nfsd: don't ignore the return code of svc_proc_register()

Currently, nfsd_proc_stat_init() ignores the return value of
svc_proc_register(). If the procfile creation fails, then the kernel
will WARN when it tries to remove the entry later.

Fix nfsd_proc_stat_init() to return the same type of pointer as
svc_proc_register(), and fix up nfsd_net_init() to check that and fail
the nfsd_net construction if it occurs.

svc_proc_register() can fail if the dentry can't be allocated, or if an
identical dentry already exists. The second case is pretty unlikely in
the nfsd_net construction codepath, so if this happens, return -ENOMEM.

Reported-by: syzbot+e34ad04f27991521104c@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-nfs/67a47501.050a0220.19061f.05f9.GAE@google.com/
Cc: stable@vger.kernel.org # v6.9
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfsctl.c

@@ -2202,8 +2202,14 @@ static __net_init int nfsd_net_init(struct net *net)
 					  NFSD_STATS_COUNTERS_NUM);
 	if (retval)
 		goto out_repcache_error;
+
 	memset(&nn->nfsd_svcstats, 0, sizeof(nn->nfsd_svcstats));
 	nn->nfsd_svcstats.program = &nfsd_programs[0];
+	if (!nfsd_proc_stat_init(net)) {
+		retval = -ENOMEM;
+		goto out_proc_error;
+	}
+
 	for (i = 0; i < sizeof(nn->nfsd_versions); i++)
 		nn->nfsd_versions[i] = nfsd_support_version(i);
 	for (i = 0; i < sizeof(nn->nfsd4_minorversions); i++)
@@ -2213,13 +2219,14 @@ static __net_init int nfsd_net_init(struct net *net)
 	nfsd4_init_leases_net(nn);
 	get_random_bytes(&nn->siphash_key, sizeof(nn->siphash_key));
 	seqlock_init(&nn->writeverf_lock);
-	nfsd_proc_stat_init(net);
 #if IS_ENABLED(CONFIG_NFS_LOCALIO)
 	spin_lock_init(&nn->local_clients_lock);
 	INIT_LIST_HEAD(&nn->local_clients);
 #endif
 	return 0;
 
+out_proc_error:
+	percpu_counter_destroy_many(nn->counter, NFSD_STATS_COUNTERS_NUM);
 out_repcache_error:
 	nfsd_idmap_shutdown(net);
 out_idmap_error:

fs/nfsd/stats.c

@@ -73,11 +73,11 @@ static int nfsd_show(struct seq_file *seq, void *v)
 
 DEFINE_PROC_SHOW_ATTRIBUTE(nfsd);
 
-void nfsd_proc_stat_init(struct net *net)
+struct proc_dir_entry *nfsd_proc_stat_init(struct net *net)
 {
 	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
 
-	svc_proc_register(net, &nn->nfsd_svcstats, &nfsd_proc_ops);
+	return svc_proc_register(net, &nn->nfsd_svcstats, &nfsd_proc_ops);
 }
 
 void nfsd_proc_stat_shutdown(struct net *net)

fs/nfsd/stats.h

@@ -10,7 +10,7 @@
 #include <uapi/linux/nfsd/stats.h>
 #include <linux/percpu_counter.h>
 
-void nfsd_proc_stat_init(struct net *net);
+struct proc_dir_entry *nfsd_proc_stat_init(struct net *net);
 void nfsd_proc_stat_shutdown(struct net *net);
 
 static inline void nfsd_stats_rc_hits_inc(struct nfsd_net *nn)