watch_queue: Add a key/keyring notification facility

Add a key/keyring change notification facility whereby notifications about
changes in key and keyring content and attributes can be received.

Firstly, an event queue needs to be created:

	pipe2(fds, O_NOTIFICATION_PIPE);
	ioctl(fds[1], IOC_WATCH_QUEUE_SET_SIZE, 256);

then a notification can be set up to report notifications via that queue:

	struct watch_notification_filter filter = {
		.nr_filters = 1,
		.filters = {
			[0] = {
				.type = WATCH_TYPE_KEY_NOTIFY,
				.subtype_filter[0] = UINT_MAX,
			},
		},
	};
	ioctl(fds[1], IOC_WATCH_QUEUE_SET_FILTER, &filter);
	keyctl_watch_key(KEY_SPEC_SESSION_KEYRING, fds[1], 0x01);

After that, records will be placed into the queue when events occur in
which keys are changed in some way.  Records are of the following format:

	struct key_notification {
		struct watch_notification watch;
		__u32	key_id;
		__u32	aux;
	} *n;

Where:

	n->watch.type will be WATCH_TYPE_KEY_NOTIFY.

	n->watch.subtype will indicate the type of event, such as
	NOTIFY_KEY_REVOKED.

	n->watch.info & WATCH_INFO_LENGTH will indicate the length of the
	record.

	n->watch.info & WATCH_INFO_ID will be the second argument to
	keyctl_watch_key(), shifted.

	n->key will be the ID of the affected key.

	n->aux will hold subtype-dependent information, such as the key
	being linked into the keyring specified by n->key in the case of
	NOTIFY_KEY_LINKED.

Note that it is permissible for event records to be of variable length -
or, at least, the length may be dependent on the subtype.  Note also that
the queue can be shared between multiple notifications of various types.

Signed-off-by: David Howells <[email protected]>
Reviewed-by: James Morris <[email protected]>

Documentation/security/keys/core.rst

@@ -1026,6 +1026,63 @@ The keyctl syscall functions are:
      written into the output buffer.  Verification returns 0 on success.
 
 
+  *  Watch a key or keyring for changes::
+
+	long keyctl(KEYCTL_WATCH_KEY, key_serial_t key, int queue_fd,
+		    const struct watch_notification_filter *filter);
+
+     This will set or remove a watch for changes on the specified key or
+     keyring.
+
+     "key" is the ID of the key to be watched.
+
+     "queue_fd" is a file descriptor referring to an open "/dev/watch_queue"
+     which manages the buffer into which notifications will be delivered.
+
+     "filter" is either NULL to remove a watch or a filter specification to
+     indicate what events are required from the key.
+
+     See Documentation/watch_queue.rst for more information.
+
+     Note that only one watch may be emplaced for any particular { key,
+     queue_fd } combination.
+
+     Notification records look like::
+
+	struct key_notification {
+		struct watch_notification watch;
+		__u32	key_id;
+		__u32	aux;
+	};
+
+     In this, watch::type will be "WATCH_TYPE_KEY_NOTIFY" and subtype will be
+     one of::
+
+	NOTIFY_KEY_INSTANTIATED
+	NOTIFY_KEY_UPDATED
+	NOTIFY_KEY_LINKED
+	NOTIFY_KEY_UNLINKED
+	NOTIFY_KEY_CLEARED
+	NOTIFY_KEY_REVOKED
+	NOTIFY_KEY_INVALIDATED
+	NOTIFY_KEY_SETATTR
+
+     Where these indicate a key being instantiated/rejected, updated, a link
+     being made in a keyring, a link being removed from a keyring, a keyring
+     being cleared, a key being revoked, a key being invalidated or a key
+     having one of its attributes changed (user, group, perm, timeout,
+     restriction).
+
+     If a watched key is deleted, a basic watch_notification will be issued
+     with "type" set to WATCH_TYPE_META and "subtype" set to
+     watch_meta_removal_notification.  The watchpoint ID will be set in the
+     "info" field.
+
+     This needs to be configured by enabling:
+
+	"Provide key/keyring change notifications" (KEY_NOTIFICATIONS)
+
+
 Kernel Services
 ===============
 

include/linux/key.h

@@ -176,6 +176,9 @@ struct key {
 		struct list_head graveyard_link;
 		struct rb_node	serial_node;
 	};
+#ifdef CONFIG_KEY_NOTIFICATIONS
+	struct watch_list	*watchers;	/* Entities watching this key for changes */
+#endif
 	struct rw_semaphore	sem;		/* change vs change sem */
 	struct key_user		*user;		/* owner of this key */
 	void			*security;	/* security data for this key */

include/uapi/linux/keyctl.h

@@ -69,6 +69,7 @@
 #define KEYCTL_RESTRICT_KEYRING		29	/* Restrict keys allowed to link to a keyring */
 #define KEYCTL_MOVE			30	/* Move keys between keyrings */
 #define KEYCTL_CAPABILITIES		31	/* Find capabilities of keyrings subsystem */
+#define KEYCTL_WATCH_KEY		32	/* Watch a key or ring of keys for changes */
 
 /* keyctl structures */
 struct keyctl_dh_params {
@@ -130,5 +131,6 @@ struct keyctl_pkey_params {
 #define KEYCTL_CAPS0_MOVE		0x80 /* KEYCTL_MOVE supported */
 #define KEYCTL_CAPS1_NS_KEYRING_NAME	0x01 /* Keyring names are per-user_namespace */
 #define KEYCTL_CAPS1_NS_KEY_TAG		0x02 /* Key indexing can include a namespace tag */
+#define KEYCTL_CAPS1_NOTIFICATIONS	0x04 /* Keys generate watchable notifications */
 
 #endif /*  _LINUX_KEYCTL_H */

include/uapi/linux/watch_queue.h

@@ -13,7 +13,8 @@
 
 enum watch_notification_type {
 	WATCH_TYPE_META		= 0,	/* Special record */
-	WATCH_TYPE__NR		= 1
+	WATCH_TYPE_KEY_NOTIFY	= 1,	/* Key change event notification */
+	WATCH_TYPE__NR		= 2
 };
 
 enum watch_meta_notification_subtype {
@@ -75,4 +76,29 @@ struct watch_notification_removal {
 	__u64	id;		/* Type-dependent identifier */
 };
 
+/*
+ * Type of key/keyring change notification.
+ */
+enum key_notification_subtype {
+	NOTIFY_KEY_INSTANTIATED	= 0, /* Key was instantiated (aux is error code) */
+	NOTIFY_KEY_UPDATED	= 1, /* Key was updated */
+	NOTIFY_KEY_LINKED	= 2, /* Key (aux) was added to watched keyring */
+	NOTIFY_KEY_UNLINKED	= 3, /* Key (aux) was removed from watched keyring */
+	NOTIFY_KEY_CLEARED	= 4, /* Keyring was cleared */
+	NOTIFY_KEY_REVOKED	= 5, /* Key was revoked */
+	NOTIFY_KEY_INVALIDATED	= 6, /* Key was invalidated */
+	NOTIFY_KEY_SETATTR	= 7, /* Key's attributes got changed */
+};
+
+/*
+ * Key/keyring notification record.
+ * - watch.type = WATCH_TYPE_KEY_NOTIFY
+ * - watch.subtype = enum key_notification_type
+ */
+struct key_notification {
+	struct watch_notification watch;
+	__u32	key_id;		/* The key/keyring affected */
+	__u32	aux;		/* Per-type auxiliary data */
+};
+
 #endif /* _UAPI_LINUX_WATCH_QUEUE_H */

security/keys/Kconfig

@@ -116,3 +116,12 @@ config KEY_DH_OPERATIONS
 	 in the kernel.
 
 	 If you are unsure as to whether this is required, answer N.
+
+config KEY_NOTIFICATIONS
+	bool "Provide key/keyring change notifications"
+	depends on KEYS && WATCH_QUEUE
+	help
+	  This option provides support for getting change notifications on keys
+	  and keyrings on which the caller has View permission.  This makes use
+	  of the /dev/watch_queue misc device to handle the notification
+	  buffer and provides KEYCTL_WATCH_KEY to enable/disable watches.

security/keys/compat.c

@@ -156,6 +156,9 @@ COMPAT_SYSCALL_DEFINE5(keyctl, u32, option,
 	case KEYCTL_CAPABILITIES:
 		return keyctl_capabilities(compat_ptr(arg2), arg3);
 
+	case KEYCTL_WATCH_KEY:
+		return keyctl_watch_key(arg2, arg3, arg4);
+
 	default:
 		return -EOPNOTSUPP;
 	}

security/keys/gc.c

@@ -131,6 +131,11 @@ static noinline void key_gc_unused_keys(struct list_head *keys)
 		kdebug("- %u", key->serial);
 		key_check(key);
 
+#ifdef CONFIG_KEY_NOTIFICATIONS
+		remove_watch_list(key->watchers, key->serial);
+		key->watchers = NULL;
+#endif
+
 		/* Throw away the key data if the key is instantiated */
 		if (state == KEY_IS_POSITIVE && key->type->destroy)
 			key->type->destroy(key);

security/keys/internal.h

@@ -15,6 +15,7 @@
 #include <linux/task_work.h>
 #include <linux/keyctl.h>
 #include <linux/refcount.h>
+#include <linux/watch_queue.h>
 #include <linux/compat.h>
 #include <linux/mm.h>
 #include <linux/vmalloc.h>
@@ -99,7 +100,8 @@ extern int __key_link_begin(struct key *keyring,
 			    const struct keyring_index_key *index_key,
 			    struct assoc_array_edit **_edit);
 extern int __key_link_check_live_key(struct key *keyring, struct key *key);
-extern void __key_link(struct key *key, struct assoc_array_edit **_edit);
+extern void __key_link(struct key *keyring, struct key *key,
+		       struct assoc_array_edit **_edit);
 extern void __key_link_end(struct key *keyring,
 			   const struct keyring_index_key *index_key,
 			   struct assoc_array_edit *edit);
@@ -183,6 +185,23 @@ extern int key_task_permission(const key_ref_t key_ref,
 			       const struct cred *cred,
 			       key_perm_t perm);
 
+static inline void notify_key(struct key *key,
+			      enum key_notification_subtype subtype, u32 aux)
+{
+#ifdef CONFIG_KEY_NOTIFICATIONS
+	struct key_notification n = {
+		.watch.type	= WATCH_TYPE_KEY_NOTIFY,
+		.watch.subtype	= subtype,
+		.watch.info	= watch_sizeof(n),
+		.key_id		= key_serial(key),
+		.aux		= aux,
+	};
+
+	post_watch_notification(key->watchers, &n.watch, current_cred(),
+				n.key_id);
+#endif
+}
+
 /*
  * Check to see whether permission is granted to use a key in the desired way.
  */
@@ -333,6 +352,15 @@ static inline long keyctl_pkey_e_d_s(int op,
 
 extern long keyctl_capabilities(unsigned char __user *_buffer, size_t buflen);
 
+#ifdef CONFIG_KEY_NOTIFICATIONS
+extern long keyctl_watch_key(key_serial_t, int, int);
+#else
+static inline long keyctl_watch_key(key_serial_t key_id, int watch_fd, int watch_id)
+{
+	return -EOPNOTSUPP;
+}
+#endif
+
 /*
  * Debugging key validation
  */

security/keys/key.c

@@ -444,6 +444,7 @@ static int __key_instantiate_and_link(struct key *key,
 			/* mark the key as being instantiated */
 			atomic_inc(&key->user->nikeys);
 			mark_key_instantiated(key, 0);
+			notify_key(key, NOTIFY_KEY_INSTANTIATED, 0);
 
 			if (test_and_clear_bit(KEY_FLAG_USER_CONSTRUCT, &key->flags))
 				awaken = 1;
@@ -453,7 +454,7 @@ static int __key_instantiate_and_link(struct key *key,
 				if (test_bit(KEY_FLAG_KEEP, &keyring->flags))
 					set_bit(KEY_FLAG_KEEP, &key->flags);
 
-				__key_link(key, _edit);
+				__key_link(keyring, key, _edit);
 			}
 
 			/* disable the authorisation key */
@@ -601,6 +602,7 @@ int key_reject_and_link(struct key *key,
 		/* mark the key as being negatively instantiated */
 		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);
 
@@ -611,7 +613,7 @@ int key_reject_and_link(struct key *key,
 
 		/* and link it into the destination keyring */
 		if (keyring && link_ret == 0)
-			__key_link(key, &edit);
+			__key_link(keyring, key, &edit);
 
 		/* disable the authorisation key */
 		if (authkey)
@@ -764,9 +766,11 @@ static inline key_ref_t __key_update(key_ref_t key_ref,
 	down_write(&key->sem);
 
 	ret = key->type->update(key, prep);
-	if (ret == 0)
+	if (ret == 0) {
 		/* Updating a negative key positively instantiates it */
 		mark_key_instantiated(key, 0);
+		notify_key(key, NOTIFY_KEY_UPDATED, 0);
+	}
 
 	up_write(&key->sem);
 
@@ -1023,9 +1027,11 @@ int key_update(key_ref_t key_ref, const void *payload, size_t plen)
 	down_write(&key->sem);
 
 	ret = key->type->update(key, &prep);
-	if (ret == 0)
+	if (ret == 0) {
 		/* Updating a negative key positively instantiates it */
 		mark_key_instantiated(key, 0);
+		notify_key(key, NOTIFY_KEY_UPDATED, 0);
+	}
 
 	up_write(&key->sem);
 
@@ -1057,15 +1063,17 @@ void key_revoke(struct key *key)
 	 *   instantiated
 	 */
 	down_write_nested(&key->sem, 1);
-	if (!test_and_set_bit(KEY_FLAG_REVOKED, &key->flags) &&
-	    key->type->revoke)
-		key->type->revoke(key);
-
-	/* set the death time to no more than the expiry time */
-	time = ktime_get_real_seconds();
-	if (key->revoked_at == 0 || key->revoked_at > time) {
-		key->revoked_at = time;
-		key_schedule_gc(key->revoked_at + key_gc_delay);
+	if (!test_and_set_bit(KEY_FLAG_REVOKED, &key->flags)) {
+		notify_key(key, NOTIFY_KEY_REVOKED, 0);
+		if (key->type->revoke)
+			key->type->revoke(key);
+
+		/* set the death time to no more than the expiry time */
+		time = ktime_get_real_seconds();
+		if (key->revoked_at == 0 || key->revoked_at > time) {
+			key->revoked_at = time;
+			key_schedule_gc(key->revoked_at + key_gc_delay);
+		}
 	}
 
 	up_write(&key->sem);
@@ -1087,8 +1095,10 @@ void key_invalidate(struct key *key)
 
 	if (!test_bit(KEY_FLAG_INVALIDATED, &key->flags)) {
 		down_write_nested(&key->sem, 1);
-		if (!test_and_set_bit(KEY_FLAG_INVALIDATED, &key->flags))
+		if (!test_and_set_bit(KEY_FLAG_INVALIDATED, &key->flags)) {
+			notify_key(key, NOTIFY_KEY_INVALIDATED, 0);
 			key_schedule_gc_links();
+		}
 		up_write(&key->sem);
 	}
 }