[PATCH 5.15.y] ext4: fix use-after-free in update_super_work when racing with umount

[Sasha Levin <sashal@kernel.org>]

From: Jiayuan Chen <jiayuan.chen@shopee.com>

[ Upstream commit d15e4b0a418537aafa56b2cb80d44add83e83697 ]

Commit b98535d09179 ("ext4: fix bug_on in start_this_handle during umount
filesystem") moved ext4_unregister_sysfs() before flushing s_sb_upd_work
to prevent new error work from being queued via /proc/fs/ext4/xx/mb_groups
reads during unmount. However, this introduced a use-after-free because
update_super_work calls ext4_notify_error_sysfs() -> sysfs_notify() which
accesses the kobject's kernfs_node after it has been freed by kobject_del()
in ext4_unregister_sysfs():

  update_super_work                ext4_put_super
  -----------------                --------------
                                   ext4_unregister_sysfs(sb)
                                     kobject_del(&sbi->s_kobj)
                                       __kobject_del()
                                         sysfs_remove_dir()
                                           kobj->sd = NULL
                                         sysfs_put(sd)
                                           kernfs_put()  // RCU free
  ext4_notify_error_sysfs(sbi)
    sysfs_notify(&sbi->s_kobj)
      kn = kobj->sd              // stale pointer
      kernfs_get(kn)             // UAF on freed kernfs_node
                                   ext4_journal_destroy()
                                     flush_work(&sbi->s_sb_upd_work)

Instead of reordering the teardown sequence, fix this by making
ext4_notify_error_sysfs() detect that sysfs has already been torn down
by checking s_kobj.state_in_sysfs, and skipping the sysfs_notify() call
in that case. A dedicated mutex (s_error_notify_mutex) serializes
ext4_notify_error_sysfs() against kobject_del() in ext4_unregister_sysfs()
to prevent TOCTOU races where the kobject could be deleted between the
state_in_sysfs check and the sysfs_notify() call.

Fixes: b98535d09179 ("ext4: fix bug_on in start_this_handle during umount filesystem")
Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260319120336.157873-1-jiayuan.chen@linux.dev
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org
[ adapted mutex_init placement ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/ext4/ext4.h  |  1 +
 fs/ext4/super.c |  1 +
 fs/ext4/sysfs.c | 10 +++++++++-
 3 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index d2158866a4b52..d99eabf846b8e 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1540,6 +1540,7 @@ struct ext4_sb_info {
 	struct proc_dir_entry *s_proc;
 	struct kobject s_kobj;
 	struct completion s_kobj_unregister;
+	struct mutex s_error_notify_mutex; /* protects sysfs_notify vs kobject_del */
 	struct super_block *s_sb;
 	struct buffer_head *s_mmp_bh;
 
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 05f1f9b7ad0c4..170bc58b61577 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -4621,6 +4621,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 
 	timer_setup(&sbi->s_err_report, print_daily_error_info, 0);
 	spin_lock_init(&sbi->s_error_lock);
+	mutex_init(&sbi->s_error_notify_mutex);
 	INIT_WORK(&sbi->s_error_work, flush_stashed_error_work);
 
 	/* Register extent status tree shrinker */
diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
index aa07b78ba9104..492f5351e267b 100644
--- a/fs/ext4/sysfs.c
+++ b/fs/ext4/sysfs.c
@@ -513,7 +513,10 @@ static struct kobj_type ext4_feat_ktype = {
 
 void ext4_notify_error_sysfs(struct ext4_sb_info *sbi)
 {
-	sysfs_notify(&sbi->s_kobj, NULL, "errors_count");
+	mutex_lock(&sbi->s_error_notify_mutex);
+	if (sbi->s_kobj.state_in_sysfs)
+		sysfs_notify(&sbi->s_kobj, NULL, "errors_count");
+	mutex_unlock(&sbi->s_error_notify_mutex);
 }
 
 static struct kobject *ext4_root;
@@ -526,8 +529,10 @@ int ext4_register_sysfs(struct super_block *sb)
 	int err;
 
 	init_completion(&sbi->s_kobj_unregister);
+	mutex_lock(&sbi->s_error_notify_mutex);
 	err = kobject_init_and_add(&sbi->s_kobj, &ext4_sb_ktype, ext4_root,
 				   "%s", sb->s_id);
+	mutex_unlock(&sbi->s_error_notify_mutex);
 	if (err) {
 		kobject_put(&sbi->s_kobj);
 		wait_for_completion(&sbi->s_kobj_unregister);
@@ -560,7 +565,10 @@ void ext4_unregister_sysfs(struct super_block *sb)
 
 	if (sbi->s_proc)
 		remove_proc_subtree(sb->s_id, ext4_proc_root);
+
+	mutex_lock(&sbi->s_error_notify_mutex);
 	kobject_del(&sbi->s_kobj);
+	mutex_unlock(&sbi->s_error_notify_mutex);
 }
 
 int __init ext4_init_sysfs(void)
-- 
2.53.0