Subject: Re: [PATCH net] sctp: fix race on protocol/netns initialization

From: Vlad Yasevich <[email-masked]>
Date: Wed, 09 Sep 2015 16:30:35 -0400

On 09/09/2015 04:03 PM, Marcelo Ricardo Leitner wrote:
> Consider sctp module is unloaded and is being requested because an user
> is creating a sctp socket.
> 
> During initialization, sctp will add the new protocol type and then
> initialize pernet subsys:
> 
>         status = sctp_v4_protosw_init();
>         if (status)
>                 goto err_protosw_init;
> 
>         status = sctp_v6_protosw_init();
>         if (status)
>                 goto err_v6_protosw_init;
> 
>         status = register_pernet_subsys(&sctp_net_ops);
> 
> The problem is that after those calls to sctp_v{4,6}_protosw_init(), it
> is possible for userspace to create SCTP sockets like if the module is
> already fully loaded. If that happens, one of the possible effects is
> that we will have readers for net->sctp.local_addr_list list earlier
> than expected and sctp_net_init() does not take precautions while
> dealing with that list, leading to a potential panic but not limited to
> that, as sctp_sock_init() will copy a bunch of blank/partially
> initialized values from net->sctp.
> 
> The race happens like this:
> 
>      CPU 0                           |  CPU 1
>   socket()                           |
>    __sock_create                     | socket()
>     inet_create                      |  __sock_create
>      list_for_each_entry_rcu(        |
>         answer, &inetsw[sock->type], |
>         list) {                      |   inet_create
>       /* no hits */                  |
>      if (unlikely(err)) {            |
>       ...                            |
>       request_module()               |
>       /* socket creation is blocked  |
>        * the module is fully loaded  |
>        */                            |
>        sctp_init                     |
>         sctp_v4_protosw_init         |
>          inet_register_protosw       |
>           list_add_rcu(&p->list,     |
>                        last_perm);   |
>                                      |  list_for_each_entry_rcu(
>                                      |     answer, &inetsw[sock->type],
>         sctp_v6_protosw_init         |     list) {
>                                      |     /* hit, so assumes protocol
>                                      |      * is already loaded
>                                      |      */
>                                      |  /* socket creation continues
>                                      |   * before netns is initialized
>                                      |   */
>         register_pernet_subsys       |
> 
> Inverting the initialization order between register_pernet_subsys() and
> sctp_v4_protosw_init() is not possible because register_pernet_subsys()
> will create a control sctp socket, so the protocol must be already
> visible by then. Deferring the socket creation to a work-queue is not
> good specially because we loose the ability to handle its errors.
> 
> So the fix then is to invert the initialization order inside
> register_pernet_subsys() so that the control socket is created by last
> and also block socket creation if netns initialization wasn't yet
> performed.
> 

not sure how much I like that...  Wouldn't it be better
to pull the control socket initialization stuff out into its
own function that does something like

for_each_net_rcu()
	init_control_socket(net, ...)

Or may be even pull the control socket creation
stuff completely into its own per-net ops operations structure
and initialize it after the the protosw stuff has been done.

-vlad

> Fixes: 4db67e808640 ("sctp: Make the address lists per network namespace")
> Signed-off-by: Marcelo Ricardo Leitner <[email-masked]>
> ---
>  net/sctp/protocol.c | 18 +++++++++++-------
>  net/sctp/socket.c   |  4 ++++
>  2 files changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/net/sctp/protocol.c b/net/sctp/protocol.c
> index 4345790ad3266c353eeac5398593c2a9ce4effda..d8f78165768a75f93f4ce4120dd5475b6a623aaf 100644
> --- a/net/sctp/protocol.c
> +++ b/net/sctp/protocol.c
> @@ -1271,12 +1271,6 @@ static int __net_init sctp_net_init(struct net *net)
>  
>  	sctp_dbg_objcnt_init(net);
>  
> -	/* Initialize the control inode/socket for handling OOTB packets.  */
> -	if ((status = sctp_ctl_sock_init(net))) {
> -		pr_err("Failed to initialize the SCTP control sock\n");
> -		goto err_ctl_sock_init;
> -	}
> -
>  	/* Initialize the local address list. */
>  	INIT_LIST_HEAD(&net->sctp.local_addr_list);
>  	spin_lock_init(&net->sctp.local_addr_lock);
> @@ -1284,11 +1278,21 @@ static int __net_init sctp_net_init(struct net *net)
>  
>  	/* Initialize the address event list */
>  	INIT_LIST_HEAD(&net->sctp.addr_waitq);
> -	INIT_LIST_HEAD(&net->sctp.auto_asconf_splist);
>  	spin_lock_init(&net->sctp.addr_wq_lock);
>  	net->sctp.addr_wq_timer.expires = 0;
>  	setup_timer(&net->sctp.addr_wq_timer, sctp_addr_wq_timeout_handler,
>  		    (unsigned long)net);
> +	/* sctp_init_sock() will use this to know that netns is
> +	 * nearly all initialized but already good to go.
> +	 */
> +	INIT_LIST_HEAD(&net->sctp.auto_asconf_splist);
> +
> +	/* Initialize the control inode/socket for handling OOTB packets.  */
> +	status = sctp_ctl_sock_init(net);
> +	if (status) {
> +		pr_err("Failed to initialize the SCTP control sock\n");
> +		goto err_ctl_sock_init;
> +	}
>  
>  	return 0;
>  
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index 17bef01b9aa3e7f75328d39fc976f9e80d641e92..45b94deec93d0c7c1612a16922348cf2a7e65ec5 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -3993,6 +3993,10 @@ static int sctp_init_sock(struct sock *sk)
>  
>  	pr_debug("%s: sk:%p\n", __func__, sk);
>  
> +	/* Validate if netns is already initialized. */
> +	if (!net->sctp.auto_asconf_splist.prev)
> +		return -ENOPROTOOPT;
> +
>  	sp = sctp_sk(sk);
>  
>  	/* Initialize the SCTP per socket area.  */
>