Re: [PATCH net] kcm: Fix kernel NULL pointer dereference in requeue_rx_msgs

[Cong Wang <xiyou.wangcong@gmail.com>]

On Sat, Nov 12, 2022 at 08:04:23PM +0800, Wang Hai wrote:
> In kcm_rcv_strparser(), the skb is queued to the kcm that is currently
> being reserved, and if the queue is full, unreserve_rx_kcm() will be
> called. At this point, if KCM_RECV_DISABLE is set, then unreserve_rx_kcm()
> will requeue received messages for the current kcm socket to other kcm
> sockets. The kcm sock lock is not held during this time, and as long as
> someone calls kcm_recvmsg, it will concurrently unlink the same skb, which
> ill result in a null pointer reference.
> 
> cpu0 			cpu1		        cpu2
> kcm_rcv_strparser
>  reserve_rx_kcm
>                         kcm_setsockopt
>                          kcm_recv_disable
>                           kcm->rx_disabled = 1;
>   kcm_queue_rcv_skb
>   unreserve_rx_kcm
>    requeue_rx_msgs                              kcm_recvmsg
>     __skb_dequeue
>      __skb_unlink(skb)				  skb_unlink(skb)
>                                                   //double unlink skb
> 

We will hold skb queue lock after my patch, so this will not happen after
applying my patch below?
https://lore.kernel.org/netdev/20221114005119.597905-1-xiyou.wangcong@gmail.com/

Thanks.