[PATCH net-next v3 02/16] l2tp: add RCU read lock to protect tunnel ptr in ip socket destroy

[James Chapman <jchapman@katalix.com>]

If an L2TPIP socket is closed, add RCU protection when we deref
sk_user_data to prevent races with another thread closing the same
tunnel.

refcount_t: increment on 0; use-after-free.
WARNING: CPU: 2 PID: 2892 at lib/refcount.c:153 refcount_inc+0x2b/0x30
Modules linked in:
CPU: 2 PID: 2892 Comm: pppol2tp_chaos Not tainted 4.15.0-rc9+ #1
Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
RIP: 0010:refcount_inc+0x2b/0x30
RSP: 0018:ffff880014147b50 EFLAGS: 00010282
RAX: 000000000000002b RBX: ffff8800194785c0 RCX: 0000000000000000
RDX: 0000000000000001 RSI: ffff88001ad1f758 RDI: ffff88001ad1f758
RBP: ffff880014147b50 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000001 R12: ffff8800194785c8
R13: ffff8800194785c0 R14: ffff88001a2c6c20 R15: ffff880013a9d580
FS:  0000000000000000(0000) GS:ffff88001ad00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fbc17ea7000 CR3: 0000000003022000 CR4: 00000000000006e0
Call Trace:
 l2tp_tunnel_delete+0x34/0x60
 l2tp_ip_destroy_sock+0x6d/0x80
 sk_common_release+0x19/0xd0
 l2tp_ip_close+0x89/0xa0
 inet_release+0x37/0x60
 sock_release+0x1a/0x70
 sock_close+0xd/0x20
 __fput+0xed/0x1f0
 ____fput+0x9/0x10
 task_work_run+0x77/0xb0
 do_exit+0x311/0xcf0
 do_group_exit+0x47/0xc0
 get_signal+0x343/0x8e0
 do_signal+0x23/0x780
 ? find_held_lock+0x39/0xb0
 exit_to_usermode_loop+0x4a/0x93
 syscall_return_slowpath+0x102/0x150
 entry_SYSCALL_64_fastpath+0x98/0x9a
RIP: 0033:0x7fbc172d7fcf
RSP: 002b:00007ffd524cd4d8 EFLAGS: 00000246 ORIG_RAX: 000000000000000e
RAX: 0000000000000000 RBX: 0000000000000006 RCX: 00007fbc172d7fcf
RDX: 0000000000000000 RSI: 00007ffd524cd460 RDI: 0000000000000002
RBP: 0000000000404930 R08: 0000000000000000 R09: 00007ffd524cd460
R10: 0000000000000008 R11: 0000000000000246 R12: 000000000000001e
R13: 0000000000404a03 R14: 0000000000000000 R15: 0000000000000000
Code: 55 48 89 e5 e8 87 ff ff ff 84 c0 74 02 5d c3 80 3d 97 87 bb 01 00 75 f5 48 c7 c7 58 3e cc 82 c6 05 87 87 bb 01

Fixes: 0d76751fad ("l2tp: Add L2TPv3 IP encapsulation (no UDP) support")
Signed-off-by: James Chapman <jchapman@katalix.com>
---
 net/l2tp/l2tp_ip.c  | 5 ++++-
 net/l2tp/l2tp_ip6.c | 5 ++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/l2tp/l2tp_ip.c b/net/l2tp/l2tp_ip.c
index ff61124fdf59..42f3c2f72bf4 100644
--- a/net/l2tp/l2tp_ip.c
+++ b/net/l2tp/l2tp_ip.c
@@ -234,15 +234,18 @@ static void l2tp_ip_close(struct sock *sk, long timeout)
 static void l2tp_ip_destroy_sock(struct sock *sk)
 {
 	struct sk_buff *skb;
-	struct l2tp_tunnel *tunnel = l2tp_sock_to_tunnel(sk);
+	struct l2tp_tunnel *tunnel;
 
 	while ((skb = __skb_dequeue_tail(&sk->sk_write_queue)) != NULL)
 		kfree_skb(skb);
 
+	rcu_read_lock();
+	tunnel = rcu_dereference_sk_user_data(sk);
 	if (tunnel) {
 		l2tp_tunnel_closeall(tunnel);
 		sock_put(sk);
 	}
+	rcu_read_unlock();
 
 	sk_refcnt_debug_dec(sk);
 }
diff --git a/net/l2tp/l2tp_ip6.c b/net/l2tp/l2tp_ip6.c
index 192344688c06..be4a3eee85a9 100644
--- a/net/l2tp/l2tp_ip6.c
+++ b/net/l2tp/l2tp_ip6.c
@@ -248,16 +248,19 @@ static void l2tp_ip6_close(struct sock *sk, long timeout)
 
 static void l2tp_ip6_destroy_sock(struct sock *sk)
 {
-	struct l2tp_tunnel *tunnel = l2tp_sock_to_tunnel(sk);
+	struct l2tp_tunnel *tunnel;
 
 	lock_sock(sk);
 	ip6_flush_pending_frames(sk);
 	release_sock(sk);
 
+	rcu_read_lock();
+	tunnel = rcu_dereference_sk_user_data(sk);
 	if (tunnel) {
 		l2tp_tunnel_closeall(tunnel);
 		sock_put(sk);
 	}
+	rcu_read_unlock();
 
 	inet6_destroy_sock(sk);
 }
-- 
1.9.1