[PATCH net v2 1/2] mptcp: prevent MPC handshake on port-based signal endpoints

From: Matthieu Baerts (NGI0)
Date: Mon Oct 14 2024 - 10:08:19 EST

From: Paolo Abeni <pabeni@xxxxxxxxxx>

Syzkaller reported a lockdep splat:

============================================
WARNING: possible recursive locking detected
6.11.0-rc6-syzkaller-00019-g67784a74e258 #0 Not tainted
--------------------------------------------
syz-executor364/5113 is trying to acquire lock:
ffff8880449f1958 (k-slock-AF_INET){+.-.}-{2:2}, at: spin_lock include/linux/spinlock.h:351 [inline]
ffff8880449f1958 (k-slock-AF_INET){+.-.}-{2:2}, at: sk_clone_lock+0x2cd/0xf40 net/core/sock.c:2328

but task is already holding lock:
ffff88803fe3cb58 (k-slock-AF_INET){+.-.}-{2:2}, at: spin_lock include/linux/spinlock.h:351 [inline]
ffff88803fe3cb58 (k-slock-AF_INET){+.-.}-{2:2}, at: sk_clone_lock+0x2cd/0xf40 net/core/sock.c:2328

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(k-slock-AF_INET);
lock(k-slock-AF_INET);

*** DEADLOCK ***

May be due to missing lock nesting notation

7 locks held by syz-executor364/5113:
#0: ffff8880449f0e18 (sk_lock-AF_INET){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1607 [inline]
#0: ffff8880449f0e18 (sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_sendmsg+0x153/0x1b10 net/mptcp/protocol.c:1806
#1: ffff88803fe39ad8 (k-sk_lock-AF_INET){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1607 [inline]
#1: ffff88803fe39ad8 (k-sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_sendmsg_fastopen+0x11f/0x530 net/mptcp/protocol.c:1727
#2: ffffffff8e938320 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire include/linux/rcupdate.h:326 [inline]
#2: ffffffff8e938320 (rcu_read_lock){....}-{1:2}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
#2: ffffffff8e938320 (rcu_read_lock){....}-{1:2}, at: __ip_queue_xmit+0x5f/0x1b80 net/ipv4/ip_output.c:470
#3: ffffffff8e938320 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire include/linux/rcupdate.h:326 [inline]
#3: ffffffff8e938320 (rcu_read_lock){....}-{1:2}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
#3: ffffffff8e938320 (rcu_read_lock){....}-{1:2}, at: ip_finish_output2+0x45f/0x1390 net/ipv4/ip_output.c:228
#4: ffffffff8e938320 (rcu_read_lock){....}-{1:2}, at: local_lock_acquire include/linux/local_lock_internal.h:29 [inline]
#4: ffffffff8e938320 (rcu_read_lock){....}-{1:2}, at: process_backlog+0x33b/0x15b0 net/core/dev.c:6104
#5: ffffffff8e938320 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire include/linux/rcupdate.h:326 [inline]
#5: ffffffff8e938320 (rcu_read_lock){....}-{1:2}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
#5: ffffffff8e938320 (rcu_read_lock){....}-{1:2}, at: ip_local_deliver_finish+0x230/0x5f0 net/ipv4/ip_input.c:232
#6: ffff88803fe3cb58 (k-slock-AF_INET){+.-.}-{2:2}, at: spin_lock include/linux/spinlock.h:351 [inline]
#6: ffff88803fe3cb58 (k-slock-AF_INET){+.-.}-{2:2}, at: sk_clone_lock+0x2cd/0xf40 net/core/sock.c:2328

stack backtrace:
CPU: 0 UID: 0 PID: 5113 Comm: syz-executor364 Not tainted 6.11.0-rc6-syzkaller-00019-g67784a74e258 #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
Call Trace:
<IRQ>
__dump_stack lib/dump_stack.c:93 [inline]
dump_stack_lvl+0x241/0x360 lib/dump_stack.c:119
check_deadlock kernel/locking/lockdep.c:3061 [inline]
validate_chain+0x15d3/0x5900 kernel/locking/lockdep.c:3855
__lock_acquire+0x137a/0x2040 kernel/locking/lockdep.c:5142
lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5759
__raw_spin_lock include/linux/spinlock_api_smp.h:133 [inline]
_raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:154
spin_lock include/linux/spinlock.h:351 [inline]
sk_clone_lock+0x2cd/0xf40 net/core/sock.c:2328
mptcp_sk_clone_init+0x32/0x13c0 net/mptcp/protocol.c:3279
subflow_syn_recv_sock+0x931/0x1920 net/mptcp/subflow.c:874
tcp_check_req+0xfe4/0x1a20 net/ipv4/tcp_minisocks.c:853
tcp_v4_rcv+0x1c3e/0x37f0 net/ipv4/tcp_ipv4.c:2267
ip_protocol_deliver_rcu+0x22e/0x440 net/ipv4/ip_input.c:205
ip_local_deliver_finish+0x341/0x5f0 net/ipv4/ip_input.c:233
NF_HOOK+0x3a4/0x450 include/linux/netfilter.h:314
NF_HOOK+0x3a4/0x450 include/linux/netfilter.h:314
__netif_receive_skb_one_core net/core/dev.c:5661 [inline]
__netif_receive_skb+0x2bf/0x650 net/core/dev.c:5775
process_backlog+0x662/0x15b0 net/core/dev.c:6108
__napi_poll+0xcb/0x490 net/core/dev.c:6772
napi_poll net/core/dev.c:6841 [inline]
net_rx_action+0x89b/0x1240 net/core/dev.c:6963
handle_softirqs+0x2c4/0x970 kernel/softirq.c:554
do_softirq+0x11b/0x1e0 kernel/softirq.c:455
</IRQ>
<TASK>
__local_bh_enable_ip+0x1bb/0x200 kernel/softirq.c:382
local_bh_enable include/linux/bottom_half.h:33 [inline]
rcu_read_unlock_bh include/linux/rcupdate.h:908 [inline]
__dev_queue_xmit+0x1763/0x3e90 net/core/dev.c:4450
dev_queue_xmit include/linux/netdevice.h:3105 [inline]
neigh_hh_output include/net/neighbour.h:526 [inline]
neigh_output include/net/neighbour.h:540 [inline]
ip_finish_output2+0xd41/0x1390 net/ipv4/ip_output.c:235
ip_local_out net/ipv4/ip_output.c:129 [inline]
__ip_queue_xmit+0x118c/0x1b80 net/ipv4/ip_output.c:535
__tcp_transmit_skb+0x2544/0x3b30 net/ipv4/tcp_output.c:1466
tcp_rcv_synsent_state_process net/ipv4/tcp_input.c:6542 [inline]
tcp_rcv_state_process+0x2c32/0x4570 net/ipv4/tcp_input.c:6729
tcp_v4_do_rcv+0x77d/0xc70 net/ipv4/tcp_ipv4.c:1934
sk_backlog_rcv include/net/sock.h:1111 [inline]
__release_sock+0x214/0x350 net/core/sock.c:3004
release_sock+0x61/0x1f0 net/core/sock.c:3558
mptcp_sendmsg_fastopen+0x1ad/0x530 net/mptcp/protocol.c:1733
mptcp_sendmsg+0x1884/0x1b10 net/mptcp/protocol.c:1812
sock_sendmsg_nosec net/socket.c:730 [inline]
__sock_sendmsg+0x1a6/0x270 net/socket.c:745
____sys_sendmsg+0x525/0x7d0 net/socket.c:2597
___sys_sendmsg net/socket.c:2651 [inline]
__sys_sendmmsg+0x3b2/0x740 net/socket.c:2737
__do_sys_sendmmsg net/socket.c:2766 [inline]
__se_sys_sendmmsg net/socket.c:2763 [inline]
__x64_sys_sendmmsg+0xa0/0xb0 net/socket.c:2763
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f04fb13a6b9
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 01 1a 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffd651f42d8 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f04fb13a6b9
RDX: 0000000000000001 RSI: 0000000020000d00 RDI: 0000000000000004
RBP: 00007ffd651f4310 R08: 0000000000000001 R09: 0000000000000001
R10: 0000000020000080 R11: 0000000000000246 R12: 00000000000f4240
R13: 00007f04fb187449 R14: 00007ffd651f42f4 R15: 00007ffd651f4300
</TASK>

As noted by Cong Wang, the splat is false positive, but the code
path leading to the report is an unexpected one: a client is
attempting an MPC handshake towards the in-kernel listener created
by the in-kernel PM for a port based signal endpoint.

Such connection will be never accepted; many of them can make the
listener queue full and preventing the creation of MPJ subflow via
such listener - its intended role.

Explicitly detect this scenario at initial-syn time and drop the
incoming MPC request.

Fixes: 1729cf186d8a ("mptcp: create the listening socket for new port")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: syzbot+f4aacdfef2c6a6529c3e@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=f4aacdfef2c6a6529c3e
Cc: Cong Wang <cong.wang@xxxxxxxxxxxxx>
Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
Reviewed-by: Mat Martineau <martineau@xxxxxxxxxx>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
---
net/mptcp/mib.c | 1 +
net/mptcp/mib.h | 1 +
net/mptcp/pm_netlink.c | 1 +
net/mptcp/protocol.h | 1 +
net/mptcp/subflow.c | 11 +++++++++++
5 files changed, 15 insertions(+)

diff --git a/net/mptcp/mib.c b/net/mptcp/mib.c
index ad88bd3c58dffed8335eedb43ca6290418e3c4f4..19eb9292bd6093a760b41f98c1774fd2490c48e3 100644
--- a/net/mptcp/mib.c
+++ b/net/mptcp/mib.c
@@ -17,6 +17,7 @@ static const struct snmp_mib mptcp_snmp_list[] = {
SNMP_MIB_ITEM("MPCapableFallbackSYNACK", MPTCP_MIB_MPCAPABLEACTIVEFALLBACK),
SNMP_MIB_ITEM("MPCapableSYNTXDrop", MPTCP_MIB_MPCAPABLEACTIVEDROP),
SNMP_MIB_ITEM("MPCapableSYNTXDisabled", MPTCP_MIB_MPCAPABLEACTIVEDISABLED),
+ SNMP_MIB_ITEM("MPCapableEndpAttempt", MPTCP_MIB_MPCAPABLEENDPATTEMPT),
SNMP_MIB_ITEM("MPFallbackTokenInit", MPTCP_MIB_TOKENFALLBACKINIT),
SNMP_MIB_ITEM("MPTCPRetrans", MPTCP_MIB_RETRANSSEGS),
SNMP_MIB_ITEM("MPJoinNoTokenFound", MPTCP_MIB_JOINNOTOKEN),
diff --git a/net/mptcp/mib.h b/net/mptcp/mib.h
index 3206cdda8bb1067f9a8354fd45deed86b67ac7da..128282982843a07614a46f9b2c2f7c708306c769 100644
--- a/net/mptcp/mib.h
+++ b/net/mptcp/mib.h
@@ -12,6 +12,7 @@ enum linux_mptcp_mib_field {
MPTCP_MIB_MPCAPABLEACTIVEFALLBACK, /* Client-side fallback during 3-way handshake */
MPTCP_MIB_MPCAPABLEACTIVEDROP, /* Client-side fallback due to a MPC drop */
MPTCP_MIB_MPCAPABLEACTIVEDISABLED, /* Client-side disabled due to past issues */
+ MPTCP_MIB_MPCAPABLEENDPATTEMPT, /* Prohibited MPC to port-based endp */
MPTCP_MIB_TOKENFALLBACKINIT, /* Could not init/allocate token */
MPTCP_MIB_RETRANSSEGS, /* Segments retransmitted at the MPTCP-level */
MPTCP_MIB_JOINNOTOKEN, /* Received MP_JOIN but the token was not found */
diff --git a/net/mptcp/pm_netlink.c b/net/mptcp/pm_netlink.c
index f6f0a38a0750f82bc909f02a75beec980d951f1f..1a78998fe1f49510ef1f487c8cd5b369a37fb20d 100644
--- a/net/mptcp/pm_netlink.c
+++ b/net/mptcp/pm_netlink.c
@@ -1121,6 +1121,7 @@ static int mptcp_pm_nl_create_listen_socket(struct sock *sk,
*/
inet_sk_state_store(newsk, TCP_LISTEN);
lock_sock(ssk);
+ WRITE_ONCE(mptcp_subflow_ctx(ssk)->pm_listener, true);
err = __inet_listen_sk(ssk, backlog);
if (!err)
mptcp_event_pm_listener(ssk, MPTCP_EVENT_LISTENER_CREATED);
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 74417aae08d0f488bc358ce6cd931540650bb7be..568a72702b080d7610425ce5c3a409c7b88da13a 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -535,6 +535,7 @@ struct mptcp_subflow_context {
__unused : 8;
bool data_avail;
bool scheduled;
+ bool pm_listener; /* a listener managed by the kernel PM? */
u32 remote_nonce;
u64 thmac;
u32 local_nonce;
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 25dde81bcb7575958635aaf14a5b8e9a5005e05f..6170f2fff71e4f9d64837f2ebf4d81bba224fafb 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -132,6 +132,13 @@ static void subflow_add_reset_reason(struct sk_buff *skb, u8 reason)
}
}

+static int subflow_reset_req_endp(struct request_sock *req, struct sk_buff *skb)
+{
+ SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_MPCAPABLEENDPATTEMPT);
+ subflow_add_reset_reason(skb, MPTCP_RST_EPROHIBIT);
+ return -EPERM;
+}
+
/* Init mptcp request socket.
*
* Returns an error code if a JOIN has failed and a TCP reset
@@ -165,6 +172,8 @@ static int subflow_check_req(struct request_sock *req,
if (opt_mp_capable) {
SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_MPCAPABLEPASSIVE);

+ if (unlikely(listener->pm_listener))
+ return subflow_reset_req_endp(req, skb);
if (opt_mp_join)
return 0;
} else if (opt_mp_join) {
@@ -172,6 +181,8 @@ static int subflow_check_req(struct request_sock *req,

if (mp_opt.backup)
SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_JOINSYNBACKUPRX);
+ } else if (unlikely(listener->pm_listener)) {
+ return subflow_reset_req_endp(req, skb);
}

if (opt_mp_capable && listener->request_mptcp) {

--
2.45.2