /* file B */

The window is the mutex_lock() itself: the writer sleeps in it while the
migration reassigns ctx->file. The list_add_tail() then runs on file B's
event_list holding only file A's mutex:

list_add corruption. prev->next should be next (ffff888101320f30),
but was ffff88814a08c418. (prev=ffff88814a075c18).
kernel BUG at lib/list_debug.c:32!
Call Trace:
ucma_write_cm_event+0x36e/0x5e0

and file A's mut is left held forever, wedging its next writer in D state.
The uevent is also stranded on a list ucma_cleanup_ctx_events() will not
walk, so it outlives its context. /dev/infiniband/rdma_cm is 0666 and no
RDMA device is involved, so an unprivileged user reaches all of this.

Take the handler lock, as ucma_cleanup_mc_events() does; ctx->cm_id is
pinned by the ucma_get_ctx() reference.