Forwarded: Re: [syzbot] [rdma?] KASAN: slab-use-after-free Read in ucma_create_uevent

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Subject: Re: [syzbot] [rdma?] KASAN: slab-use-after-free Read in ucma_create_uevent
Author: lizhi.xu@xxxxxxxxxxxxx

#syz test

diff --git a/drivers/infiniband/core/ucma.c b/drivers/infiniband/core/ucma.c
index 6e700b974033..89c444c6f317 100644
--- a/drivers/infiniband/core/ucma.c
+++ b/drivers/infiniband/core/ucma.c
@@ -109,6 +109,7 @@ struct ucma_multicast {
 	u8			join_state;
 	struct list_head	list;
 	struct sockaddr_storage	addr;
+	atomic_t		ref;
 };
 
 struct ucma_event {
@@ -257,6 +258,12 @@ static void ucma_copy_ud_event(struct ib_device *device,
 	dst->qkey = src->qkey;
 }
 
+static void ucma_put_mc(struct ucma_multicast *mc)
+{
+	if (mc && atomic_dec_and_test(&mc->ref))
+		kfree(mc);
+}
+
 static struct ucma_event *ucma_create_uevent(struct ucma_context *ctx,
 					     struct rdma_cm_event *event)
 {
@@ -274,6 +281,7 @@ static struct ucma_event *ucma_create_uevent(struct ucma_context *ctx,
 			     event->param.ud.private_data;
 		uevent->resp.uid = uevent->mc->uid;
 		uevent->resp.id = uevent->mc->id;
+		ucma_put_mc(uevent->mc);
 		break;
 	default:
 		uevent->resp.uid = ctx->uid;
@@ -1471,6 +1479,7 @@ static ssize_t ucma_process_join(struct ucma_file *file,
 	mc->ctx = ctx;
 	mc->join_state = join_state;
 	mc->uid = cmd->uid;
+	atomic_set(&mc->ref, 1);
 	memcpy(&mc->addr, addr, cmd->addr_size);
 
 	xa_lock(&multicast_table);
@@ -1489,6 +1498,7 @@ static ssize_t ucma_process_join(struct ucma_file *file,
 	mutex_unlock(&ctx->mutex);
 	if (ret)
 		goto err_xa_erase;
+	atomic_inc(&mc->ref);
 
 	resp.id = mc->id;
 	if (copy_to_user(u64_to_user_ptr(cmd->response),
@@ -1513,7 +1523,7 @@ static ssize_t ucma_process_join(struct ucma_file *file,
 	__xa_erase(&multicast_table, mc->id);
 err_free_mc:
 	xa_unlock(&multicast_table);
-	kfree(mc);
+	ucma_put_mc(mc);
 err_put_ctx:
 	ucma_put_ctx(ctx);
 	return ret;