sunrpc: init gssp_lock before publishing proc entry

create_use_gss_proxy_proc_entry() publishes /proc/net/rpc/use-gss-proxy
via proc_create_data() before init_gssp_clnt() runs mutex_init() on
sn->gssp_lock.  Once the dentry is linked under proc_subdir_lock it is
immediately reachable from userspace, so a write that lands in the
window drives set_gssp_clnt() into mutex_lock() on a zero-initialized
struct mutex.

    create_use_gss_proxy_proc_entry(net)
      proc_create_data("use-gss-proxy", ...)   /* dentry live */
      init_gssp_clnt(sn)
        mutex_init(&sn->gssp_lock)             /* too late */

    write_gssp()
      set_gssp_clnt(net)
        mutex_lock(&sn->gssp_lock)             /* uninitialized */
        gssp_rpc_create(...)
        sn->gssp_clnt = clnt
        mutex_unlock(&sn->gssp_lock)

The window spans only the two statements between proc_create_data()
returning and init_gssp_clnt(), so a writer reaches it only if the
registering thread is preempted there while another task is already
opening the freshly published file.  register_pernet_subsys() runs in
preemptible context under pernet_ops_rwsem, so that preemption is
possible, and the window widens on auth_rpcgss module load, when the
proc entry is created for every live net namespace whose tasks are
already running.  A writer that wins the race locks a zero-filled
struct mutex.  On CONFIG_DEBUG_MUTEXES the missing magic value trips a
"lock used without init" splat; on a production kernel the fast path
acquires the lock via CMPXCHG(owner, 0, current).  In the latter case
a second writer that arrives before init_gssp_clnt() re-zeroes owner
can enter set_gssp_clnt() concurrently, shut down the first writer's
clnt while it is still in use, and leak the loser's clnt.

Fix by initializing sn->gssp_lock in sunrpc_init_net() so its lifetime
matches the sunrpc_net it lives in.  sn->gssp_clnt is already NULL from
the kzalloc that backs net_generic storage, so the lazy helper is no
longer needed; drop init_gssp_clnt(), its prototype, and the call from
create_use_gss_proxy_proc_entry().  sunrpc.ko is a build-time
dependency of auth_rpcgss.ko, so sunrpc_init_net() has always run on
every netns before any auth_gss pernet init can publish the proc
entry.

Fixes: 030d794bf4 ("SUNRPC: Use gssproxy upcall for server RPCGSS authentication.")
Cc: stable@vger.kernel.org
Assisted-by: kres:claude-opus-4-7
Signed-off-by: Chris Mason <clm@meta.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260530-tier2-local-v2-1-5a0fd532db57@oracle.com
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/auth_gss/gss_rpc_upcall.c

@@ -121,12 +121,6 @@ static int gssp_rpc_create(struct net *net, struct rpc_clnt **_clnt)
 	return result;
 }
 
-void init_gssp_clnt(struct sunrpc_net *sn)
-{
-	mutex_init(&sn->gssp_lock);
-	sn->gssp_clnt = NULL;
-}
-
 int set_gssp_clnt(struct net *net)
 {
 	struct sunrpc_net *sn = net_generic(net, sunrpc_net_id);

net/sunrpc/auth_gss/gss_rpc_upcall.h

@@ -29,7 +29,6 @@ int gssp_accept_sec_context_upcall(struct net *net,
 				struct gssp_upcall_data *data);
 void gssp_free_upcall_data(struct gssp_upcall_data *data);
 
-void init_gssp_clnt(struct sunrpc_net *);
 int set_gssp_clnt(struct net *);
 void clear_gssp_clnt(struct sunrpc_net *);
 

net/sunrpc/auth_gss/svcauth_gss.c

@@ -1468,7 +1468,6 @@ static int create_use_gss_proxy_proc_entry(struct net *net)
 			      &use_gss_proxy_proc_ops, net);
 	if (!*p)
 		return -ENOMEM;
-	init_gssp_clnt(sn);
 	return 0;
 }
 

net/sunrpc/sunrpc_syms.c

@@ -57,6 +57,7 @@ static __net_init int sunrpc_init_net(struct net *net)
 	INIT_LIST_HEAD(&sn->all_clients);
 	spin_lock_init(&sn->rpc_client_lock);
 	spin_lock_init(&sn->rpcb_clnt_lock);
+	mutex_init(&sn->gssp_lock);
 	return 0;
 
 err_pipefs: