[PATCH net v2] l2tp: only return sessions of the given tunnel in l2tp_session_get()

[PATCH net v2] l2tp: only return sessions of the given tunnel in l2tp_session_get()

[Hengyu Liang]

Commit 5f77c18ea556 ("l2tp: use IDR for all session lookups") made
pppol2tp_connect() and the netlink session commands find sessions with
l2tp_session_get(), passing the socket of the tunnel they name.

L2TPv3 session IDs are unique per network namespace, except that
sessions carried over UDP are only unique per UDP socket, the socket of
their tunnel. However, l2tp_v3_session_get() only checks the socket when
another session has the same session ID. Otherwise it returns the
session, whichever tunnel it belongs to. As of now, a session command
that names the wrong tunnel acts on a session of another tunnel, and a
PPPoL2TP socket for a second L2TPv3 tunnel fails with EEXIST, because it
finds session 0 of the first tunnel.

The issue can be reproduced with:

    ip l2tp add tunnel tunnel_id 3 peer_tunnel_id 3 encap udp \
        local 127.0.0.1 remote 127.0.0.1 udp_sport 5003 udp_dport 6003
    ip l2tp add tunnel tunnel_id 4 peer_tunnel_id 4 encap udp \
        local 127.0.0.1 remote 127.0.0.1 udp_sport 5004 udp_dport 6004
    ip l2tp add session tunnel_id 3 session_id 7 peer_session_id 7
    ip l2tp del session tunnel_id 4 session_id 7
    ip l2tp show session

Before that commit, the delete fails with "No such device", and session
7 of tunnel 3 is still listed. After that commit, the delete succeeds,
and session 7 of tunnel 3 is gone.

This patch will make l2tp_session_get() return NULL when the L2TPv3
session it finds does not belong to the given socket. The receive path
calls l2tp_v3_session_get() directly and is not changed.

Fixes: 5f77c18ea556 ("l2tp: use IDR for all session lookups")
Cc: stable@vger.kernel.org
Signed-off-by: Hengyu Liang <hengyul@cs.unc.edu>
---
Changes in v2:
- Compare the session's tunnel socket with sk instead of the tunnel ID
  (Guillaume).
- Describe session IDs as unique per netns, and per UDP socket for UDP
  encap, in the commit message.
- v1: https://lore.kernel.org/netdev/20261025145831.2133617-1-hengyul@cs.unc.edu/

Tested with the commands above, the same session ID in two UDP tunnels
(still allowed) and in two IP tunnels (still refused), and two PPPoL2TP
tunnel sockets.

 net/l2tp/l2tp_core.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 7e042a3e648c..0cd3e38bf662 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -315,10 +315,19 @@ EXPORT_SYMBOL_GPL(l2tp_v2_session_get);
 struct l2tp_session *l2tp_session_get(const struct net *net, struct sock *sk, int pver,
 				      u32 tunnel_id, u32 session_id)
 {
+	struct l2tp_session *session;
+
 	if (pver == L2TP_HDR_VER_2)
 		return l2tp_v2_session_get(net, tunnel_id, session_id);
-	else
-		return l2tp_v3_session_get(net, sk, session_id);
+
+	/* l2tp_v3_session_get() doesn't check sk if the ID hasn't collided */
+	session = l2tp_v3_session_get(net, sk, session_id);
+	if (session && session->tunnel->sock != sk) {
+		l2tp_session_put(session);
+		return NULL;
+	}
+
+	return session;
 }
 EXPORT_SYMBOL_GPL(l2tp_session_get);
 

base-commit: 7076f684d5132e6a6145e1da2fa74771ecd052e5
-- 
2.53.0

Re: [PATCH net v2] l2tp: only return sessions of the given tunnel in l2tp_session_get()

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - How the issue was discovered, e.g. hit in production, hit during
   development, syzbot report, manual code inspection, LLM or static
   analysis tool scan.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
other reasons, please include the information in the commit messages
then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.

Re: [PATCH net v2] l2tp: only return sessions of the given tunnel in l2tp_session_get()

[Guillaume Nault]

On Thu, Oct 29, 2026 at 11:26:17PM -0400, Hengyu Liang wrote:
> Commit 5f77c18ea556 ("l2tp: use IDR for all session lookups") made
> pppol2tp_connect() and the netlink session commands find sessions with
> l2tp_session_get(), passing the socket of the tunnel they name.
> 
> L2TPv3 session IDs are unique per network namespace, except that
> sessions carried over UDP are only unique per UDP socket, the socket of
> their tunnel. However, l2tp_v3_session_get() only checks the socket when
> another session has the same session ID. Otherwise it returns the
> session, whichever tunnel it belongs to. As of now, a session command
> that names the wrong tunnel acts on a session of another tunnel, and a
> PPPoL2TP socket for a second L2TPv3 tunnel fails with EEXIST, because it
> finds session 0 of the first tunnel.
> 
> The issue can be reproduced with:
> 
>     ip l2tp add tunnel tunnel_id 3 peer_tunnel_id 3 encap udp \
>         local 127.0.0.1 remote 127.0.0.1 udp_sport 5003 udp_dport 6003
>     ip l2tp add tunnel tunnel_id 4 peer_tunnel_id 4 encap udp \
>         local 127.0.0.1 remote 127.0.0.1 udp_sport 5004 udp_dport 6004
>     ip l2tp add session tunnel_id 3 session_id 7 peer_session_id 7
>     ip l2tp del session tunnel_id 4 session_id 7
>     ip l2tp show session
> 
> Before that commit, the delete fails with "No such device", and session
> 7 of tunnel 3 is still listed. After that commit, the delete succeeds,
> and session 7 of tunnel 3 is gone.
> 
> This patch will make l2tp_session_get() return NULL when the L2TPv3
> session it finds does not belong to the given socket. The receive path
> calls l2tp_v3_session_get() directly and is not changed.
> 

Acked-by: Guillaume Nault <gnault@redhat.com>

Re: [PATCH net v2] l2tp: only return sessions of the given tunnel in l2tp_session_get()

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Paolo Abeni <pabeni@redhat.com>:

On Thu, 29 Oct 2026 23:26:17 -0400 you wrote:
> Commit 5f77c18ea556 ("l2tp: use IDR for all session lookups") made
> pppol2tp_connect() and the netlink session commands find sessions with
> l2tp_session_get(), passing the socket of the tunnel they name.
> 
> L2TPv3 session IDs are unique per network namespace, except that
> sessions carried over UDP are only unique per UDP socket, the socket of
> their tunnel. However, l2tp_v3_session_get() only checks the socket when
> another session has the same session ID. Otherwise it returns the
> session, whichever tunnel it belongs to. As of now, a session command
> that names the wrong tunnel acts on a session of another tunnel, and a
> PPPoL2TP socket for a second L2TPv3 tunnel fails with EEXIST, because it
> finds session 0 of the first tunnel.
> 
> [...]

Here is the summary with links:
  - [net,v2] l2tp: only return sessions of the given tunnel in l2tp_session_get()
    https://git.kernel.org/netdev/net/c/a75facf4bb3e

You are awesome, thank you!
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