[PATCH 6.18 030/227] l2tp: Fix memleak in l2tp_udp_encap_recv().

Greg Kroah-Hartman
2026-01-28T15:53:21Z

From: Kuniyuki Iwashima <kuniyu@google.com>

[ Upstream commit 4d10edfd1475b69dbd4c47f34b61a3772ece83ca ]

syzbot reported memleak of struct l2tp_session, l2tp_tunnel,
sock, etc. [0]

The cited commit moved down the validation of the protocol
version in l2tp_udp_encap_recv().

The new place requires an extra error handling to avoid the
memleak.

Let's call l2tp_session_put() there.

[0]:
BUG: memory leak
unreferenced object 0xffff88810a290200 (size 512):
comm "syz.0.17", pid 6086, jiffies 4294944299
hex dump (first 32 bytes):
7d eb 04 0c 00 00 00 00 01 00 00 00 00 00 00 00 }...............
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
backtrace (crc babb6a4f):
kmemleak_alloc_recursive include/linux/kmemleak.h:44 [inline]
slab_post_alloc_hook mm/slub.c:4958 [inline]
slab_alloc_node mm/slub.c:5263 [inline]
__do_kmalloc_node mm/slub.c:5656 [inline]
__kmalloc_noprof+0x3e0/0x660 mm/slub.c:5669
kmalloc_noprof include/linux/slab.h:961 [inline]
kzalloc_noprof include/linux/slab.h:1094 [inline]
l2tp_session_create+0x3a/0x3b0 net/l2tp/l2tp_core.c:1778
pppol2tp_connect+0x48b/0x920 net/l2tp/l2tp_ppp.c:755
__sys_connect_file+0x7a/0xb0 net/socket.c:2089
__sys_connect+0xde/0x110 net/socket.c:2108
__do_sys_connect net/socket.c:2114 [inline]
__se_sys_connect net/socket.c:2111 [inline]
__x64_sys_connect+0x1c/0x30 net/socket.c:2111
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xa4/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: 364798056f518 ("l2tp: Support different protocol versions with same IP/port quadruple")
Reported-by: syzbot+2c42ea4485b29beb0643@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/696693f2.a70a0220.245e30.0001.GAE@google.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/20260113185446.2533333-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/l2tp/l2tp_core.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 0710281dd95aa..d6f4bef0236dc 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1086,8 +1086,10 @@ int l2tp_udp_encap_recv(struct sock *sk, struct sk_buff *skb)
tunnel = session->tunnel;

/* Check protocol version */
- if (version != tunnel->version)
+ if (version != tunnel->version) {
+ l2tp_session_put(session);
goto invalid;
+ }

if (version == L2TP_HDR_VER_3 &&
l2tp_v3_ensure_opt_in_linear(session, skb, &ptr, &optr)) {
--
2.51.0