tracefs: Reset permissions on remount if permissions are options

There's an inconsistency with the way permissions are handled in tracefs.
Because the permissions are generated when accessed, they default to the
root inode's permission if they were never set by the user. If the user
sets the permissions, then a flag is set and the permissions are saved via
the inode (for tracefs files) or an internal attribute field (for
eventfs).

But if a remount happens that specify the permissions, all the files that
were not changed by the user gets updated, but the ones that were are not.
If the user were to remount the file system with a given permission, then
all files and directories within that file system should be updated.

This can cause security issues if a file's permission was updated but the
admin forgot about it. They could incorrectly think that remounting with
permissions set would update all files, but miss some.

For example:

 # cd /sys/kernel/tracing
 # chgrp 1002 current_tracer
 # ls -l
[..]
 -rw-r-----  1 root root 0 May  1 21:25 buffer_size_kb
 -rw-r-----  1 root root 0 May  1 21:25 buffer_subbuf_size_kb
 -r--r-----  1 root root 0 May  1 21:25 buffer_total_size_kb
 -rw-r-----  1 root lkp  0 May  1 21:25 current_tracer
 -rw-r-----  1 root root 0 May  1 21:25 dynamic_events
 -r--r-----  1 root root 0 May  1 21:25 dyn_ftrace_total_info
 -r--r-----  1 root root 0 May  1 21:25 enabled_functions

Where current_tracer now has group "lkp".

 # mount -o remount,gid=1001 .
 # ls -l
 -rw-r-----  1 root tracing 0 May  1 21:25 buffer_size_kb
 -rw-r-----  1 root tracing 0 May  1 21:25 buffer_subbuf_size_kb
 -r--r-----  1 root tracing 0 May  1 21:25 buffer_total_size_kb
 -rw-r-----  1 root lkp     0 May  1 21:25 current_tracer
 -rw-r-----  1 root tracing 0 May  1 21:25 dynamic_events
 -r--r-----  1 root tracing 0 May  1 21:25 dyn_ftrace_total_info
 -r--r-----  1 root tracing 0 May  1 21:25 enabled_functions

Everything changed but the "current_tracer".

Add a new link list that keeps track of all the tracefs_inodes which has
the permission flags that tell if the file/dir should use the root inode's
permission or not. Then on remount, clear all the flags so that the
default behavior of using the root inode's permission is done for all
files and directories.

Link: https://lore.kernel.org/linux-trace-kernel/20240502200905.529542160@goodmis.org

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Fixes: 8186fff7ab649 ("tracefs/eventfs: Use root and instance inodes as default ownership")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

1 files changed, 29 insertions, 0 deletions

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index cc8b838bbe62..15a2a9c3c62b 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -308,6 +308,35 @@ static const struct file_operations eventfs_file_operations = {
 	.llseek		= generic_file_llseek,
 };
 
+/*
+ * On a remount of tracefs, if UID or GID options are set, then
+ * the mount point inode permissions should be used.
+ * Reset the saved permission flags appropriately.
+ */
+void eventfs_remount(struct tracefs_inode *ti, bool update_uid, bool update_gid)
+{
+	struct eventfs_inode *ei = ti->private;
+
+	if (!ei)
+		return;
+
+	if (update_uid)
+		ei->attr.mode &= ~EVENTFS_SAVE_UID;
+
+	if (update_gid)
+		ei->attr.mode &= ~EVENTFS_SAVE_GID;
+
+	if (!ei->entry_attrs)
+		return;
+
+	for (int i = 0; i < ei->nr_entries; i++) {
+		if (update_uid)
+			ei->entry_attrs[i].mode &= ~EVENTFS_SAVE_UID;
+		if (update_gid)
+			ei->entry_attrs[i].mode &= ~EVENTFS_SAVE_GID;
+	}
+}
+
 /* Return the evenfs_inode of the "events" directory */
 static struct eventfs_inode *eventfs_find_events(struct dentry *dentry)
 {