[PATCH] nbd: defer config unlock in nbd_genl_connect

[PATCH] nbd: defer config unlock in nbd_genl_connect

[Zheng Qixing]

From: Zheng Qixing <zhengqixing@huawei.com>

There is one use-after-free warning when running NBD_CMD_CONNECT and
NBD_CLEAR_SOCK:

nbd_genl_connect
  nbd_alloc_and_init_config // config_refs=1
  nbd_start_device // config_refs=2
  set NBD_RT_HAS_CONFIG_REF			open nbd // config_refs=3
  recv_work done // config_refs=2
						NBD_CLEAR_SOCK // config_refs=1
						close nbd // config_refs=0
  refcount_inc -> uaf

------------[ cut here ]------------
refcount_t: addition on 0; use-after-free.
WARNING: CPU: 24 PID: 1014 at lib/refcount.c:25 refcount_warn_saturate+0x12e/0x290
 nbd_genl_connect+0x16d0/0x1ab0
 genl_family_rcv_msg_doit+0x1f3/0x310
 genl_rcv_msg+0x44a/0x790

The issue can be easily reproduced by adding a small delay before
refcount_inc(&nbd->config_refs) in nbd_genl_connect():

        mutex_unlock(&nbd->config_lock);
        if (!ret) {
                set_bit(NBD_RT_HAS_CONFIG_REF, &config->runtime_flags);
+               printk("before sleep\n");
+               mdelay(5 * 1000);
+               printk("after sleep\n");
                refcount_inc(&nbd->config_refs);
                nbd_connect_reply(info, nbd->index);
        }

Fixes: e46c7287b1c2 ("nbd: add a basic netlink interface")
Signed-off-by: Zheng Qixing <zhengqixing@huawei.com>
---
 drivers/block/nbd.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 215fc18115b7..a05ff68e58d0 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -2241,12 +2241,13 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
 
 	ret = nbd_start_device(nbd);
 out:
-	mutex_unlock(&nbd->config_lock);
 	if (!ret) {
 		set_bit(NBD_RT_HAS_CONFIG_REF, &config->runtime_flags);
 		refcount_inc(&nbd->config_refs);
 		nbd_connect_reply(info, nbd->index);
 	}
+	mutex_unlock(&nbd->config_lock);
+
 	nbd_config_put(nbd);
 	if (put_dev)
 		nbd_put(nbd);
-- 
2.39.2

Re: [PATCH] nbd: defer config unlock in nbd_genl_connect

[Yu Kuai]

在 2025/11/10 20:49, Zheng Qixing 写道:

> There is one use-after-free warning when running NBD_CMD_CONNECT and
> NBD_CLEAR_SOCK:
>
> nbd_genl_connect
>    nbd_alloc_and_init_config // config_refs=1
>    nbd_start_device // config_refs=2
>    set NBD_RT_HAS_CONFIG_REF			open nbd // config_refs=3
>    recv_work done // config_refs=2
> 						NBD_CLEAR_SOCK // config_refs=1
> 						close nbd // config_refs=0
>    refcount_inc -> uaf
>
> ------------[ cut here ]------------
> refcount_t: addition on 0; use-after-free.
> WARNING: CPU: 24 PID: 1014 at lib/refcount.c:25 refcount_warn_saturate+0x12e/0x290
>   nbd_genl_connect+0x16d0/0x1ab0
>   genl_family_rcv_msg_doit+0x1f3/0x310
>   genl_rcv_msg+0x44a/0x790
>
> The issue can be easily reproduced by adding a small delay before
> refcount_inc(&nbd->config_refs) in nbd_genl_connect():
>
>          mutex_unlock(&nbd->config_lock);
>          if (!ret) {
>                  set_bit(NBD_RT_HAS_CONFIG_REF, &config->runtime_flags);
> +               printk("before sleep\n");
> +               mdelay(5 * 1000);
> +               printk("after sleep\n");
>                  refcount_inc(&nbd->config_refs);
>                  nbd_connect_reply(info, nbd->index);
>          }
>
> Fixes: e46c7287b1c2 ("nbd: add a basic netlink interface")
> Signed-off-by: Zheng Qixing<zhengqixing@huawei.com>
> ---
>   drivers/block/nbd.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)

LGTM
Reviewed-by: Yu Kuai <yukuai@fnnas.com>

Re: [PATCH] nbd: defer config unlock in nbd_genl_connect

[Jens Axboe]

On Mon, 10 Nov 2025 20:49:20 +0800, Zheng Qixing wrote:
> There is one use-after-free warning when running NBD_CMD_CONNECT and
> NBD_CLEAR_SOCK:
> 
> nbd_genl_connect
>   nbd_alloc_and_init_config // config_refs=1
>   nbd_start_device // config_refs=2
>   set NBD_RT_HAS_CONFIG_REF			open nbd // config_refs=3
>   recv_work done // config_refs=2
> 						NBD_CLEAR_SOCK // config_refs=1
> 						close nbd // config_refs=0
>   refcount_inc -> uaf
> 
> [...]

Applied, thanks!

[1/1] nbd: defer config unlock in nbd_genl_connect
      commit: 1649714b930f9ea6233ce0810ba885999da3b5d4

Best regards,
-- 
Jens Axboe