Re: [PATCH net] vsock/loopback: use only sk_buff_head.lock to protect the packet queue

[Bobby Eshleman]

On Fri, Mar 24, 2023 at 12:54:50PM +0100, Stefano Garzarella wrote:
> pkt_list_lock was used before commit 71dc9ec9ac7d ("virtio/vsock:
> replace virtio_vsock_pkt with sk_buff") to protect the packet queue.
> After that commit we switched to sk_buff and we are using
> sk_buff_head.lock in almost every place to protect the packet queue
> except in vsock_loopback_work() when we call skb_queue_splice_init().
> 
> As reported by syzbot, this caused unlocked concurrent access to the
> packet queue between vsock_loopback_work() and
> vsock_loopback_cancel_pkt() since it is not holding pkt_list_lock.
> 
> With the introduction of sk_buff_head, pkt_list_lock is redundant and
> can cause confusion, so let's remove it and use sk_buff_head.lock
> everywhere to protect the packet queue access.
> 
> Fixes: 71dc9ec9ac7d ("virtio/vsock: replace virtio_vsock_pkt with sk_buff")
> Cc: bobby.eshleman@xxxxxxxxxxxxx
> Reported-and-tested-by: syzbot+befff0a9536049e7902e@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> ---
>  net/vmw_vsock/vsock_loopback.c | 10 ++--------
>  1 file changed, 2 insertions(+), 8 deletions(-)
> 
> diff --git a/net/vmw_vsock/vsock_loopback.c b/net/vmw_vsock/vsock_loopback.c
> index 671e03240fc5..89905c092645 100644
> --- a/net/vmw_vsock/vsock_loopback.c
> +++ b/net/vmw_vsock/vsock_loopback.c
> @@ -15,7 +15,6 @@
>  struct vsock_loopback {
>  	struct workqueue_struct *workqueue;
>  
> -	spinlock_t pkt_list_lock; /* protects pkt_list */
>  	struct sk_buff_head pkt_queue;
>  	struct work_struct pkt_work;
>  };
> @@ -32,9 +31,7 @@ static int vsock_loopback_send_pkt(struct sk_buff *skb)
>  	struct vsock_loopback *vsock = &the_vsock_loopback;
>  	int len = skb->len;
>  
> -	spin_lock_bh(&vsock->pkt_list_lock);
>  	skb_queue_tail(&vsock->pkt_queue, skb);
> -	spin_unlock_bh(&vsock->pkt_list_lock);
>  
>  	queue_work(vsock->workqueue, &vsock->pkt_work);
>  
> @@ -113,9 +110,9 @@ static void vsock_loopback_work(struct work_struct *work)
>  
>  	skb_queue_head_init(&pkts);
>  
> -	spin_lock_bh(&vsock->pkt_list_lock);
> +	spin_lock_bh(&vsock->pkt_queue.lock);
>  	skb_queue_splice_init(&vsock->pkt_queue, &pkts);
> -	spin_unlock_bh(&vsock->pkt_list_lock);
> +	spin_unlock_bh(&vsock->pkt_queue.lock);
>  
>  	while ((skb = __skb_dequeue(&pkts))) {
>  		virtio_transport_deliver_tap_pkt(skb);
> @@ -132,7 +129,6 @@ static int __init vsock_loopback_init(void)
>  	if (!vsock->workqueue)
>  		return -ENOMEM;
>  
> -	spin_lock_init(&vsock->pkt_list_lock);
>  	skb_queue_head_init(&vsock->pkt_queue);
>  	INIT_WORK(&vsock->pkt_work, vsock_loopback_work);
>  
> @@ -156,9 +152,7 @@ static void __exit vsock_loopback_exit(void)
>  
>  	flush_work(&vsock->pkt_work);
>  
> -	spin_lock_bh(&vsock->pkt_list_lock);
>  	virtio_vsock_skb_queue_purge(&vsock->pkt_queue);
> -	spin_unlock_bh(&vsock->pkt_list_lock);
>  
>  	destroy_workqueue(vsock->workqueue);
>  }
> -- 
> 2.39.2
> 

Makes sense to me. Thanks for getting to this so fast.

Best,
Bobby

Reviewed-by: Bobby Eshleman <bobby.eshleman@xxxxxxxxxxxxx>