[PATCH 6.18.y] NFSD: Guard admin state-revocation walks with NFSD_NET_UP

From: Chuck Lever <cel@kernel.org>

[ Upstream commit 2f3e6638aebc0ab8afb8b4e9816ea9a1cad85378 ]

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.

Reported-by: XIAO WU <redacted>
Fixes: 1ac3629bf012 ("nfsd: prepare for supporting admin-revocation of state")
Cc: stable@vger.kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260621162551.2469460-1-cel@kernel.org (local)
Signed-off-by: Chuck Lever <cel@kernel.org>
[ adapted NFSD_NET_UP flag check to the older nfsd_net_up boolean ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/nfsd/nfsctl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
index 1da5c40d6722a..eb90e31440d68 100644
--- a/fs/nfsd/nfsctl.c
+++ b/fs/nfsd/nfsctl.c

@@ -298,7 +298,7 @@ static ssize_t write_unlock_fs(struct file *file, char *buf, size_t size)
 	error = nlmsvc_unlock_all_by_sb(path.dentry->d_sb);
 	mutex_lock(&nfsd_mutex);
 	nn = net_generic(netns(file), nfsd_net_id);
-	if (nn->nfsd_serv)
+	if (nn->nfsd_net_up)
 		nfsd4_revoke_states(nn, path.dentry->d_sb);
 	else
 		error = -EINVAL;

-- 
2.53.0