[PATCH v2] eventfs: Test for ei->is_freed when accessing ei->dentry

[Steven Rostedt <rostedt@goodmis.org>]

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

The eventfs_inode (ei) is protected by SRCU, but the ei->dentry is not. It
is protected by the eventfs_mutex. Anytime the eventfs_mutex is released,
and access to the ei->dentry needs to be done, it should first check if
ei->is_freed is set under the eventfs_mutex. If it is, then the ei->dentry
is invalid and must not be used. The ei->dentry must only be accessed
under the eventfs_mutex and after checking if ei->is_freed is set.

When the ei is being freed, it will (under the eventfs_mutex) set is_freed
and at the same time move the dentry to a free list to be cleared after
the eventfs_mutex is released. This means that any access to the
ei->dentry must check first if ei->is_freed is set, because if it is, then
the dentry is on its way to be freed.

Also add comments to describe this better.

Link: https://lore.kernel.org/all/CA+G9fYt6pY+tMZEOg=SoEywQOe19fGP3uR15SGowkdK+_X85Cg@mail.gmail.com/
Link: https://lore.kernel.org/all/CA+G9fYuDP3hVQ3t7FfrBAjd_WFVSurMgCepTxunSJf=MTe=6aA@mail.gmail.com/

Fixes: 5790b1fb3d672 ("eventfs: Remove eventfs_file and just use eventfs_inode")
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reported-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---

Changes since v1: https://lore.kernel.org/all/20231028163749.0d3429a1@rorschach.local.home/

 - Add comment about ei->is_freed is a union along with ei->rcu and
   ei->del_list so that others can find where ei->is_freed is set and
   not get confused about why ei->dentry is being removed but ei->is_freed
   isn't mentioned.

 - And fixed change log to remove the double "Reported-by".

 fs/tracefs/event_inode.c | 65 +++++++++++++++++++++++++++++++++-------
 fs/tracefs/internal.h    |  3 +-
 2 files changed, 56 insertions(+), 12 deletions(-)

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 4d2da7480e5f..45bddce7c747 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -24,7 +24,20 @@
 #include <linux/delay.h>
 #include "internal.h"
 
+/*
+ * eventfs_mutex protects the eventfs_inode (ei) dentry. Any access
+ * to the ei->dentry must be done under this mutex and after checking
+ * if ei->is_freed is not set. The ei->dentry is released under the
+ * mutex at the same time ei->is_freed is set. If ei->is_freed is set
+ * then the ei->dentry is invalid.
+ */
 static DEFINE_MUTEX(eventfs_mutex);
+
+/*
+ * The eventfs_inode (ei) itself is protected by SRCU. It is released from
+ * its parent's list and will have is_freed set (under eventfs_mutex).
+ * After the SRCU grace period is over, the ei may be freed.
+ */
 DEFINE_STATIC_SRCU(eventfs_srcu);
 
 static struct dentry *eventfs_root_lookup(struct inode *dir,
@@ -234,6 +247,10 @@ create_file_dentry(struct eventfs_inode *ei, struct dentry **e_dentry,
 	bool invalidate = false;
 
 	mutex_lock(&eventfs_mutex);
+	if (ei->is_freed) {
+		mutex_unlock(&eventfs_mutex);
+		return NULL;
+	}
 	/* If the e_dentry already has a dentry, use it */
 	if (*e_dentry) {
 		/* lookup does not need to up the ref count */
@@ -307,6 +324,8 @@ static void eventfs_post_create_dir(struct eventfs_inode *ei)
 	struct eventfs_inode *ei_child;
 	struct tracefs_inode *ti;
 
+	lockdep_assert_held(&eventfs_mutex);
+
 	/* srcu lock already held */
 	/* fill parent-child relation */
 	list_for_each_entry_srcu(ei_child, &ei->children, list,
@@ -320,6 +339,7 @@ static void eventfs_post_create_dir(struct eventfs_inode *ei)
 
 /**
  * create_dir_dentry - Create a directory dentry for the eventfs_inode
+ * @pei: The eventfs_inode parent of ei.
  * @ei: The eventfs_inode to create the directory for
  * @parent: The dentry of the parent of this directory
  * @lookup: True if this is called by the lookup code
@@ -327,12 +347,17 @@ static void eventfs_post_create_dir(struct eventfs_inode *ei)
  * This creates and attaches a directory dentry to the eventfs_inode @ei.
  */
 static struct dentry *
-create_dir_dentry(struct eventfs_inode *ei, struct dentry *parent, bool lookup)
+create_dir_dentry(struct eventfs_inode *pei, struct eventfs_inode *ei,
+		  struct dentry *parent, bool lookup)
 {
 	bool invalidate = false;
 	struct dentry *dentry = NULL;
 
 	mutex_lock(&eventfs_mutex);
+	if (pei->is_freed || ei->is_freed) {
+		mutex_unlock(&eventfs_mutex);
+		return NULL;
+	}
 	if (ei->dentry) {
 		/* If the dentry already has a dentry, use it */
 		dentry = ei->dentry;
@@ -435,7 +460,7 @@ static struct dentry *eventfs_root_lookup(struct inode *dir,
 	 */
 	mutex_lock(&eventfs_mutex);
 	ei = READ_ONCE(ti->private);
-	if (ei)
+	if (ei && !ei->is_freed)
 		ei_dentry = READ_ONCE(ei->dentry);
 	mutex_unlock(&eventfs_mutex);
 
@@ -449,7 +474,7 @@ static struct dentry *eventfs_root_lookup(struct inode *dir,
 		if (strcmp(ei_child->name, name) != 0)
 			continue;
 		ret = simple_lookup(dir, dentry, flags);
-		create_dir_dentry(ei_child, ei_dentry, true);
+		create_dir_dentry(ei, ei_child, ei_dentry, true);
 		created = true;
 		break;
 	}
@@ -583,7 +608,7 @@ static int dcache_dir_open_wrapper(struct inode *inode, struct file *file)
 
 	list_for_each_entry_srcu(ei_child, &ei->children, list,
 				 srcu_read_lock_held(&eventfs_srcu)) {
-		d = create_dir_dentry(ei_child, parent, false);
+		d = create_dir_dentry(ei, ei_child, parent, false);
 		if (d) {
 			ret = add_dentries(&dentries, d, cnt);
 			if (ret < 0)
@@ -637,6 +662,13 @@ static int dcache_readdir_wrapper(struct file *file, struct dir_context *ctx)
 	return ret;
 }
 
+static void free_ei(struct eventfs_inode *ei)
+{
+	kfree_const(ei->name);
+	kfree(ei->d_children);
+	kfree(ei);
+}
+
 /**
  * eventfs_create_dir - Create the eventfs_inode for this directory
  * @name: The name of the directory to create.
@@ -700,12 +732,20 @@ struct eventfs_inode *eventfs_create_dir(const char *name, struct eventfs_inode
 	ei->nr_entries = size;
 	ei->data = data;
 	INIT_LIST_HEAD(&ei->children);
+	INIT_LIST_HEAD(&ei->list);
 
 	mutex_lock(&eventfs_mutex);
-	list_add_tail(&ei->list, &parent->children);
-	ei->d_parent = parent->dentry;
+	if (!parent->is_freed) {
+		list_add_tail(&ei->list, &parent->children);
+		ei->d_parent = parent->dentry;
+	}
 	mutex_unlock(&eventfs_mutex);
 
+	/* Was the parent freed? */
+	if (list_empty(&ei->list)) {
+		free_ei(ei);
+		ei = NULL;
+	}
 	return ei;
 }
 
@@ -787,13 +827,11 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
 	return ERR_PTR(-ENOMEM);
 }
 
-static void free_ei(struct rcu_head *head)
+static void free_rcu_ei(struct rcu_head *head)
 {
 	struct eventfs_inode *ei = container_of(head, struct eventfs_inode, rcu);
 
-	kfree_const(ei->name);
-	kfree(ei->d_children);
-	kfree(ei);
+	free_ei(ei);
 }
 
 /**
@@ -880,7 +918,12 @@ void eventfs_remove_dir(struct eventfs_inode *ei)
 		for (i = 0; i < ei->nr_entries; i++)
 			unhook_dentry(&ei->d_children[i], &dentry_list);
 		unhook_dentry(&ei->dentry, &dentry_list);
-		call_srcu(&eventfs_srcu, &ei->rcu, free_ei);
+		/*
+		 * Note, ei->is_freed is a union along with ei->rcu
+		 * and ei->del_list. When the ei is added to either
+		 * of those lists, it automatically sets ei->is_freed.
+		 */
+		call_srcu(&eventfs_srcu, &ei->rcu, free_rcu_ei);
 	}
 	mutex_unlock(&eventfs_mutex);
 
diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
index 64fde9490f52..21a1fa682b74 100644
--- a/fs/tracefs/internal.h
+++ b/fs/tracefs/internal.h
@@ -30,7 +30,7 @@ struct eventfs_inode {
 	const struct eventfs_entry	*entries;
 	const char			*name;
 	struct list_head		children;
-	struct dentry			*dentry;
+	struct dentry			*dentry; /* Check is_freed to access */
 	struct dentry			*d_parent;
 	struct dentry			**d_children;
 	void				*data;
@@ -39,6 +39,7 @@ struct eventfs_inode {
 	 * @del_list:	list of eventfs_inode to delete
 	 * @rcu:	eventfs_inode to delete in RCU
 	 * @is_freed:	node is freed if one of the above is set
+	 *   Note if is_freed is set, then dentry is corrupted.
 	 */
 	union {
 		struct list_head	del_list;
-- 
2.42.0