GHSA-rhhp-94ch-9rf5

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

sock_map: fix a NULL pointer dereference in sock_map_link_update_prog()

The following race condition could trigger a NULL pointer dereference:

sock_map_link_detach(): sock_map_link_update_prog():
mutex_lock(&sockmap_mutex);
...
sockmap_link->map = NULL;
mutex_unlock(&sockmap_mutex);
mutex_lock(&sockmap_mutex);
...
sock_map_prog_link_lookup(sockmap_link->map);
mutex_unlock(&sockmap_mutex);
<continue>

Fix it by adding a NULL pointer check. In this specific case, it makes
no sense to update a link which is being released.