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

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Commit b98535d091 ("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: b98535d091 ("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
This commit is contained in:
Jiayuan Chen
2026-03-19 20:03:35 +08:00
committed by Theodore Ts'o
parent 496bb99b7e
commit d15e4b0a41
3 changed files with 11 additions and 1 deletions
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fs/ext4/ext4.h
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@@ -1570,6 +1570,7 @@ 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;