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

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Message-ID: <hzxqc3tbxc7bd6s3qv3hyocxvhuh4zszkvkqkvrmefhc5qrlez@5yroq663nxpj>
Date: Thu, 19 Jun 2025 16:57:34 +0200
From: Jan Kara <jack@...e.cz>
To: avinashlalotra <abinashlalotra@...il.com>
Cc: jack@...e.cz, 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

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