[PATCH 6.1 273/312] ext4: fix use-after-free in update_super_work when racing with umount

Greg Kroah-Hartman
2026-04-08T18:18:34Z

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/ext4/ext4.h | 1 +
fs/ext4/super.c | 1 +
fs/ext4/sysfs.c | 10 +++++++++-
3 files changed, 11 insertions(+), 1 deletion(-)

--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1557,6 +1557,7 @@ 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;

--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -5263,6 +5263,7 @@ static int __ext4_fill_super(struct fs_c

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 */
--- a/fs/ext4/sysfs.c
+++ b/fs/ext4/sysfs.c
@@ -515,7 +515,10 @@ 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;
@@ -528,8 +531,10 @@ int ext4_register_sysfs(struct super_blo
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);
@@ -562,7 +567,10 @@ void ext4_unregister_sysfs(struct super_

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)