keys, dns: Allow key types (eg. DNS) to be reclaimed immediately on expiry

If a key has an expiration time, then when that time passes, the key is
left around for a certain amount of time before being collected (5 mins by
default) so that EKEYEXPIRED can be returned instead of ENOKEY.  This is a
problem for DNS keys because we want to redo the DNS lookup immediately at
that point.

Fix this by allowing key types to be marked such that keys of that type
don't have this extra period, but are reclaimed as soon as they expire and
turn this on for dns_resolver-type keys.  To make this easier to handle,
key->expiry is changed to be permanent if TIME64_MAX rather than 0.

Furthermore, give such new-style negative DNS results a 1s default expiry
if no other expiry time is set rather than allowing it to stick around
indefinitely.  This shouldn't be zero as ls will follow a failing stat call
immediately with a second with AT_SYMLINK_NOFOLLOW added.

Fixes: 1a4240f4 ("DNS: Separate out CIFS DNS Resolver code")
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Markus Suvanto <markus.suvanto@gmail.com>
cc: Wang Lei <wang840925@gmail.com>
cc: Jeff Layton <jlayton@redhat.com>
cc: Steve French <smfrench@gmail.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jarkko Sakkinen <jarkko@kernel.org>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: linux-afs@lists.infradead.org
cc: linux-cifs@vger.kernel.org
cc: linux-nfs@vger.kernel.org
cc: ceph-devel@vger.kernel.org
cc: keyrings@vger.kernel.org
cc: netdev@vger.kernel.org

include/linux/key-type.h

@@ -73,6 +73,7 @@ struct key_type {
 
 	unsigned int flags;
 #define KEY_TYPE_NET_DOMAIN	0x00000001 /* Keys of this type have a net namespace domain */
+#define KEY_TYPE_INSTANT_REAP	0x00000002 /* Keys of this type don't have a delay after expiring */
 
 	/* vet a description */
 	int (*vet_description)(const char *description);

net/dns_resolver/dns_key.c

@@ -91,6 +91,7 @@ const struct cred *dns_resolver_cache;
 static int
 dns_resolver_preparse(struct key_preparsed_payload *prep)
 {
+	const struct dns_server_list_v1_header *v1;
 	const struct dns_payload_header *bin;
 	struct user_key_payload *upayload;
 	unsigned long derrno;
@@ -122,6 +123,13 @@ dns_resolver_preparse(struct key_preparsed_payload *prep)
 			return -EINVAL;
 		}
 
+		v1 = (const struct dns_server_list_v1_header *)bin;
+		if ((v1->status != DNS_LOOKUP_GOOD &&
+		     v1->status != DNS_LOOKUP_GOOD_WITH_BAD)) {
+			if (prep->expiry == TIME64_MAX)
+				prep->expiry = ktime_get_real_seconds() + 1;
+		}
+
 		result_len = datalen;
 		goto store_result;
 	}
@@ -314,7 +322,7 @@ static long dns_resolver_read(const struct key *key,
 
 struct key_type key_type_dns_resolver = {
 	.name		= "dns_resolver",
-	.flags		= KEY_TYPE_NET_DOMAIN,
+	.flags		= KEY_TYPE_NET_DOMAIN | KEY_TYPE_INSTANT_REAP,
 	.preparse	= dns_resolver_preparse,
 	.free_preparse	= dns_resolver_free_preparse,
 	.instantiate	= generic_key_instantiate,

security/keys/gc.c

@@ -66,6 +66,19 @@ void key_schedule_gc(time64_t gc_at)
 	}
 }
 
+/*
+ * Set the expiration time on a key.
+ */
+void key_set_expiry(struct key *key, time64_t expiry)
+{
+	key->expiry = expiry;
+	if (expiry != TIME64_MAX) {
+		if (!(key->type->flags & KEY_TYPE_INSTANT_REAP))
+			expiry += key_gc_delay;
+		key_schedule_gc(expiry);
+	}
+}
+
 /*
  * Schedule a dead links collection run.
  */
@@ -176,7 +189,6 @@ static void key_garbage_collector(struct work_struct *work)
 	static u8 gc_state;		/* Internal persistent state */
 #define KEY_GC_REAP_AGAIN	0x01	/* - Need another cycle */
 #define KEY_GC_REAPING_LINKS	0x02	/* - We need to reap links */
-#define KEY_GC_SET_TIMER	0x04	/* - We need to restart the timer */
 #define KEY_GC_REAPING_DEAD_1	0x10	/* - We need to mark dead keys */
 #define KEY_GC_REAPING_DEAD_2	0x20	/* - We need to reap dead key links */
 #define KEY_GC_REAPING_DEAD_3	0x40	/* - We need to reap dead keys */
@@ -184,21 +196,17 @@ static void key_garbage_collector(struct work_struct *work)
 
 	struct rb_node *cursor;
 	struct key *key;
-	time64_t new_timer, limit;
+	time64_t new_timer, limit, expiry;
 
 	kenter("[%lx,%x]", key_gc_flags, gc_state);
 
 	limit = ktime_get_real_seconds();
-	if (limit > key_gc_delay)
-		limit -= key_gc_delay;
-	else
-		limit = key_gc_delay;
 
 	/* Work out what we're going to be doing in this pass */
 	gc_state &= KEY_GC_REAPING_DEAD_1 | KEY_GC_REAPING_DEAD_2;
 	gc_state <<= 1;
 	if (test_and_clear_bit(KEY_GC_KEY_EXPIRED, &key_gc_flags))
-		gc_state |= KEY_GC_REAPING_LINKS | KEY_GC_SET_TIMER;
+		gc_state |= KEY_GC_REAPING_LINKS;
 
 	if (test_and_clear_bit(KEY_GC_REAP_KEYTYPE, &key_gc_flags))
 		gc_state |= KEY_GC_REAPING_DEAD_1;
@@ -233,8 +241,11 @@ static void key_garbage_collector(struct work_struct *work)
 			}
 		}
 
-		if (gc_state & KEY_GC_SET_TIMER) {
-			if (key->expiry > limit && key->expiry < new_timer) {
+		expiry = key->expiry;
+		if (expiry != TIME64_MAX) {
+			if (!(key->type->flags & KEY_TYPE_INSTANT_REAP))
+				expiry += key_gc_delay;
+			if (expiry > limit && expiry < new_timer) {
 				kdebug("will expire %x in %lld",
 				       key_serial(key), key->expiry - limit);
 				new_timer = key->expiry;
@@ -276,7 +287,7 @@ static void key_garbage_collector(struct work_struct *work)
 	 */
 	kdebug("pass complete");
 
-	if (gc_state & KEY_GC_SET_TIMER && new_timer != (time64_t)TIME64_MAX) {
+	if (new_timer != TIME64_MAX) {
 		new_timer += key_gc_delay;
 		key_schedule_gc(new_timer);
 	}

security/keys/internal.h

@@ -167,6 +167,7 @@ extern unsigned key_gc_delay;
 extern void keyring_gc(struct key *keyring, time64_t limit);
 extern void keyring_restriction_gc(struct key *keyring,
 				   struct key_type *dead_type);
+void key_set_expiry(struct key *key, time64_t expiry);
 extern void key_schedule_gc(time64_t gc_at);
 extern void key_schedule_gc_links(void);
 extern void key_gc_keytype(struct key_type *ktype);
@@ -215,10 +216,18 @@ extern struct key *key_get_instantiation_authkey(key_serial_t target_id);
  */
 static inline bool key_is_dead(const struct key *key, time64_t limit)
 {
+	time64_t expiry = key->expiry;
+
+	if (expiry != TIME64_MAX) {
+		if (!(key->type->flags & KEY_TYPE_INSTANT_REAP))
+			expiry += key_gc_delay;
+		if (expiry <= limit)
+			return true;
+	}
+
 	return
 		key->flags & ((1 << KEY_FLAG_DEAD) |
 			      (1 << KEY_FLAG_INVALIDATED)) ||
-		(key->expiry > 0 && key->expiry <= limit) ||
 		key->domain_tag->removed;
 }
 

security/keys/key.c

@@ -294,6 +294,7 @@ struct key *key_alloc(struct key_type *type, const char *desc,
 	key->uid = uid;
 	key->gid = gid;
 	key->perm = perm;
+	key->expiry = TIME64_MAX;
 	key->restrict_link = restrict_link;
 	key->last_used_at = ktime_get_real_seconds();
 
@@ -463,10 +464,7 @@ static int __key_instantiate_and_link(struct key *key,
 			if (authkey)
 				key_invalidate(authkey);
 
-			if (prep->expiry != TIME64_MAX) {
-				key->expiry = prep->expiry;
-				key_schedule_gc(prep->expiry + key_gc_delay);
-			}
+			key_set_expiry(key, prep->expiry);
 		}
 	}
 
@@ -606,8 +604,7 @@ int key_reject_and_link(struct key *key,
 		atomic_inc(&key->user->nikeys);
 		mark_key_instantiated(key, -error);
 		notify_key(key, NOTIFY_KEY_INSTANTIATED, -error);
-		key->expiry = ktime_get_real_seconds() + timeout;
-		key_schedule_gc(key->expiry + key_gc_delay);
+		key_set_expiry(key, ktime_get_real_seconds() + timeout);
 
 		if (test_and_clear_bit(KEY_FLAG_USER_CONSTRUCT, &key->flags))
 			awaken = 1;
@@ -723,16 +720,14 @@ struct key_type *key_type_lookup(const char *type)
 
 void key_set_timeout(struct key *key, unsigned timeout)
 {
-	time64_t expiry = 0;
+	time64_t expiry = TIME64_MAX;
 
 	/* make the changes with the locks held to prevent races */
 	down_write(&key->sem);
 
 	if (timeout > 0)
 		expiry = ktime_get_real_seconds() + timeout;
-
-	key->expiry = expiry;
-	key_schedule_gc(key->expiry + key_gc_delay);
+	key_set_expiry(key, expiry);
 
 	up_write(&key->sem);
 }

security/keys/proc.c

@@ -198,7 +198,7 @@ static int proc_keys_show(struct seq_file *m, void *v)
 
 	/* come up with a suitable timeout value */
 	expiry = READ_ONCE(key->expiry);
-	if (expiry == 0) {
+	if (expiry == TIME64_MAX) {
 		memcpy(xbuf, "perm", 5);
 	} else if (now >= expiry) {
 		memcpy(xbuf, "expd", 5);