linux-kernel - Re: [PATCH v2] fsnotify: initialize destroy_next to avoid KMSAN uninit-value warning

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Message-ID: <CAJZ91LB0Sq7X3QUhNOPRXNC8YDgCfZQaPvEn_AP8=JEcfXEe=A@mail.gmail.com>
Date: Fri, 20 Jun 2025 00:16:12 +0530
From: Abinash <abinashlalotra@...il.com>
To: Jan Kara <jack@...e.cz>
Cc: amir73il@...il.com, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, avinashlalotra <abinashsinghlalotra@...il.com>, 
	syzbot+aaeb1646d01d0358cb2a@...kaller.appspotmail.com
Subject: Re: [PATCH v2] fsnotify: initialize destroy_next to avoid KMSAN
 uninit-value warning

Thank you for your response.

You are right — it doesn't make sense at first glance.

I went through the code again and tried to understand the connector's
lifecycle more clearly.
The destroy_next pointer is initialized when detaching the connector
from the inode and before the connector is destroyed.
This bug would only be possible if the connector allocated by
fsnotify_attach_connector_to_object() were to somehow
end up directly in the connector_destroy_list, which it does not.
Before reaching that list, in fsnotify_put_mark(), the function
fsnotify_detach_connector_from_object() nullifies the obj pointer.
After that, the destroy_next pointer is explicitly set to the head of
connector_destroy_list.

So there's no scenario where a connector from
fsnotify_attach_connector_to_object() can land
in connector_destroy_list without destroy_next being initialized...

This suggests that KMSAN might be confused, as the following loop:

while (conn) {
    free = conn;
    conn = conn->destroy_next;
    kmem_cache_free(fsnotify_mark_connector_cachep, free);
}

should not involve any uninitialized value access.


Thank You
regards
Abinash


On Thu, 19 Jun 2025 at 20:27, Jan Kara <jack@...e.cz> wrote:
>
> On Thu 19-06-25 16:21:17, avinashlalotra wrote:
> > KMSAN reported an uninitialized value use in
> > fsnotify_connector_destroy_workfn(), specifically when accessing
> > `conn->destroy_next`:
> >
> >     BUG: KMSAN: uninit-value in fsnotify_connector_destroy_workfn+0x108/0x160
> >     Uninit was created at:
> >      slab_alloc_node mm/slub.c:4197 [inline]
> >      kmem_cache_alloc_noprof+0x81b/0xec0 mm/slub.c:4204
> >      fsnotify_attach_connector_to_object fs/notify/mark.c:663
> >
> > The struct fsnotify_mark_connector was allocated using
> > kmem_cache_alloc(), but the `destroy_next` field was never initialized,
> > leading to a use of uninitialized memory when the work function later
> > traversed the destroy list.
> >
> > Fix this by explicitly initializing `destroy_next` to NULL immediately
> > after allocation.
> >
> > Reported-by: syzbot+aaeb1646d01d0358cb2a@...kaller.appspotmail.com
> > Signed-off-by: abinashlalotra <abinashsinghlalotra@...il.com>
>
> This doesn't make sense. If you checked definition of
> fsnotify_mark_connector you'd see that destroy_next is in union with void
> *obj:
>
>         union {
>                 /* Object pointer [lock] */
>                 void *obj;
>                 /* Used listing heads to free after srcu period expires */
>                 struct fsnotify_mark_connector *destroy_next;
>         };
>
> and we do initialize 'obj' pointer in
> fsnotify_attach_connector_to_object(). So this report was caused either by
> some other memory corruption or KMSAN getting utterly confused...
>
>                                                                 Honza
>
> >
> > ---
> > v2: Corrected the syzbot Reported-by email address.
> > ---
> >  fs/notify/mark.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/fs/notify/mark.c b/fs/notify/mark.c
> > index 798340db69d7..28013046f732 100644
> > --- a/fs/notify/mark.c
> > +++ b/fs/notify/mark.c
> > @@ -665,6 +665,7 @@ static int fsnotify_attach_connector_to_object(fsnotify_connp_t *connp,
> >               return -ENOMEM;
> >       spin_lock_init(&conn->lock);
> >       INIT_HLIST_HEAD(&conn->list);
> > +     conn->destroy_next = NULL;
> >       conn->flags = 0;
> >       conn->prio = 0;
> >       conn->type = obj_type;
> > --
> > 2.43.0
> >
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR