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Subject[PATCH 4.9 12/28] l2tp: do not accept arbitrary sockets
4.9-stable review patch.  If anyone has any objections, please let me know.


From: Eric Dumazet <>

[ Upstream commit 17cfe79a65f98abe535261856c5aef14f306dff7 ]

syzkaller found an issue caused by lack of sufficient checks
in l2tp_tunnel_create()

RAW sockets can not be considered as UDP ones for instance.

In another patch, we shall replace all pr_err() by less intrusive
pr_debug() so that syzkaller can find other bugs faster.
Acked-by: Guillaume Nault <>
Acked-by: James Chapman <>

BUG: KASAN: slab-out-of-bounds in setup_udp_tunnel_sock+0x3ee/0x5f0 net/ipv4/udp_tunnel.c:69
dst_release: dst:00000000d53d0d0f refcnt:-1
Write of size 1 at addr ffff8801d013b798 by task syz-executor3/6242

CPU: 1 PID: 6242 Comm: syz-executor3 Not tainted 4.16.0-rc2+ #253
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0x194/0x24d lib/dump_stack.c:53
print_address_description+0x73/0x250 mm/kasan/report.c:256
kasan_report_error mm/kasan/report.c:354 [inline]
kasan_report+0x23b/0x360 mm/kasan/report.c:412
__asan_report_store1_noabort+0x17/0x20 mm/kasan/report.c:435
setup_udp_tunnel_sock+0x3ee/0x5f0 net/ipv4/udp_tunnel.c:69
l2tp_tunnel_create+0x1354/0x17f0 net/l2tp/l2tp_core.c:1596
pppol2tp_connect+0x14b1/0x1dd0 net/l2tp/l2tp_ppp.c:707
SYSC_connect+0x213/0x4a0 net/socket.c:1640
SyS_connect+0x24/0x30 net/socket.c:1621
do_syscall_64+0x280/0x940 arch/x86/entry/common.c:287

Fixes: fd558d186df2 ("l2tp: Split pppol2tp patch into separate l2tp and ppp parts")
Signed-off-by: Eric Dumazet <>
Reported-by: syzbot <>
Signed-off-by: David S. Miller <>
Signed-off-by: Greg Kroah-Hartman <>
net/l2tp/l2tp_core.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1612,9 +1612,14 @@ int l2tp_tunnel_create(struct net *net,
encap = cfg->encap;

/* Quick sanity checks */
+ if (sk->sk_type != SOCK_DGRAM) {
+ pr_debug("tunl %hu: fd %d wrong socket type\n",
+ tunnel_id, fd);
+ goto err;
+ }
switch (encap) {
if (sk->sk_protocol != IPPROTO_UDP) {
pr_err("tunl %hu: fd %d wrong protocol, got %d, expected %d\n",
tunnel_id, fd, sk->sk_protocol, IPPROTO_UDP);
@@ -1622,7 +1627,6 @@ int l2tp_tunnel_create(struct net *net,
if (sk->sk_protocol != IPPROTO_L2TP) {
pr_err("tunl %hu: fd %d wrong protocol, got %d, expected %d\n",
tunnel_id, fd, sk->sk_protocol, IPPROTO_L2TP);

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