[PATCH v4 12/14] digest_cache: Notify digest cache events

[Roberto Sassu]

From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

Introduce digest_cache_register_notifier() and
digest_cache_unregister_notifier() to let the subscribers know about events
occurring on digest caches. Subscribers should provide a callback, to be
invoked for every notification.

Introduce a notification list for each digest cache, and populate it with
inodes passed to digest_cache_get(), when dig_user of that inode is still
NULL so that there are no duplicates.

Then, on setting the RESET bit, emit a DIGEST_CACHE_RESET notification,
passing the affected digest cache and inodes, so that subscribers can
eventually invalidate a cached security decision on that inode.

On a file digest cache reset, emit a DIGEST_CACHE_RESET notification also
for the parent directory digest cache and the inodes using it, since
requestors might have looked up digests through that digest cache. Those
requestors will see changes by performing another lookup.

On setting the RESET_USER bit, emit a notification only for the inode
signaled by the LSM hook, since only the link between that inode and its
digest cache is changing (due to changing the security.digest_list xattr).

Finally, free the notification list with digest_cache_notify_inodes_free(),
when the digest cache is freed.

Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
---
 include/linux/digest_cache.h     |  34 ++++++++
 security/digest_cache/Makefile   |   2 +-
 security/digest_cache/internal.h |  26 ++++++
 security/digest_cache/main.c     |  12 +++
 security/digest_cache/notifier.c | 135 +++++++++++++++++++++++++++++++
 security/digest_cache/reset.c    |  42 +++++++++-
 6 files changed, 248 insertions(+), 3 deletions(-)
 create mode 100644 security/digest_cache/notifier.c

diff --git a/include/linux/digest_cache.h b/include/linux/digest_cache.h
index 9db8128513ca..950f3a58a861 100644
--- a/include/linux/digest_cache.h
+++ b/include/linux/digest_cache.h
@@ -15,6 +15,28 @@
 
 struct digest_cache;
 
+/**
+ * enum digest_cache_event - Events occurring on a digest cache
+ *
+ * This enum lists all the events occurring on a digest cache, to be notified
+ * outside the digest_cache LSM.
+ */
+enum digest_cache_event {
+	DIGEST_CACHE_RESET,
+};
+
+/**
+ * struct digest_cache_event_data - Information on digest cache events
+ * @digest_cache: Digest cache
+ * @inode: Inode for which the digest cache was requested
+ *
+ * This structure holds information about events occurring on a digest cache.
+ */
+struct digest_cache_event_data {
+	struct digest_cache *digest_cache;
+	struct inode *inode;
+};
+
 /**
  * typedef digest_cache_found_t - Digest cache reference as numeric value
  *
@@ -48,6 +70,8 @@ int digest_cache_verif_set(struct file *file, const char *verif_id, void *data,
 			   size_t size);
 void *digest_cache_verif_get(struct digest_cache *digest_cache,
 			     const char *verif_id);
+int digest_cache_register_notifier(struct notifier_block *nb);
+int digest_cache_unregister_notifier(struct notifier_block *nb);
 
 #else
 static inline struct digest_cache *digest_cache_get(struct dentry *dentry)
@@ -79,5 +103,15 @@ static inline void *digest_cache_verif_get(struct digest_cache *digest_cache,
 	return NULL;
 }
 
+static inline int digest_cache_register_notifier(struct notifier_block *nb)
+{
+	return -EOPNOTSUPP;
+}
+
+static inline int digest_cache_unregister_notifier(struct notifier_block *nb)
+{
+	return -EOPNOTSUPP;
+}
+
 #endif /* CONFIG_SECURITY_DIGEST_CACHE */
 #endif /* _LINUX_DIGEST_CACHE_H */
diff --git a/security/digest_cache/Makefile b/security/digest_cache/Makefile
index 3d5e600a2c45..7577b099c170 100644
--- a/security/digest_cache/Makefile
+++ b/security/digest_cache/Makefile
@@ -5,7 +5,7 @@
 obj-$(CONFIG_SECURITY_DIGEST_CACHE) += digest_cache.o
 
 digest_cache-y := main.o secfs.o htable.o populate.o modsig.o verif.o dir.o \
-		  reset.o
+		  reset.o notifier.o
 
 digest_cache-y += parsers/tlv.o
 digest_cache-y += parsers/rpm.o
diff --git a/security/digest_cache/internal.h b/security/digest_cache/internal.h
index c816929c4743..d930132cc963 100644
--- a/security/digest_cache/internal.h
+++ b/security/digest_cache/internal.h
@@ -21,6 +21,19 @@
 #define RESET			4	/* Digest cache to be recreated. */
 #define RESET_USER		5	/* Dig_user pointer to be released. */
 
+/**
+ * struct notify_inode - Structure with inode for which notification is emitted
+ * @list: Linked list
+ * @inode: Inode for which a notification is emitted
+ *
+ * This structure contains an inode for which a notification of a digest cache
+ * event is emitted.
+ */
+struct notify_inode {
+	struct list_head list;
+	struct inode *inode;
+};
+
 /**
  * struct readdir_callback - Structure to store information for dir iteration
  * @ctx: Context structure
@@ -126,6 +139,7 @@ struct htable {
  * @path_str: Path of the digest list the digest cache was created from
  * @flags: Control flags
  * @verif_data: Verification data regarding the digest list
+ * @notify_inodes: List of inodes for which a notification is emitted
  * @mutex: Protect digest cache modifications
  *
  * This structure represents a cache of digests extracted from a digest list.
@@ -137,6 +151,7 @@ struct digest_cache {
 	char *path_str;
 	unsigned long flags;
 	struct list_head verif_data;
+	struct list_head notify_inodes;
 	struct mutex mutex;
 };
 
@@ -261,4 +276,15 @@ void digest_cache_inode_post_setxattr(struct dentry *dentry, const char *name,
 void digest_cache_inode_post_removexattr(struct dentry *dentry,
 					 const char *name);
 
+/* notifier.c */
+int digest_cache_notify_inode_add(struct digest_cache *digest_cache,
+				  struct inode *inode);
+void digest_cache_notify_inodes_free(struct digest_cache *digest_cache);
+void digest_cache_notify(struct digest_cache *digest_cache, struct inode *inode,
+			 enum digest_cache_event event);
+void digest_cache_notify_multiple(struct digest_cache *digest_cache,
+				  enum digest_cache_event event);
+int digest_cache_register_notifier(struct notifier_block *nb);
+int digest_cache_unregister_notifier(struct notifier_block *nb);
+
 #endif /* _DIGEST_CACHE_INTERNAL_H */
diff --git a/security/digest_cache/main.c b/security/digest_cache/main.c
index ce3518a33c80..6fe0864f938f 100644
--- a/security/digest_cache/main.c
+++ b/security/digest_cache/main.c
@@ -51,6 +51,7 @@ static struct digest_cache *digest_cache_alloc_init(char *path_str,
 	INIT_LIST_HEAD(&digest_cache->htables);
 	INIT_LIST_HEAD(&digest_cache->verif_data);
 	INIT_LIST_HEAD(&digest_cache->dir_entries);
+	INIT_LIST_HEAD(&digest_cache->notify_inodes);
 	mutex_init(&digest_cache->mutex);
 
 	pr_debug("New digest cache %s (ref count: %d)\n",
@@ -70,6 +71,7 @@ static void digest_cache_free(struct digest_cache *digest_cache)
 	digest_cache_htable_free(digest_cache);
 	digest_cache_verif_free(digest_cache);
 	digest_cache_dir_free(digest_cache);
+	digest_cache_notify_inodes_free(digest_cache);
 	mutex_destroy(&digest_cache->mutex);
 
 	pr_debug("Freed digest cache %s\n", digest_cache->path_str);
@@ -398,6 +400,7 @@ struct digest_cache *digest_cache_get(struct dentry *dentry)
 	struct digest_cache_security *dig_sec;
 	struct digest_cache *digest_cache = NULL;
 	struct inode *inode = d_backing_inode(dentry);
+	int ret;
 
 	if (!digest_cache_enabled)
 		return NULL;
@@ -420,6 +423,15 @@ struct digest_cache *digest_cache_get(struct dentry *dentry)
 		/* Consume extra reference from digest_cache_create(). */
 		dig_sec->dig_user = digest_cache_new(dentry);
 		up_read(&default_path_sem);
+
+		if (dig_sec->dig_user) {
+			ret = digest_cache_notify_inode_add(dig_sec->dig_user,
+							    inode);
+			if (ret < 0) {
+				digest_cache_put(dig_sec->dig_user);
+				dig_sec->dig_user = NULL;
+			}
+		}
 	}
 
 	if (dig_sec->dig_user)
diff --git a/security/digest_cache/notifier.c b/security/digest_cache/notifier.c
new file mode 100644
index 000000000000..06e4730e4434
--- /dev/null
+++ b/security/digest_cache/notifier.c
@@ -0,0 +1,135 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2024 Huawei Technologies Duesseldorf GmbH
+ *
+ * Author: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
+ *
+ * Implement the notifier of the digest_cache LSM.
+ */
+
+#define pr_fmt(fmt) "DIGEST CACHE: "fmt
+
+#include "internal.h"
+
+static BLOCKING_NOTIFIER_HEAD(chain);
+
+static const char * const events_str[] = {
+	[DIGEST_CACHE_RESET] = __stringify(DIGEST_CACHE_RESET),
+};
+
+/**
+ * digest_cache_notify_inode_add - Add inode to digest cache notification list
+ * @digest_cache: Digest cache
+ * @inode: Inode to add
+ *
+ * This function adds an inode to the digest cache notification list, so that
+ * notifications are emitted for that inode.
+ *
+ * Return: Zero on success, -ENOMEM on error.
+ */
+int digest_cache_notify_inode_add(struct digest_cache *digest_cache,
+				  struct inode *inode)
+{
+	struct notify_inode *notify_inode;
+
+	notify_inode = kmalloc(sizeof(*notify_inode), GFP_KERNEL);
+	if (!notify_inode)
+		return -ENOMEM;
+
+	notify_inode->inode = inode;
+
+	mutex_lock(&digest_cache->mutex);
+	list_add_tail(&notify_inode->list, &digest_cache->notify_inodes);
+	pr_debug("Added inode %lu to notification list of digest cache %s\n",
+		 inode->i_ino, digest_cache->path_str);
+	mutex_unlock(&digest_cache->mutex);
+	return 0;
+}
+
+/**
+ * digest_cache_notify_inodes_free - Free digest cache notification list
+ * @digest_cache: Digest cache
+ *
+ * This function removes all inodes from the notification list of the passed
+ * digest cache and frees the memory. Does not need locking, since it is called
+ * only at the time the digest cache is freed.
+ */
+void digest_cache_notify_inodes_free(struct digest_cache *digest_cache)
+{
+	struct notify_inode *p, *q;
+
+	list_for_each_entry_safe(p, q, &digest_cache->notify_inodes, list) {
+		list_del(&p->list);
+		pr_debug("Removed inode %lu from notification list of digest cache %s\n",
+			 p->inode->i_ino, digest_cache->path_str);
+		kfree(p);
+	}
+}
+
+/**
+ * digest_cache_notify - Emit notification for an inode
+ * @digest_cache: Digest cache
+ * @inode: Inode for which a digest cache is used
+ * @event: Event to notify
+ *
+ * This function emits a notification of a digest cache event for an affected
+ * inode.
+ */
+void digest_cache_notify(struct digest_cache *digest_cache, struct inode *inode,
+			 enum digest_cache_event event)
+{
+	struct digest_cache_event_data event_data = {
+		.digest_cache = digest_cache,
+		.inode = inode,
+	};
+
+	pr_debug("Notify event %s for inode %lu using digest cache %s\n",
+		 events_str[event], inode->i_ino, digest_cache->path_str);
+
+	blocking_notifier_call_chain(&chain, event, &event_data);
+}
+
+/**
+ * digest_cache_notify_multiple - Emit notification for all inodes in the list
+ * @digest_cache: Digest cache
+ * @event: Event to notify
+ *
+ * This function emits a notification for all inodes in the notification list
+ * of the passed digest cache.
+ */
+void digest_cache_notify_multiple(struct digest_cache *digest_cache,
+				  enum digest_cache_event event)
+{
+	struct notify_inode *p;
+
+	list_for_each_entry(p, &digest_cache->notify_inodes, list)
+		digest_cache_notify(digest_cache, p->inode, event);
+}
+
+/**
+ * digest_cache_register_notifier() - Register a digest cache notifier
+ * @nb: Notifier block with the callback
+ *
+ * This function registers a new notifier for events occurring on digest caches.
+ *
+ * Return: Zero on success, -EEXIST on error.
+ */
+int digest_cache_register_notifier(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_register(&chain, nb);
+}
+EXPORT_SYMBOL(digest_cache_register_notifier);
+
+/**
+ * digest_cache_unregister_notifier() - Unregister a digest cache notifier
+ * @nb: Notifier block with the callback
+ *
+ * This function unregisters a previously registered notifier.
+ *
+ * Return: Zero on success, -ENOENT on error.
+ */
+int digest_cache_unregister_notifier(struct notifier_block *nb)
+{
+	return blocking_notifier_chain_unregister(&chain, nb);
+}
+EXPORT_SYMBOL(digest_cache_unregister_notifier);
diff --git a/security/digest_cache/reset.c b/security/digest_cache/reset.c
index e07222b0e0e8..bb3b4108ba3b 100644
--- a/security/digest_cache/reset.c
+++ b/security/digest_cache/reset.c
@@ -10,6 +10,29 @@
 #define pr_fmt(fmt) "DIGEST CACHE: "fmt
 #include "internal.h"
 
+/**
+ * digest_cache_notify_dir_entry_reset - Notify dir entry reset
+ * @dir: Directory containing the digest lists
+ *
+ * Emit a notification for the directory containing the digest lists about a
+ * reset occurring on a directory entry.
+ */
+
+static void digest_cache_notify_dir_entry_reset(struct inode *dir)
+{
+	struct digest_cache_security *dir_dig_sec;
+
+	dir_dig_sec = digest_cache_get_security(dir);
+	if (unlikely(!dir_dig_sec))
+		return;
+
+	mutex_lock(&dir_dig_sec->dig_owner_mutex);
+	if (dir_dig_sec->dig_owner)
+		digest_cache_notify_multiple(dir_dig_sec->dig_owner,
+					     DIGEST_CACHE_RESET);
+	mutex_unlock(&dir_dig_sec->dig_owner_mutex);
+}
+
 /**
  * digest_cache_reset_owner - Reset dig_owner
  * @inode: Inode of the digest list/directory containing the digest list
@@ -19,6 +42,9 @@
  * (if unset), so that digest_cache_get() and digest_cache_create() respectively
  * release and clear dig_user and dig_owner in the inode security blob. This
  * causes new callers of digest_cache_get() to get a new digest cache.
+ *
+ * After setting RESET, it emits a notification for all inodes using the digest
+ * cache.
  */
 static void digest_cache_reset_owner(struct inode *inode, const char *reason)
 {
@@ -30,9 +56,12 @@ static void digest_cache_reset_owner(struct inode *inode, const char *reason)
 
 	mutex_lock(&dig_sec->dig_owner_mutex);
 	if (dig_sec->dig_owner &&
-	    !test_and_set_bit(RESET, &dig_sec->dig_owner->flags))
+	    !test_and_set_bit(RESET, &dig_sec->dig_owner->flags)) {
 		pr_debug("Resetting %s (dig_owner), reason: %s\n",
 			 dig_sec->dig_owner->path_str, reason);
+		digest_cache_notify_multiple(dig_sec->dig_owner,
+					     DIGEST_CACHE_RESET);
+	}
 	mutex_unlock(&dig_sec->dig_owner_mutex);
 }
 
@@ -55,9 +84,12 @@ static void digest_cache_reset_user(struct inode *inode, const char *reason)
 
 	mutex_lock(&dig_sec->dig_user_mutex);
 	if (dig_sec->dig_user &&
-	    !test_and_set_bit(RESET_USER, &dig_sec->dig_user->flags))
+	    !test_and_set_bit(RESET_USER, &dig_sec->dig_user->flags)) {
 		pr_debug("Resetting %s (dig_user), reason: %s\n",
 			 dig_sec->dig_user->path_str, reason);
+		digest_cache_notify(dig_sec->dig_user, inode,
+				    DIGEST_CACHE_RESET);
+	}
 	mutex_unlock(&dig_sec->dig_user_mutex);
 }
 
@@ -74,11 +106,14 @@ static void digest_cache_reset_user(struct inode *inode, const char *reason)
 int digest_cache_path_truncate(const struct path *path)
 {
 	struct inode *inode = d_backing_inode(path->dentry);
+	struct inode *dir = d_backing_inode(path->dentry->d_parent);
 
 	if (!S_ISREG(inode->i_mode))
 		return 0;
 
 	digest_cache_reset_owner(inode, "file_truncate");
+	/* Dir digest cache users should know a dir entry changed. */
+	digest_cache_notify_dir_entry_reset(dir);
 	return 0;
 }
 
@@ -101,6 +136,9 @@ void digest_cache_file_release(struct file *file)
 	digest_cache_reset_owner(file_inode(file), "file_file_release");
 	if (file->f_mode & FMODE_CREATED)
 		digest_cache_reset_owner(dir, "dir_file_release");
+	else
+		/* Dir digest cache users should know a dir entry changed. */
+		digest_cache_notify_dir_entry_reset(dir);
 }
 
 /**
-- 
2.34.1