CVE-2025-21731

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

nbd: dont 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.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/6bef6222a3f6c7adb6396f77f25a3579d821b09a
• https://git.kernel.org/stable/c/844b8cdc681612ff24df62cdefddeab5772fadf1
• https://git.kernel.org/stable/c/9793bd5ae4bdbdb2dde401a3cab94a6bfd05e302
• https://git.kernel.org/stable/c/a8ee6ecde2b7bfb58c8a3afe8a9d2b848f580739
• https://git.kernel.org/stable/c/d208d2c52b652913b5eefc8ca434b0d6b757f68f
• https://git.kernel.org/stable/c/e3be8862d73cac833e0fb7602636c19c6cb94b11
• https://git.kernel.org/stable/c/e70a578487a47d7cf058904141e586684d1c3381
• https://git.kernel.org/stable/c/e7343fa33751cb07c1c56b666bf37cfca357130e