linux-kernel - Re: [PATCH net] kcm: Fix kernel NULL pointer dereference in requeue_rx

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Date:   Sun, 13 Nov 2022 16:55:15 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Wang Hai <wanghai38@...wei.com>
Cc:     davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, cong.wang@...edance.com, f.fainelli@...il.com,
        tom@...bertland.com, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH net] kcm: Fix kernel NULL pointer dereference in
 requeue_rx_msgs

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.