Description

In the Linux kernel, the following vulnerability has been resolved:

nbd: don't allow reconnect after disconnect

Following process can cause nbd_config UAF:

1) grab nbd_config temporarily;

2) nbd_genl_disconnect() flush all recv_work() and release the
initial reference:

nbd_genl_disconnect
nbd_disconnect_and_put
nbd_disconnect
flush_workqueue(nbd->recv_workq)
if (test_and_clear_bit(NBD_RT_HAS_CONFIG_REF, ...))
nbd_config_put
-> due to step 1), reference is still not zero

3) nbd_genl_reconfigure() queue recv_work() again;

nbd_genl_reconfigure
config = nbd_get_config_unlocked(nbd)
if (!config)
-> succeed
if (!test_bit(NBD_RT_BOUND, ...))
-> succeed
nbd_reconnect_socket
queue_work(nbd->recv_workq, &args->work)

4) step 1) release the reference;

5) Finially, recv_work() will trigger UAF:

recv_work
nbd_config_put(nbd)
-> nbd_config is freed
atomic_dec(&config->recv_threads)
-> UAF

Fix the problem by clearing NBD_RT_BOUND in nbd_genl_disconnect(), so
that nbd_genl_reconfigure() will fail.

Classification

CVE ID: CVE-2025-21731

Affected Products

Vendor: Linux, Linux

Product: Linux, Linux

Exploit Prediction Scoring System (EPSS)

EPSS Score: 0.02% (probability of being exploited)

EPSS Percentile: 1.85% (scored less or equal to compared to others)

EPSS Date: 2025-03-27 (when was this score calculated)

References

https://nvd.nist.gov/vuln/detail/CVE-2025-21731
https://git.kernel.org/stable/c/e7343fa33751cb07c1c56b666bf37cfca357130e
https://git.kernel.org/stable/c/d208d2c52b652913b5eefc8ca434b0d6b757f68f
https://git.kernel.org/stable/c/a8ee6ecde2b7bfb58c8a3afe8a9d2b848f580739
https://git.kernel.org/stable/c/9793bd5ae4bdbdb2dde401a3cab94a6bfd05e302
https://git.kernel.org/stable/c/844b8cdc681612ff24df62cdefddeab5772fadf1

Timeline