Re: [PATCH v2 net] l2tp: Allow duplicate session creation with UDP

[James Chapman <jchapman@katalix.com>]

On 03/02/2020 23:24, Ridge Kennedy wrote:
> In the past it was possible to create multiple L2TPv3 sessions with the
> same session id as long as the sessions belonged to different tunnels.
> The resulting sessions had issues when used with IP encapsulated tunnels,
> but worked fine with UDP encapsulated ones. Some applications began to
> rely on this behaviour to avoid having to negotiate unique session ids.
>
> Some time ago a change was made to require session ids to be unique across
> all tunnels, breaking the applications making use of this "feature".
>
> This change relaxes the duplicate session id check to allow duplicates
> if both of the colliding sessions belong to UDP encapsulated tunnels.
>
> Fixes: dbdbc73b4478 ("l2tp: fix duplicate session creation")
> Signed-off-by: Ridge Kennedy <ridge.kennedy@alliedtelesis.co.nz>

Acked-by: James Chapman <jchapman@katalix.com>


> ---
>  net/l2tp/l2tp_core.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> index f82ea12bac37..425b95eb7e87 100644
> --- a/net/l2tp/l2tp_core.c
> +++ b/net/l2tp/l2tp_core.c
> @@ -322,8 +322,13 @@ int l2tp_session_register(struct l2tp_session *session,
>  
>  		spin_lock_bh(&pn->l2tp_session_hlist_lock);
>  
> +		/* IP encap expects session IDs to be globally unique, while
> +		 * UDP encap doesn't.
> +		 */
>  		hlist_for_each_entry(session_walk, g_head, global_hlist)
> -			if (session_walk->session_id == session->session_id) {
> +			if (session_walk->session_id == session->session_id &&
> +			    (session_walk->tunnel->encap == L2TP_ENCAPTYPE_IP ||
> +			     tunnel->encap == L2TP_ENCAPTYPE_IP)) {
>  				err = -EEXIST;
>  				goto err_tlock_pnlock;
>  			}