Since L2TP hooks on sockets opened by userspace using sk_user_data, we
may race with other socket families that attempt to use the same
socket.
This problem was discovered by syzbot using AF_KCM. KCM has since been
modified to use only TCP sockets to avoid hitting this issue but we
should prevent such races in L2TP anyway.
Fixes: c8fffcea0a079 ("l2tp: Refactor l2tp core driver to make use of the
common UDP tunnel function")
Reported-by: syzbot+8865eaff7f9acd593...@syzkaller.appspotmail.com
Kernel BUG at net/l2tp/l2tp_ppp.c:176!
invalid opcode: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
(ftrace buffer empty)
Modules linked in:
CPU: 1 PID: 3503 Comm: syzkaller938388 Not tainted 4.15.0-rc7+ #181
Hardware name: Google Google Compute Engine/Google Compute Engine
RIP: 0010:pppol2tp_sock_to_session net/l2tp/l2tp_ppp.c:176 [inline]
RIP: 0010:pppol2tp_sendmsg+0x512/0x670 net/l2tp/l2tp_ppp.c:304
RSP: 0018:ffff8801d4887438 EFLAGS: 00010293
RAX: ffff8801bfef2180 RBX: ffff8801bff88440 RCX: ffffffff84ffbca2
RDX: 0000000000000000 RSI: ffff8801d4887598 RDI: ffff8801bff88820
RBP: ffff8801d48874a8 R08: 0000000000000000 R09: 1ffff1003a910e17
R10: 0000000000000003 R11: 0000000000000001 R12: ffff8801bfff9bc0
R13: 0000000000000000 R14: 0000000000008000 R15: 0000000000000000
FS: 0000000001194880(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020ea0000 CR3: 00000001bfecf001 CR4: 00000000001606e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
sock_sendmsg_nosec net/socket.c:628 [inline]
sock_sendmsg+0xca/0x110 net/socket.c:638
kernel_sendmsg+0x47/0x60 net/socket.c:646
sock_no_sendpage+0x1cc/0x280 net/core/sock.c:2581
kernel_sendpage+0xbf/0xe0 net/socket.c:3349
kcm_write_msgs+0x404/0x1b80 net/kcm/kcmsock.c:646
kcm_sendmsg+0x148d/0x22d0 net/kcm/kcmsock.c:1035
sock_sendmsg_nosec net/socket.c:628 [inline]
sock_sendmsg+0xca/0x110 net/socket.c:638
___sys_sendmsg+0x767/0x8b0 net/socket.c:2018
__sys_sendmsg+0xe5/0x210 net/socket.c:2052
SYSC_sendmsg net/socket.c:2063 [inline]
SyS_sendmsg+0x2d/0x50 net/socket.c:2059
entry_SYSCALL_64_fastpath+0x23/0x9a
RIP: 0033:0x440159
RSP: 002b:00007ffe74df8288 EFLAGS: 00000217 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: ffffffffffffffff RCX: 0000000000440159
RDX: 0000000000000000 RSI: 00000000201fcfc8 RDI: 0000000000000005
RBP: 00000000006ca018 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000217 R12: 0000000000401ac0
R13: 0000000000401b50 R14: 0000000000000000 R15: 0000000000000000
Code: c5 61 70 fc 48 8b 7d d0 e8 7c c2 5b fd 84 c0 74 0d e8 b3 61 70 fc 48 89
df e8 3b 49 2f ff 41 bd f7 ff ff ff eb 86 e8 9e 61 70 fc <0f> 0b 41 bd 95 ff ff
ff e9 74 ff ff ff e8 ec 32 a8 fc e9 77 fb
RIP: pppol2tp_sock_to_session net/l2tp/l2tp_ppp.c:176 [inline] RSP:
ffff8801d4887438
RIP: pppol2tp_sendmsg+0x512/0x670 net/l2tp/l2tp_ppp.c:304 RSP: ffff8801d4887438
---
net/l2tp/l2tp_core.c | 21 ++++++++++++++++++---
net/l2tp/l2tp_ppp.c | 8 ++++++--
2 files changed, 24 insertions(+), 5 deletions(-)
diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 194a7483bb93..de7dce64173f 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1216,6 +1216,7 @@ static void l2tp_tunnel_destruct(struct sock *sk)
/* Disable udp encapsulation */
+ write_lock_bh(&sk->sk_callback_lock);
switch (tunnel->encap) {
case L2TP_ENCAPTYPE_UDP:
/* No longer an encapsulation socket. See net/ipv4/udp.c */
@@ -1229,7 +1230,8 @@ static void l2tp_tunnel_destruct(struct sock *sk)
/* Remove hooks into tunnel socket */
sk->sk_destruct = tunnel->old_sk_destruct;
- sk->sk_user_data = NULL;
+ rcu_assign_sk_user_data(sk, NULL);
+ write_unlock_bh(&sk->sk_callback_lock);
/* Remove the tunnel struct from the tunnel list */
pn = l2tp_pernet(tunnel->l2tp_net);
@@ -1583,6 +1585,20 @@ int l2tp_tunnel_create(struct net *net, int fd, int
version, u32 tunnel_id, u32
}
#endif
+ /* Assign socket sk_user_data. Must be done with
+ * sk_callback_lock. Bail if sk_user_data is already assigned.
+ */
+ write_lock_bh(&sk->sk_callback_lock);
+ if (sk->sk_user_data) {
+ err = -EALREADY;
+ write_unlock_bh(&sk->sk_callback_lock);
+ kfree(tunnel);
+ tunnel = NULL;
+ goto err;
+ }
+ rcu_assign_sk_user_data(sk, tunnel);
+ write_unlock_bh(&sk->sk_callback_lock);
+
/* Mark socket as an encapsulation socket. See net/ipv4/udp.c */
tunnel->encap = encap;
if (encap == L2TP_ENCAPTYPE_UDP) {
@@ -1594,8 +1610,6 @@ int l2tp_tunnel_create(struct net *net, int fd, int
version, u32 tunnel_id, u32
udp_cfg.encap_destroy = l2tp_udp_encap_destroy;
setup_udp_tunnel_sock(net, sock, &udp_cfg);
- } else {
- sk->sk_user_data = tunnel;
}
/* Hook on the tunnel socket destructor so that we can cleanup
@@ -1603,6 +1617,7 @@ int l2tp_tunnel_create(struct net *net, int fd, int
version, u32 tunnel_id, u32
*/
tunnel->old_sk_destruct = sk->sk_destruct;
sk->sk_destruct = &l2tp_tunnel_destruct;
+
tunnel->sock = sk;
tunnel->fd = fd;
lockdep_set_class_and_name(&sk->sk_lock.slock, &l2tp_socket_class,
"l2tp_sock");
diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index 59f246d7b290..fe5a0043dd32 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -443,7 +443,9 @@ static void pppol2tp_session_destruct(struct sock *sk)
skb_queue_purge(&sk->sk_write_queue);
if (session) {
- sk->sk_user_data = NULL;
+ write_lock_bh(&sk->sk_callback_lock);
+ rcu_assign_sk_user_data(sk, NULL);
+ write_unlock_bh(&sk->sk_callback_lock);
BUG_ON(session->magic != L2TP_SESSION_MAGIC);
l2tp_session_dec_refcount(session);
}
@@ -796,7 +798,9 @@ static int pppol2tp_connect(struct socket *sock, struct
sockaddr *uservaddr,
out_no_ppp:
/* This is how we get the session context from the socket. */
- sk->sk_user_data = session;
+ write_lock_bh(&sk->sk_callback_lock);
+ rcu_assign_sk_user_data(sk, session);
+ write_unlock_bh(&sk->sk_callback_lock);
rcu_assign_pointer(ps->sk, sk);
mutex_unlock(&ps->sk_lock);
--
1.9.1
