Re: [PATCHv2 net] sctp: hold transport before accessing its asoc in sctp_hash_transport

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

On Thu, Nov 29, 2018 at 02:49:28PM +0800, Xin Long wrote:
> In sctp_hash_transport, it dereferences a transport's asoc only under
> rcu_read_lock. Without holding the transport, its asoc could be freed
> already, which leads to a use-after-free panic.
> 
> A similar fix as Commit bab1be79a516 ("sctp: hold transport before
> accessing its asoc in sctp_transport_get_next") is needed to hold
> the transport before accessing its asoc in sctp_hash_transport.
> 
> Note that as rhlist keeps the lists to a small size, this extra
> atomic operation won't cause a noticeable latency on inserting
> a transport. Yet it's not in a datapath.
> 
> v1->v2:
>   - improve the changelog.
> 
> Fixes: cd2b70875058 ("sctp: check duplicate node before inserting a new transport")
> Reported-by: syzbot+0b05d8aa7cb185107483@syzkaller.appspotmail.com
> Signed-off-by: Xin Long <lucien.xin@gmail.com>

Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>

> ---
>  net/sctp/input.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sctp/input.c b/net/sctp/input.c
> index ce7351c..c2c0816 100644
> --- a/net/sctp/input.c
> +++ b/net/sctp/input.c
> @@ -896,11 +896,16 @@ int sctp_hash_transport(struct sctp_transport *t)
>  	list = rhltable_lookup(&sctp_transport_hashtable, &arg,
>  			       sctp_hash_params);
>  
> -	rhl_for_each_entry_rcu(transport, tmp, list, node)
> +	rhl_for_each_entry_rcu(transport, tmp, list, node) {
> +		if (!sctp_transport_hold(transport))
> +			continue;
>  		if (transport->asoc->ep == t->asoc->ep) {
> +			sctp_transport_put(transport);
>  			rcu_read_unlock();
>  			return -EEXIST;
>  		}
> +		sctp_transport_put(transport);
> +	}
>  	rcu_read_unlock();
>  
>  	err = rhltable_insert_key(&sctp_transport_hashtable, &arg,
> -- 
> 2.1.0
>