In the Linux kernel, the following vulnerability has been resolved: NFSD: Guard admin state-revocation walks with NFSD_NET_UP Writing to /

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

NFSD: Guard admin state-revocation walks with NFSD_NET_UP

Writing to /proc/fs/nfsd/unlock_filesystem, or sending the
NFSD_CMD_UNLOCK_FILESYSTEM or NFSD_CMD_UNLOCK_EXPORT netlink command,
walks the NFSv4 client hash tables to revoke open state and cancel
async COPY operations. All three handlers gate that walk on
nn->nfsd_serv, but a listener added via portlist or netlink
listener_set sets nn->nfsd_serv before any nfsd thread starts.
nfsd_startup_net() has not yet allocated nn->conf_id_hashtbl, so the
walkers dereference a NULL table. A local administrator with
CAP_SYS_ADMIN can crash the kernel this way without ever starting the
server.

nn->nfsd_serv is set when the service is created, which precedes
table allocation. NFSD_NET_UP instead brackets the window where the
tables are live: set at the end of nfsd_startup_net() and cleared in
nfsd_shutdown_net() after they are freed, both under nfsd_mutex.
Gating the three unlock paths on NFSD_NET_UP fixes the startup-time
NULL dereference while preserving the earlier post-shutdown
use-after-free fix.