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

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

[Ridge Kennedy]

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>
---
 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;
 			}
-- 
2.25.0

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

[James Chapman]

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;
>  			}

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

[David Miller]

From: Ridge Kennedy <ridge.kennedy@alliedtelesis.co.nz>
Date: Tue,  4 Feb 2020 12:24:00 +1300

> 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>

Applied and queued up for -stable, thank you.

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

[Guillaume Nault]

On Tue, Feb 04, 2020 at 12:24:00PM +1300, 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.
> 
Thanks Ridge.

Even though it's already applied,
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Tested-by: Guillaume Nault <gnault@redhat.com>