Subject: Re: [PATCH net,v4] netlink: fix potential sleeping issue in mqueue_flush_file

From: Pablo Neira Ayuso <[email-masked]>
Date: Mon, 22 Jan 2024 09:56:55 +0100

On Mon, Jan 22, 2024 at 09:18:07AM +0800, Zhengchao Shao wrote:
> I analyze the potential sleeping issue of the following processes:
> Thread A                                Thread B
> ...                                     netlink_create  //ref = 1
> do_mq_notify                            ...
>   sock = netlink_getsockbyfilp          ...     //ref = 2
>   info->notify_sock = sock;             ...
> ...                                     netlink_sendmsg
> ...                                       skb = netlink_alloc_large_skb  //skb->head is vmalloced
> ...                                       netlink_unicast
> ...                                         sk = netlink_getsockbyportid //ref = 3
> ...                                         netlink_sendskb
> ...                                           __netlink_sendskb
> ...                                             skb_queue_tail //put skb to sk_receive_queue
> ...                                         sock_put //ref = 2
> ...                                     ...
> ...                                     netlink_release
> ...                                       deferred_put_nlk_sk //ref = 1
> mqueue_flush_file
>   spin_lock
>   remove_notification
>     netlink_sendskb
>       sock_put  //ref = 0
>         sk_free
>           ...
>           __sk_destruct
>             netlink_sock_destruct
>               skb_queue_purge  //get skb from sk_receive_queue
>                 ...
>                 __skb_queue_purge_reason
>                   kfree_skb_reason
>                     __kfree_skb
>                     ...
>                     skb_release_all
>                       skb_release_head_state
>                         netlink_skb_destructor
>                           vfree(skb->head)  //sleeping while holding spinlock
>
> In netlink_sendmsg, if the memory pointed to by skb->head is allocated by
> vmalloc, and is put to sk_receive_queue queue, also the skb is not freed.
> When the mqueue executes flush, the sleeping bug will occur. Use
> vfree_atomic instead of vfree in netlink_skb_destructor to solve the issue.

mqueue notification is of NOTIFY_COOKIE_LEN size:

static int do_mq_notify(mqd_t mqdes, const struct sigevent *notification)
{
        [...]
                if (notification->sigev_notify == SIGEV_THREAD) {
                        long timeo;

                        /* create the notify skb */
                        nc = alloc_skb(NOTIFY_COOKIE_LEN, GFP_KERNEL);
                        if (!nc)
                                return -ENOMEM;

Do you have a reproducer?