Linux · Linux Kernel · CVE-2021-46981
**Name of the Vulnerable Software and Affected Versions**
Linux kernel versions prior to 5.11.0-rc5-00040-g76c057c84d28
**Description**
The vulnerability is related to a NULL pointer dereference in the `flush workqueue` function when disconnecting from a network block device (`/dev/nbdX`). The issue occurs when the `config refs` is 1 and the pointers in `nbd device` are still null. This can lead to a kernel crash. The vulnerability can be exploited by an attacker to cause a denial-of-service (DoS) condition.
To fix the issue, add a check for `nbd->recv workq` in the `nbd disconnect and put` function.
**Recommendations**
For Linux kernel versions prior to 5.11.0-rc5-00040-g76c057c84d28, add if `(nbd->recv workq)` to `nbd disconnect and put()`.