[PATCH -next v3 1/6] nbd: call genl_unregister_family() first in nbd_cleanup()

Yu Kuai
2022-05-21T07:24:19Z

Otherwise there may be race between module removal and the handling of
netlink command, which can lead to the oops as shown below:

BUG: kernel NULL pointer dereference, address: 0000000000000098
Oops: 0002 [#1] SMP PTI
CPU: 1 PID: 31299 Comm: nbd-client Tainted: G E 5.14.0-rc4
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
RIP: 0010:down_write+0x1a/0x50
Call Trace:
start_creating+0x89/0x130
debugfs_create_dir+0x1b/0x130
nbd_start_device+0x13d/0x390 [nbd]
nbd_genl_connect+0x42f/0x748 [nbd]
genl_family_rcv_msg_doit.isra.0+0xec/0x150
genl_rcv_msg+0xe5/0x1e0
netlink_rcv_skb+0x55/0x100
genl_rcv+0x29/0x40
netlink_unicast+0x1a8/0x250
netlink_sendmsg+0x21b/0x430
____sys_sendmsg+0x2a4/0x2d0
___sys_sendmsg+0x81/0xc0
__sys_sendmsg+0x62/0xb0
__x64_sys_sendmsg+0x1f/0x30
do_syscall_64+0x3b/0xc0
entry_SYSCALL_64_after_hwframe+0x44/0xae
Modules linked in: nbd(E-)

Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
---
drivers/block/nbd.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index ac8b045c777c..a73e853f5833 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -2528,6 +2528,12 @@ static void __exit nbd_cleanup(void)
struct nbd_device *nbd;
LIST_HEAD(del_list);

+ /*
+ * Unregister netlink interface prior to waiting
+ * for the completion of netlink commands.
+ */
+ genl_unregister_family(&nbd_genl_family);
+
nbd_dbg_close();

mutex_lock(&nbd_index_mutex);
@@ -2546,7 +2552,6 @@ static void __exit nbd_cleanup(void)
destroy_workqueue(nbd_del_wq);

idr_destroy(&nbd_index_idr);
- genl_unregister_family(&nbd_genl_family);
unregister_blkdev(NBD_MAJOR, "nbd");
}

--
2.31.1