netdev - Re: [PATCH net] net/x25: Fix null-ptr-deref caused by x25

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Date:   Sat, 26 Mar 2022 15:14:55 +0800 (GMT+08:00)
From:   周多明 <duoming@....edu.cn>
To:     netdev@...r.kernel.org
Cc:     linux-x25@...r.kernel.org, linux-kernel@...r.kernel.org,
        ms@....tdt.de, davem@...emloft.net, kuba@...nel.org,
        pabeni@...hat.com, tanxin.ctf@...il.com, linma@....edu.cn,
        xiyuyang19@...an.edu.cn
Subject: Re: [PATCH net] net/x25: Fix null-ptr-deref caused by
 x25_disconnect


I am sorry, this patch has a problem. I will send the correct version later.

> The previous commit 4becb7ee5b3d ("net/x25: Fix x25_neigh refcnt leak when
> x25 disconnect") adds decrement of refcount of x25->neighbour and sets
> x25->neighbour to NULL in x25_disconnect(), but when the link layer is
> terminating, it could cause null-ptr-deref bugs in x25_sendmsg(),
> x25_recvmsg() and x25_connect(). One of the bugs is shown below.
> 
> x25_link_terminated()          | x25_recvmsg()
>  x25_kill_by_neigh()           |  ...
>   x25_disconnect()             |  lock_sock(sk)
>    ...                         |  ...
>    x25->neighbour = NULL //(1) |
>    ...                         |  x25->neighbour->extended //(2)
> 
> We set NULL to x25->neighbour in position (1) and dereference
> x25->neighbour in position (2), which could cause null-ptr-deref bug.
> 
> This patch adds lock_sock(sk) in x25_disconnect() in order to synchronize
> with x25_sendmsg(), x25_recvmsg() and x25_connect(). What`s more, the sk
> held by lock_sock() is not NULL, because it is extracted from x25_list
> and uses x25_list_lock to synchronize.
> 
> Fixes: 4becb7ee5b3d ("net/x25: Fix x25_neigh refcnt leak when x25 disconnect")
> Signed-off-by: Duoming Zhou <duoming@....edu.cn>
> ---
>  net/x25/x25_subr.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/x25/x25_subr.c b/net/x25/x25_subr.c
> index 0285aaa1e93..4e19752bdd0 100644
> --- a/net/x25/x25_subr.c
> +++ b/net/x25/x25_subr.c
> @@ -360,7 +360,9 @@ void x25_disconnect(struct sock *sk, int reason, unsigned char cause,
>  	if (x25->neighbour) {
>  		read_lock_bh(&x25_list_lock);
>  		x25_neigh_put(x25->neighbour);
> +		lock_sock(sk);
>  		x25->neighbour = NULL;
> +		release_sock(sk);
>  		read_unlock_bh(&x25_list_lock);
>  	}
>  }
> -- 
> 2.17.1