netdev - Re: [PATCH net] tipc: fix missing rtnl lock protection during setting link properties

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Date:   Wed, 03 Jan 2018 10:48:50 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     ying.xue@...driver.com
Cc:     netdev@...r.kernel.org, jon.maloy@...csson.com,
        syzkaller-bugs@...glegroups.com,
        tipc-discussion@...ts.sourceforge.net
Subject: Re: [PATCH net] tipc: fix missing rtnl lock protection during
 setting link properties

From: Ying Xue <ying.xue@...driver.com>
Date: Mon, 1 Jan 2018 18:24:01 +0800

> diff --git a/net/tipc/netlink_compat.c b/net/tipc/netlink_compat.c
> index e48f0b2..0fb12a4 100644
> --- a/net/tipc/netlink_compat.c
> +++ b/net/tipc/netlink_compat.c
> @@ -720,17 +720,21 @@ static int tipc_nl_compat_link_set(struct tipc_nl_compat_cmd_doit *cmd,
>  
>  	lc = (struct tipc_link_config *)TLV_DATA(msg->req);
>  
> +	rtnl_lock();
>  	media = tipc_media_find(lc->name);
>  	if (media) {
>  		cmd->doit = &tipc_nl_media_set;
> +		rtnl_unlock();
>  		return tipc_nl_compat_media_set(skb, msg);
>  	}
>  
>  	bearer = tipc_bearer_find(msg->net, lc->name);
>  	if (bearer) {
>  		cmd->doit = &tipc_nl_bearer_set;
> +		rtnl_unlock();
>  		return tipc_nl_compat_bearer_set(skb, msg);
>  	}
> +	rtnl_unlock();
>  
>  	return __tipc_nl_compat_link_set(skb, msg);
>  }

As soon as you drop the RTNL lock, the media or bearer entry can be
removed from the tables.

This invalidates what you do next, whether it's
tipc_nl_compat_media_set(), tipc_nl_compat_bearer_set(), etc.

Therefore, you have to lock down the tipc configuration state around
this entire operation, from media/bearer probe to the building of the
netlink message(s).

Either this entire code path must execute with the bearer/media entry
present, or without.  If you drop the RTNL mutex in the middle, this
invariant is not held.