eventfs: Fix file and directory uid and gid ownership

It was reported that when mounting the tracefs file system with a gid
other than root, the ownership did not carry down to the eventfs directory
due to the dynamic nature of it.

A fix was done to solve this, but it had two issues.

(a) if the attr passed into update_inode_attr() was NULL, it didn't do
    anything. This is true for files that have not had a chown or chgrp
    done to itself or any of its sibling files, as the attr is allocated
    for all children when any one needs it.

 # umount /sys/kernel/tracing
 # mount -o rw,seclabel,relatime,gid=1000 -t tracefs nodev /mnt

 # ls -ld /mnt/events/sched
drwxr-xr-x 28 root rostedt 0 Dec 21 13:12 /mnt/events/sched/

 # ls -ld /mnt/events/sched/sched_switch
drwxr-xr-x 2 root rostedt 0 Dec 21 13:12 /mnt/events/sched/sched_switch/

But when checking the files:

 # ls -l /mnt/events/sched/sched_switch
total 0
-rw-r----- 1 root root 0 Dec 21 13:12 enable
-rw-r----- 1 root root 0 Dec 21 13:12 filter
-r--r----- 1 root root 0 Dec 21 13:12 format
-r--r----- 1 root root 0 Dec 21 13:12 hist
-r--r----- 1 root root 0 Dec 21 13:12 id
-rw-r----- 1 root root 0 Dec 21 13:12 trigger

(b) When the attr does not denote the UID or GID, it defaulted to using
    the parent uid or gid. This is incorrect as changing the parent
    uid or gid will automatically change all its children.

 # chgrp tracing /mnt/events/timer

 # ls -ld /mnt/events/timer
drwxr-xr-x 2 root tracing 0 Dec 21 14:34 /mnt/events/timer

 # ls -l /mnt/events/timer
total 0
-rw-r----- 1 root root    0 Dec 21 14:35 enable
-rw-r----- 1 root root    0 Dec 21 14:35 filter
drwxr-xr-x 2 root tracing 0 Dec 21 14:35 hrtimer_cancel
drwxr-xr-x 2 root tracing 0 Dec 21 14:35 hrtimer_expire_entry
drwxr-xr-x 2 root tracing 0 Dec 21 14:35 hrtimer_expire_exit
drwxr-xr-x 2 root tracing 0 Dec 21 14:35 hrtimer_init
drwxr-xr-x 2 root tracing 0 Dec 21 14:35 hrtimer_start
drwxr-xr-x 2 root tracing 0 Dec 21 14:35 itimer_expire
drwxr-xr-x 2 root tracing 0 Dec 21 14:35 itimer_state
drwxr-xr-x 2 root tracing 0 Dec 21 14:35 tick_stop
drwxr-xr-x 2 root tracing 0 Dec 21 14:35 timer_cancel
drwxr-xr-x 2 root tracing 0 Dec 21 14:35 timer_expire_entry
drwxr-xr-x 2 root tracing 0 Dec 21 14:35 timer_expire_exit
drwxr-xr-x 2 root tracing 0 Dec 21 14:35 timer_init
drwxr-xr-x 2 root tracing 0 Dec 21 14:35 timer_start

At first it was thought that this could be easily fixed by just making the
default ownership of the superblock when it was mounted. But this does not
handle the case of:

 # chgrp tracing instances
 # mkdir instances/foo

If the superblock was used, then the group ownership would be that of what
it was when it was mounted, when it should instead be "tracing".

Instead, set a flag for the top level eventfs directory ("events") to flag
which eventfs_inode belongs to it.

Since the "events" directory's dentry and inode are never freed, it does
not need to use its attr field to restore its mode and ownership. Use the
this eventfs_inode's attr as the default ownership for all the files and
directories underneath it.

When the events eventfs_inode is created, it sets its ownership to its
parent uid and gid. As the events directory is created at boot up before
it gets mounted, this will always be uid=0 and gid=0. If it's created via
an instance, then it will take the ownership of the instance directory.

When the file system is mounted, it will update all the gids if one is
specified. This will have a callback to update the events evenfs_inode's
default entries.

When a file or directory is created under the events directory, it will
walk the ei->dentry parents until it finds the evenfs_inode that belongs
to the events directory to retrieve the default uid and gid values.

Link: https://lore.kernel.org/all/CAHk-=wiwQtUHvzwyZucDq8=Gtw+AnwScyLhpFswrQ84PjhoGsg@mail.gmail.com/
Link: https://lore.kernel.org/linux-trace-kernel/20231221190757.7eddbca9@gandalf.local.home

Cc: stable@vger.kernel.org
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Dongliang Cui <cuidongliang390@gmail.com>
Cc: Hongyu Jin  <hongyu.jin@unisoc.com>
Fixes: 0dfc852b ("eventfs: Have event files and directories default to parent uid and gid")
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Tested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

fs/tracefs/event_inode.c

@@ -113,6 +113,13 @@ static int eventfs_set_attr(struct mnt_idmap *idmap, struct dentry *dentry,
 	 * determined by the parent directory.
 	 */
 	if (dentry->d_inode->i_mode & S_IFDIR) {
+		/*
+		 * The events directory dentry is never freed, unless its
+		 * part of an instance that is deleted. It's attr is the
+		 * default for its child files and directories.
+		 * Do not update it. It's not used for its own mode or ownership
+		 */
+		if (!ei->is_events)
 			update_attr(&ei->attr, iattr);
 
 	} else {
@@ -148,28 +155,93 @@ static const struct file_operations eventfs_file_operations = {
 	.release	= eventfs_release,
 };
 
+/* Return the evenfs_inode of the "events" directory */
+static struct eventfs_inode *eventfs_find_events(struct dentry *dentry)
+{
+	struct eventfs_inode *ei;
+
+	mutex_lock(&eventfs_mutex);
+	do {
+		/* The parent always has an ei, except for events itself */
+		ei = dentry->d_parent->d_fsdata;
+
+		/*
+		 * If the ei is being freed, the ownership of the children
+		 * doesn't matter.
+		 */
+		if (ei->is_freed) {
+			ei = NULL;
+			break;
+		}
+
+		dentry = ei->dentry;
+	} while (!ei->is_events);
+	mutex_unlock(&eventfs_mutex);
+
+	return ei;
+}
+
 static void update_inode_attr(struct dentry *dentry, struct inode *inode,
 			      struct eventfs_attr *attr, umode_t mode)
 {
-	if (!attr) {
+	struct eventfs_inode *events_ei = eventfs_find_events(dentry);
+
+	if (!events_ei)
+		return;
+
 	inode->i_mode = mode;
+	inode->i_uid = events_ei->attr.uid;
+	inode->i_gid = events_ei->attr.gid;
+
+	if (!attr)
 		return;
-	}
 
 	if (attr->mode & EVENTFS_SAVE_MODE)
 		inode->i_mode = attr->mode & EVENTFS_MODE_MASK;
-	else
-		inode->i_mode = mode;
 
 	if (attr->mode & EVENTFS_SAVE_UID)
 		inode->i_uid = attr->uid;
-	else
-		inode->i_uid = d_inode(dentry->d_parent)->i_uid;
 
 	if (attr->mode & EVENTFS_SAVE_GID)
 		inode->i_gid = attr->gid;
-	else
-		inode->i_gid = d_inode(dentry->d_parent)->i_gid;
+}
+
+static void update_gid(struct eventfs_inode *ei, kgid_t gid, int level)
+{
+	struct eventfs_inode *ei_child;
+
+	/* at most we have events/system/event */
+	if (WARN_ON_ONCE(level > 3))
+		return;
+
+	ei->attr.gid = gid;
+
+	if (ei->entry_attrs) {
+		for (int i = 0; i < ei->nr_entries; i++) {
+			ei->entry_attrs[i].gid = gid;
+		}
+	}
+
+	/*
+	 * Only eventfs_inode with dentries are updated, make sure
+	 * all eventfs_inodes are updated. If one of the children
+	 * do not have a dentry, this function must traverse it.
+	 */
+	list_for_each_entry_srcu(ei_child, &ei->children, list,
+				 srcu_read_lock_held(&eventfs_srcu)) {
+		if (!ei_child->dentry)
+			update_gid(ei_child, gid, level + 1);
+	}
+}
+
+void eventfs_update_gid(struct dentry *dentry, kgid_t gid)
+{
+	struct eventfs_inode *ei = dentry->d_fsdata;
+	int idx;
+
+	idx = srcu_read_lock(&eventfs_srcu);
+	update_gid(ei, gid, 0);
+	srcu_read_unlock(&eventfs_srcu, idx);
 }
 
 /**
@@ -860,6 +932,8 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
 	struct eventfs_inode *ei;
 	struct tracefs_inode *ti;
 	struct inode *inode;
+	kuid_t uid;
+	kgid_t gid;
 
 	if (security_locked_down(LOCKDOWN_TRACEFS))
 		return NULL;
@@ -884,11 +958,20 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
 	ei->dentry = dentry;
 	ei->entries = entries;
 	ei->nr_entries = size;
+	ei->is_events = 1;
 	ei->data = data;
 	ei->name = kstrdup_const(name, GFP_KERNEL);
 	if (!ei->name)
 		goto fail;
 
+	/* Save the ownership of this directory */
+	uid = d_inode(dentry->d_parent)->i_uid;
+	gid = d_inode(dentry->d_parent)->i_gid;
+
+	/* This is used as the default ownership of the files and directories */
+	ei->attr.uid = uid;
+	ei->attr.gid = gid;
+
 	INIT_LIST_HEAD(&ei->children);
 	INIT_LIST_HEAD(&ei->list);
 
@@ -897,6 +980,8 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
 	ti->private = ei;
 
 	inode->i_mode = S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO;
+	inode->i_uid = uid;
+	inode->i_gid = gid;
 	inode->i_op = &eventfs_root_dir_inode_operations;
 	inode->i_fop = &eventfs_file_operations;
 

fs/tracefs/inode.c

@@ -210,6 +210,7 @@ static void set_gid(struct dentry *parent, kgid_t gid)
 	next = this_parent->d_subdirs.next;
 resume:
 	while (next != &this_parent->d_subdirs) {
+		struct tracefs_inode *ti;
 		struct list_head *tmp = next;
 		struct dentry *dentry = list_entry(tmp, struct dentry, d_child);
 		next = tmp->next;
@@ -218,6 +219,11 @@ static void set_gid(struct dentry *parent, kgid_t gid)
 
 		change_gid(dentry, gid);
 
+		/* If this is the events directory, update that too */
+		ti = get_tracefs(dentry->d_inode);
+		if (ti && (ti->flags & TRACEFS_EVENT_INODE))
+			eventfs_update_gid(dentry, gid);
+
 		if (!list_empty(&dentry->d_subdirs)) {
 			spin_unlock(&this_parent->d_lock);
 			spin_release(&dentry->d_lock.dep_map, _RET_IP_);

fs/tracefs/internal.h

@@ -62,6 +62,7 @@ struct eventfs_inode {
 		struct rcu_head		rcu;
 	};
 	unsigned int			is_freed:1;
+	unsigned int			is_events:1;
 	unsigned int			nr_entries:31;
 };
 
@@ -77,6 +78,7 @@ struct inode *tracefs_get_inode(struct super_block *sb);
 struct dentry *eventfs_start_creating(const char *name, struct dentry *parent);
 struct dentry *eventfs_failed_creating(struct dentry *dentry);
 struct dentry *eventfs_end_creating(struct dentry *dentry);
+void eventfs_update_gid(struct dentry *dentry, kgid_t gid);
 void eventfs_set_ei_status_free(struct tracefs_inode *ti, struct dentry *dentry);
 
 #endif /* _TRACEFS_INTERNAL_H */