IB/ucma: Fix theoretical user triggered use-after-free

Something like this:

CPU A                         CPU B
Acked-by: Sean Hefty <sean.hefty@intel.com>

========================      ================================
ucma_destroy_id()
 wait_for_completion()
                              .. anything
                                ucma_put_ctx()
                                  complete()
 .. continues ...
                              ucma_leave_multicast()
                               mutex_lock(mut)
                                 atomic_inc(ctx->ref)
                               mutex_unlock(mut)
 ucma_free_ctx()
  ucma_cleanup_multicast()
   mutex_lock(mut)
     kfree(mc)
                               rdma_leave_multicast(mc->ctx->cm_id,..

Fix it by latching the ref at 0. Once it goes to 0 mc and ctx cannot
leave the mutex(mut) protection.

The other atomic_inc in ucma_get_ctx is OK because mutex(mut) protects
it from racing with ucma_destroy_id.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Acked-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/core/ucma.c

@@ -1321,10 +1321,10 @@ static ssize_t ucma_leave_multicast(struct ucma_file *file,
 		mc = ERR_PTR(-ENOENT);
 	else if (mc->ctx->file != file)
 		mc = ERR_PTR(-EINVAL);
-	else {
+	else if (!atomic_inc_not_zero(&mc->ctx->ref))
+		mc = ERR_PTR(-ENXIO);
+	else
 		idr_remove(&multicast_idr, mc->id);
-		atomic_inc(&mc->ctx->ref);
-	}
 	mutex_unlock(&mut);
 
 	if (IS_ERR(mc)) {