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

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Date:   Tue,  4 Feb 2020 12:24:00 +1300
From:   Ridge Kennedy <ridge.kennedy@...iedtelesis.co.nz>
To:     netdev@...r.kernel.org
Cc:     gnault@...hat.com, tparkin@...alix.com, jchapman@...alix.com,
        Ridge Kennedy <ridge.kennedy@...iedtelesis.co.nz>
Subject: [PATCH v2 net] l2tp: Allow duplicate session creation with UDP

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@...iedtelesis.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