l2tp: fix lockdep splat

j-c-h · smb49 · commit 0f3eac1e2148 · 2024-09-27T11:57:26.000+02:00
BugLink: https://bugs.launchpad.net/bugs/2078428 [ Upstream commit 86a41ea ] When l2tp tunnels use a socket provided by userspace, we can hit lockdep splats like the below when data is transmitted through another (unrelated) userspace socket which then gets routed over l2tp. This issue was previously discussed here: https://lore.kernel.org/netdev/87sfialu2n.fsf@cloudflare.com/ The solution is to have lockdep treat socket locks of l2tp tunnel sockets separately than those of standard INET sockets. To do so, use a different lockdep subclass where lock nesting is possible. ============================================ WARNING: possible recursive locking detected 6.10.0+ #34 Not tainted -------------------------------------------- iperf3/771 is trying to acquire lock: ffff8881027601d8 (slock-AF_INET/1){+.-.}-{2:2}, at: l2tp_xmit_skb+0x243/0x9d0 but task is already holding lock: ffff888102650d98 (slock-AF_INET/1){+.-.}-{2:2}, at: tcp_v4_rcv+0x1848/0x1e10 other info that might help us debug this: Possible unsafe locking scenario: CPU0 ---- lock(slock-AF_INET/1); lock(slock-AF_INET/1); *** DEADLOCK *** May be due to missing lock nesting notation 10 locks held by iperf3/771: #0: ffff888102650258 (sk_lock-AF_INET){+.+.}-{0:0}, at: tcp_sendmsg+0x1a/0x40 #1: ffffffff822ac220 (rcu_read_lock){....}-{1:2}, at: __ip_queue_xmit+0x4b/0xbc0 #2: ffffffff822ac220 (rcu_read_lock){....}-{1:2}, at: ip_finish_output2+0x17a/0x1130 #3: ffffffff822ac220 (rcu_read_lock){....}-{1:2}, at: process_backlog+0x28b/0x9f0 #4: ffffffff822ac220 (rcu_read_lock){....}-{1:2}, at: ip_local_deliver_finish+0xf9/0x260 #5: ffff888102650d98 (slock-AF_INET/1){+.-.}-{2:2}, at: tcp_v4_rcv+0x1848/0x1e10 #6: ffffffff822ac220 (rcu_read_lock){....}-{1:2}, at: __ip_queue_xmit+0x4b/0xbc0 #7: ffffffff822ac220 (rcu_read_lock){....}-{1:2}, at: ip_finish_output2+0x17a/0x1130 #8: ffffffff822ac1e0 (rcu_read_lock_bh){....}-{1:2}, at: __dev_queue_xmit+0xcc/0x1450 #9: ffff888101f33258 (dev->qdisc_tx_busylock ?: &qdisc_tx_busylock#2){+...}-{2:2}, at: __dev_queue_xmit+0x513/0x1450 stack backtrace: CPU: 2 UID: 0 PID: 771 Comm: iperf3 Not tainted 6.10.0+ #34 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014 Call Trace: <IRQ> dump_stack_lvl+0x69/0xa0 dump_stack+0xc/0x20 __lock_acquire+0x135d/0x2600 ? srso_alias_return_thunk+0x5/0xfbef5 lock_acquire+0xc4/0x2a0 ? l2tp_xmit_skb+0x243/0x9d0 ? __skb_checksum+0xa3/0x540 _raw_spin_lock_nested+0x35/0x50 ? l2tp_xmit_skb+0x243/0x9d0 l2tp_xmit_skb+0x243/0x9d0 l2tp_eth_dev_xmit+0x3c/0xc0 dev_hard_start_xmit+0x11e/0x420 sch_direct_xmit+0xc3/0x640 __dev_queue_xmit+0x61c/0x1450 ? ip_finish_output2+0xf4c/0x1130 ip_finish_output2+0x6b6/0x1130 ? srso_alias_return_thunk+0x5/0xfbef5 ? __ip_finish_output+0x217/0x380 ? srso_alias_return_thunk+0x5/0xfbef5 __ip_finish_output+0x217/0x380 ip_output+0x99/0x120 __ip_queue_xmit+0xae4/0xbc0 ? srso_alias_return_thunk+0x5/0xfbef5 ? srso_alias_return_thunk+0x5/0xfbef5 ? tcp_options_write.constprop.0+0xcb/0x3e0 ip_queue_xmit+0x34/0x40 __tcp_transmit_skb+0x1625/0x1890 __tcp_send_ack+0x1b8/0x340 tcp_send_ack+0x23/0x30 __tcp_ack_snd_check+0xa8/0x530 ? srso_alias_return_thunk+0x5/0xfbef5 tcp_rcv_established+0x412/0xd70 tcp_v4_do_rcv+0x299/0x420 tcp_v4_rcv+0x1991/0x1e10 ip_protocol_deliver_rcu+0x50/0x220 ip_local_deliver_finish+0x158/0x260 ip_local_deliver+0xc8/0xe0 ip_rcv+0xe5/0x1d0 ? __pfx_ip_rcv+0x10/0x10 __netif_receive_skb_one_core+0xce/0xe0 ? process_backlog+0x28b/0x9f0 __netif_receive_skb+0x34/0xd0 ? process_backlog+0x28b/0x9f0 process_backlog+0x2cb/0x9f0 __napi_poll.constprop.0+0x61/0x280 net_rx_action+0x332/0x670 ? srso_alias_return_thunk+0x5/0xfbef5 ? find_held_lock+0x2b/0x80 ? srso_alias_return_thunk+0x5/0xfbef5 ? srso_alias_return_thunk+0x5/0xfbef5 handle_softirqs+0xda/0x480 ? __dev_queue_xmit+0xa2c/0x1450 do_softirq+0xa1/0xd0 </IRQ> <TASK> __local_bh_enable_ip+0xc8/0xe0 ? __dev_queue_xmit+0xa2c/0x1450 __dev_queue_xmit+0xa48/0x1450 ? ip_finish_output2+0xf4c/0x1130 ip_finish_output2+0x6b6/0x1130 ? srso_alias_return_thunk+0x5/0xfbef5 ? __ip_finish_output+0x217/0x380 ? srso_alias_return_thunk+0x5/0xfbef5 __ip_finish_output+0x217/0x380 ip_output+0x99/0x120 __ip_queue_xmit+0xae4/0xbc0 ? srso_alias_return_thunk+0x5/0xfbef5 ? srso_alias_return_thunk+0x5/0xfbef5 ? tcp_options_write.constprop.0+0xcb/0x3e0 ip_queue_xmit+0x34/0x40 __tcp_transmit_skb+0x1625/0x1890 tcp_write_xmit+0x766/0x2fb0 ? __entry_text_end+0x102ba9/0x102bad ? srso_alias_return_thunk+0x5/0xfbef5 ? __might_fault+0x74/0xc0 ? srso_alias_return_thunk+0x5/0xfbef5 __tcp_push_pending_frames+0x56/0x190 tcp_push+0x117/0x310 tcp_sendmsg_locked+0x14c1/0x1740 tcp_sendmsg+0x28/0x40 inet_sendmsg+0x5d/0x90 sock_write_iter+0x242/0x2b0 vfs_write+0x68d/0x800 ? __pfx_sock_write_iter+0x10/0x10 ksys_write+0xc8/0xf0 __x64_sys_write+0x3d/0x50 x64_sys_call+0xfaf/0x1f50 do_syscall_64+0x6d/0x140 entry_SYSCALL_64_after_hwframe+0x76/0x7e RIP: 0033:0x7f4d143af992 Code: c3 8b 07 85 c0 75 24 49 89 fb 48 89 f0 48 89 d7 48 89 ce 4c 89 c2 4d 89 ca 4c 8b 44 24 08 4c 8b 4c 24 10 4c 89 5c 24 08 0f 05 <c3> e9 01 cc ff ff 41 54 b8 02 00 00 0 RSP: 002b:00007ffd65032058 EFLAGS: 00000246 ORIG_RAX: 0000000000000001 RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00007f4d143af992 RDX: 0000000000000025 RSI: 00007f4d143f3bcc RDI: 0000000000000005 RBP: 00007f4d143f2b28 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000246 R12: 00007f4d143f3bcc R13: 0000000000000005 R14: 0000000000000000 R15: 00007ffd650323f0 </TASK> Fixes: 0b2c597 ("l2tp: close all race conditions in l2tp_tunnel_register()") Suggested-by: Eric Dumazet <edumazet@google.com> Reported-by: syzbot+6acef9e0a4d1f46c83d4@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=6acef9e0a4d1f46c83d4 CC: gnault@redhat.com CC: cong.wang@bytedance.com Signed-off-by: James Chapman <jchapman@katalix.com> Signed-off-by: Tom Parkin <tparkin@katalix.com> Link: https://patch.msgid.link/20240806160626.1248317-1-jchapman@katalix.com Signed-off-by: Jakub Kicinski <kuba@kernel.org> Signed-off-by: Sasha Levin <sashal@kernel.org> Signed-off-by: Koichiro Den <koichiro.den@canonical.com> Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
@@ -88,6 +88,11 @@
 /* Default trace flags */
 #define L2TP_DEFAULT_DEBUG_FLAGS	0
 
+#define L2TP_DEPTH_NESTING		2
+#if L2TP_DEPTH_NESTING == SINGLE_DEPTH_NESTING
+#error "L2TP requires its own lockdep subclass"
+#endif
+
 /* Private data stored for received packets in the skb.
  */
 struct l2tp_skb_cb {
@@ -1041,7 +1046,13 @@ static int l2tp_xmit_core(struct l2tp_session *session, struct sk_buff *skb, uns
 	IPCB(skb)->flags &= ~(IPSKB_XFRM_TUNNEL_SIZE | IPSKB_XFRM_TRANSFORMED | IPSKB_REROUTED);
 	nf_reset_ct(skb);
 
-	bh_lock_sock_nested(sk);
+	/* L2TP uses its own lockdep subclass to avoid lockdep splats caused by
+	 * nested socket calls on the same lockdep socket class. This can
+	 * happen when data from a user socket is routed over l2tp, which uses
+	 * another userspace socket.
+	 */
+	spin_lock_nested(&sk->sk_lock.slock, L2TP_DEPTH_NESTING);
+
 	if (sock_owned_by_user(sk)) {
 		kfree_skb(skb);
 		ret = NET_XMIT_DROP;
@@ -1093,7 +1104,7 @@ static int l2tp_xmit_core(struct l2tp_session *session, struct sk_buff *skb, uns
 	ret = l2tp_xmit_queue(tunnel, skb, &inet->cork.fl);
 
 out_unlock:
-	bh_unlock_sock(sk);
+	spin_unlock(&sk->sk_lock.slock);
 
 	return ret;
 }
