[RFC PATCH v2 03/11] dns_resolver: Clear .dns_resolver cache when the last client unloads

[Jarkko Sakkinen <jarkko@kernel.org>]

dns_query() caches resolved dns_resolver keys into the module-owned
.dns_resolver keyring until the TTL expires or module_exit() revokes the
keyring.

Once live keys pin their key type, cached entries keep dns_resolver.ko
pinned after its client modules unload.

Track active kernel client modules with dns_resolver_cache_get() and
dns_resolver_cache_put(), and do the necessary plumbing for:

1. nfsv4
2. cifs
3. libceph
4. kafs

When the last client unloads, clear .dns_resolver and flush key garbage
collection so dns_resolver.ko can unload immediately.

Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
---
 fs/afs/main.c                |  4 ++++
 fs/nfs/dns_resolve.h         |  7 ++++++-
 fs/smb/client/cifsfs.c       |  4 ++++
 include/linux/dns_resolver.h | 13 ++++++++++---
 net/ceph/ceph_common.c       |  7 +++++++
 net/dns_resolver/dns_key.c   | 28 ++++++++++++++++++++++++++++
 6 files changed, 59 insertions(+), 4 deletions(-)

diff --git a/fs/afs/main.c b/fs/afs/main.c
index 7a883c59976f..16ae947d371e 100644
--- a/fs/afs/main.c
+++ b/fs/afs/main.c
@@ -12,6 +12,7 @@
 #include <linux/sched.h>
 #include <linux/random.h>
 #include <linux/proc_fs.h>
+#include <linux/dns_resolver.h>
 #define CREATE_TRACE_POINTS
 #include "internal.h"
 
@@ -183,6 +184,7 @@ static int __init afs_init(void)
 	if (ret < 0)
 		goto error_net;
 
+	dns_resolver_cache_get();
 	/* register the filesystems */
 	ret = afs_fs_init();
 	if (ret < 0)
@@ -199,6 +201,7 @@ static int __init afs_init(void)
 error_proc:
 	afs_fs_exit();
 error_fs:
+	dns_resolver_cache_put();
 	unregister_pernet_device(&afs_net_ops);
 error_net:
 	destroy_workqueue(afs_lock_manager);
@@ -232,6 +235,7 @@ static void __exit afs_exit(void)
 	destroy_workqueue(afs_wq);
 	afs_clean_up_permit_cache();
 	rcu_barrier();
+	dns_resolver_cache_put();
 }
 
 module_exit(afs_exit);
diff --git a/fs/nfs/dns_resolve.h b/fs/nfs/dns_resolve.h
index fe3b172c4de1..4b9ecfb4f7fe 100644
--- a/fs/nfs/dns_resolve.h
+++ b/fs/nfs/dns_resolve.h
@@ -9,13 +9,18 @@
 
 
 #ifdef CONFIG_NFS_USE_KERNEL_DNS
+#include <linux/dns_resolver.h>
+
 static inline int nfs_dns_resolver_init(void)
 {
+	dns_resolver_cache_get();
 	return 0;
 }
 
 static inline void nfs_dns_resolver_destroy(void)
-{}
+{
+	dns_resolver_cache_put();
+}
 
 static inline int nfs_dns_resolver_cache_init(struct net *net)
 {
diff --git a/fs/smb/client/cifsfs.c b/fs/smb/client/cifsfs.c
index bf199a5ca084..9788052599ce 100644
--- a/fs/smb/client/cifsfs.c
+++ b/fs/smb/client/cifsfs.c
@@ -31,6 +31,7 @@
 #include <linux/xattr.h>
 #include <linux/mm.h>
 #include <linux/key-type.h>
+#include <linux/dns_resolver.h>
 #include <linux/fileattr.h>
 #include <uapi/linux/magic.h>
 #include <net/ipv6.h>
@@ -2160,6 +2161,7 @@ init_cifs(void)
 		goto out_cifs_swn_init;
 
 	cifs_proc_init();
+	dns_resolver_cache_get();
 
 	rc = register_filesystem(&cifs_fs_type);
 	if (rc)
@@ -2174,6 +2176,7 @@ init_cifs(void)
 	return 0;
 
 out_clean_proc:
+	dns_resolver_cache_put();
 	cifs_proc_clean();
 	exit_cifs_idmap();
 out_cifs_swn_init:
@@ -2242,6 +2245,7 @@ exit_cifs(void)
 	destroy_workqueue(serverclose_wq);
 	destroy_workqueue(cfid_put_wq);
 	destroy_workqueue(cifsiod_wq);
+	dns_resolver_cache_put();
 }
 
 MODULE_AUTHOR("Steve French");
diff --git a/include/linux/dns_resolver.h b/include/linux/dns_resolver.h
index 976cbbdb2832..cc11e09b0df3 100644
--- a/include/linux/dns_resolver.h
+++ b/include/linux/dns_resolver.h
@@ -27,8 +27,15 @@
 #include <uapi/linux/dns_resolver.h>
 
 struct net;
-extern int dns_query(struct net *net, const char *type, const char *name, size_t namelen,
-		     const char *options, char **_result, time64_t *_expiry,
-		     bool invalidate);
+#if IS_ENABLED(CONFIG_DNS_RESOLVER)
+int dns_query(struct net *net, const char *type, const char *name, size_t namelen,
+	      const char *options, char **_result, time64_t *_expiry,
+	      bool invalidate);
+void dns_resolver_cache_get(void);
+void dns_resolver_cache_put(void);
+#else
+static inline void dns_resolver_cache_get(void) { }
+static inline void dns_resolver_cache_put(void) { }
+#endif
 
 #endif /* _LINUX_DNS_RESOLVER_H */
diff --git a/net/ceph/ceph_common.c b/net/ceph/ceph_common.c
index a797c7360e3c..df62a269cb73 100644
--- a/net/ceph/ceph_common.c
+++ b/net/ceph/ceph_common.c
@@ -8,6 +8,7 @@
 #include <linux/in6.h>
 #include <linux/key.h>
 #include <keys/ceph-type.h>
+#include <linux/dns_resolver.h>
 #include <linux/module.h>
 #include <linux/mount.h>
 #include <linux/nsproxy.h>
@@ -890,6 +891,9 @@ static int __init init_ceph_lib(void)
 	if (ret < 0)
 		goto out_msgr;
 
+#ifdef CONFIG_CEPH_LIB_USE_DNS_RESOLVER
+	dns_resolver_cache_get();
+#endif
 	pr_info("loaded (mon/osd proto %d/%d)\n",
 		CEPH_MONC_PROTOCOL, CEPH_OSDC_PROTOCOL);
 
@@ -913,6 +917,9 @@ static void __exit exit_ceph_lib(void)
 	ceph_msgr_exit();
 	ceph_crypto_shutdown();
 	ceph_debugfs_cleanup();
+#ifdef CONFIG_CEPH_LIB_USE_DNS_RESOLVER
+	dns_resolver_cache_put();
+#endif
 }
 
 module_init(init_ceph_lib);
diff --git a/net/dns_resolver/dns_key.c b/net/dns_resolver/dns_key.c
index 451247864a63..63f4fac0dbc8 100644
--- a/net/dns_resolver/dns_key.c
+++ b/net/dns_resolver/dns_key.c
@@ -16,6 +16,7 @@
 #include <linux/kernel.h>
 #include <linux/keyctl.h>
 #include <linux/err.h>
+#include <linux/mutex.h>
 #include <linux/seq_file.h>
 #include <linux/dns_resolver.h>
 #include <keys/dns_resolver-type.h>
@@ -31,6 +32,8 @@ module_param_named(debug, dns_resolver_debug, uint, 0644);
 MODULE_PARM_DESC(debug, "DNS Resolver debugging mask");
 
 const struct cred *dns_resolver_cache;
+static DEFINE_MUTEX(dns_resolver_cache_mutex);
+static unsigned int dns_resolver_cache_users;
 
 #define	DNS_ERRORNO_OPTION	"dnserror"
 
@@ -306,6 +309,31 @@ static long dns_resolver_read(const struct key *key,
 	return user_read(key, buffer, buflen);
 }
 
+void dns_resolver_cache_get(void)
+{
+	mutex_lock(&dns_resolver_cache_mutex);
+	dns_resolver_cache_users++;
+	mutex_unlock(&dns_resolver_cache_mutex);
+}
+EXPORT_SYMBOL_GPL(dns_resolver_cache_get);
+
+void dns_resolver_cache_put(void)
+{
+	int ret;
+
+	mutex_lock(&dns_resolver_cache_mutex);
+	if (--dns_resolver_cache_users == 0 && dns_resolver_cache) {
+		ret = keyring_clear(dns_resolver_cache->thread_keyring);
+		if (ret)
+			pr_warn_ratelimited("dns_resolver: Failed to clear cache: %d\n",
+					    ret);
+		else
+			key_gc_flush();
+	}
+	mutex_unlock(&dns_resolver_cache_mutex);
+}
+EXPORT_SYMBOL_GPL(dns_resolver_cache_put);
+
 struct key_type key_type_dns_resolver = {
 	.name		= "dns_resolver",
 	.flags		= KEY_TYPE_NET_DOMAIN | KEY_TYPE_INSTANT_REAP,
-- 
2.47.3