ext4: fix use-after-free in update_super_work when racing with umount

From: Jiayuan Chen <[email protected]>

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 <[email protected]>

Suggested-by Jan Kara <[email protected]>

Signed-off-by Jiayuan Chen <[email protected]>

---
v2 -> v3: Fix issues reported by AI.
https://sashiko.dev/#/patchset/20260319073651.79209-1-jiayuan.chen%40linux.dev

v1 -> v2: Use state_in_sysfs instead of reordering the teardown sequence.
https://lore.kernel.org/linux-ext4/[email protected]/

---
 fs/ext4/ext4.h  |  1 +
 fs/ext4/super.c |  1 +
 fs/ext4/sysfs.c | 10 +++++++++-
 3 files changed, 11 insertions(+), 1 deletion(-)

fs/ext4/ext4.h +1 -0

index b76966dc06c0..c012e85d2515 100644
@@ -1574,6 +1574,7 @@ 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;
 

fs/ext4/super.c +1 -0

index 752f414aa06b..b2d58a9b0413 100644
@@ -5397,6 +5397,7 @@ static int __ext4_fill_super(struct fs_context *fc, struct super_block *sb)
 
 	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_sb_upd_work, update_super_work);
 
 	err = ext4_group_desc_init(sb, es, logical_sb_block, &first_not_zeroed);

fs/ext4/sysfs.c +9 -1

index d2ecc1026c0c..0503bf02aea1 100644
@@ -597,7 +597,10 @@ static const 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;

@@ -610,8 +613,10 @@ 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);

@@ -644,7 +649,10 @@ 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)

Signature · 1 line

2.43.0

From: Jiayuan Chen <[email protected]>

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)

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():

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