Re: [PATCH v1] ext4: fix use-after-free in update_super_work when racing with umount

[Jiayuan Chen]

On 3/17/26 7:38 PM, Jan Kara wrote:
> On Fri 13-03-26 14:52:04, Jiayuan Chen wrote:
> > 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:
> >
> >    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)
> >
> > The original blamed commit needed procfs removal before the work
> > flush to prevent /proc/fs/ext4/xx/mb_groups reads from queuing new error
> > work. But it bundled procfs removal and kobject_del together in
> > ext4_unregister_sysfs(), causing the sysfs kobject to be torn down too
> > early.
> >
> > The correct teardown ordering has three constraints:
> >
> >    1. procfs removal must happen before flushing s_sb_upd_work, to prevent
> >       /proc reads from queuing new error work that would BUG_ON.
> >    2. sysfs kobject removal must happen after flushing s_sb_upd_work, since
> >       the work calls sysfs_notify() which accesses the kernfs_node.
> >    3. sysfs kobject removal must happen before jbd2_journal_destroy(), since
> >       userspace could read the journal_task sysfs attribute and dereference
> >       j_task after the journal thread has been killed.
> >
> > Fix this by:
> >    - Adding ext4_sb_release_proc() to remove procfs entries early.
> >    - Splitting ext4_journal_destroy() into ext4_journal_stop_work() and
> >      ext4_journal_finish(), so that ext4_unregister_sysfs() can be placed
> >      between them to satisfy all three ordering constraints.
> >
> > Fixes: b98535d09179 ("ext4: fix bug_on in start_this_handle during umount filesystem")
> > Cc: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>
> > Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>
> Thanks for the analysis and the patch! I fully agree with your analysis but
> I think your solution shows that the teardown sequence is just too subtle
> (too many different dependencies). Ideally, we'd instead modify
> ext4_notify_error_sysfs() to detect sysfs is already torn down by
> ext4_unregister_sysfs() and do nothing. We can check
> sbi->s_kobj.state_in_sysfs for that. The only trouble with this is that
> sysfs_notify() could still race with kobject_del() so we also need some
> locking in ext4_unregister_sysfs() locking out parallel
> ext4_notify_error_sysfs() and we probably need to introduce a separate
> mutex for that. What do you think?
>
> 								Honza


Hi Honza,

Thanks for the suggestion !

I tried the approach you described — having ext4_notify_error_sysfs() 
check s_kobj.state_in_sysfs and using a
dedicated mutex to serialize against kobject_del() in
ext4_unregister_sysfs(). It is indeed much simpler than my original
approach of reordering the teardown sequence.

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index b76966dc06c0..c012e85d2515 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -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;

diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
index d2ecc1026c0c..f2505988e010 100644
--- a/fs/ext4/sysfs.c
+++ b/fs/ext4/sysfs.c
@@ -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;
@@ -610,6 +613,7 @@ int ext4_register_sysfs(struct super_block *sb)
        int err;

        init_completion(&sbi->s_kobj_unregister);
+       mutex_init(&sbi->s_error_notify_mutex);
        err = kobject_init_and_add(&sbi->s_kobj, &ext4_sb_ktype, ext4_root,
                                   "%s", sb->s_id);
        if (err) {
@@ -644,7 +648,10 @@ 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)