[PATCH v1 net] smc: Fix lockdep false-positive for IPPROTO_SMC.

Kuniyuki Iwashima
2025-04-07T17:03:58Z

SMC consists of two sockets: smc_sock and kernel TCP socket.

Currently, there are two ways of creating the sockets, and syzbot reported
a lockdep splat [0] for the newer way introduced by commit d25a92ccae6b
("net/smc: Introduce IPPROTO_SMC").

socket(AF_SMC , SOCK_STREAM, SMCPROTO_SMC or SMCPROTO_SMC6)
socket(AF_INET or AF_INET6, SOCK_STREAM, IPPROTO_SMC)

When a socket is allocated, sock_lock_init() sets a lockdep lock class to
sk->sk_lock.slock based on its protocol family. In the IPPROTO_SMC case,
AF_INET or AF_INET6 lock class is assigned to smc_sock.

The repro sets IPV6_JOIN_ANYCAST for IPv6 UDP and SMC socket and exercises
smc_switch_to_fallback() for IPPROTO_SMC.

1. smc_switch_to_fallback() is called under lock_sock() and holds
smc->clcsock_release_lock.

sk_lock-AF_INET6 -> &smc->clcsock_release_lock
(sk_lock-AF_SMC)

2. Setting IPV6_JOIN_ANYCAST to SMC holds smc->clcsock_release_lock
and calls setsockopt() for the kernel TCP socket, which holds RTNL
and the kernel socket's lock_sock().

&smc->clcsock_release_lock -> rtnl_mutex (-> k-sk_lock-AF_INET6)

3. Setting IPV6_JOIN_ANYCAST to UDP holds RTNL and lock_sock().

rtnl_mutex -> sk_lock-AF_INET6

Then, lockdep detects a false-positive circular locking,

.-> sk_lock-AF_INET6 -> &smc->clcsock_release_lock -> rtnl_mutex -.
`-----------------------------------------------------------------'

but IPPROTO_SMC should have the same locking rule as AF_SMC.

sk_lock-AF_SMC -> &smc->clcsock_release_lock -> rtnl_mutex -> k-sk_lock-AF_INET6

Let's set the same lock class for smc_sock.

Given AF_SMC uses the same lock class for SMCPROTO_SMC and SMCPROTO_SMC6,
we do not need to separate the class for AF_INET and AF_INET6.

[0]:
WARNING: possible circular locking dependency detected
6.14.0-rc3-syzkaller-00267-gff202c5028a1 #0 Not tainted

syz.4.1528/11571 is trying to acquire lock:
ffffffff8fef8de8 (rtnl_mutex){+.+.}-{4:4}, at: ipv6_sock_ac_close+0xd9/0x110 net/ipv6/anycast.c:220

but task is already holding lock:
ffff888027f596a8 (&smc->clcsock_release_lock){+.+.}-{4:4}, at: smc_clcsock_release+0x75/0xe0 net/smc/smc_close.c:30

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (&smc->clcsock_release_lock){+.+.}-{4:4}:
__mutex_lock_common kernel/locking/mutex.c:585 [inline]
__mutex_lock+0x19b/0xb10 kernel/locking/mutex.c:730
smc_switch_to_fallback+0x2d/0xa00 net/smc/af_smc.c:903
smc_sendmsg+0x13d/0x520 net/smc/af_smc.c:2781
sock_sendmsg_nosec net/socket.c:718 [inline]
__sock_sendmsg net/socket.c:733 [inline]
____sys_sendmsg+0xaaf/0xc90 net/socket.c:2573
___sys_sendmsg+0x135/0x1e0 net/socket.c:2627
__sys_sendmsg+0x16e/0x220 net/socket.c:2659
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (sk_lock-AF_INET6){+.+.}-{0:0}:
lock_sock_nested+0x3a/0xf0 net/core/sock.c:3645
lock_sock include/net/sock.h:1624 [inline]
sockopt_lock_sock net/core/sock.c:1133 [inline]
sockopt_lock_sock+0x54/0x70 net/core/sock.c:1124
do_ipv6_setsockopt+0x2160/0x4520 net/ipv6/ipv6_sockglue.c:567
ipv6_setsockopt+0xcb/0x170 net/ipv6/ipv6_sockglue.c:993
udpv6_setsockopt+0x7d/0xd0 net/ipv6/udp.c:1850
do_sock_setsockopt+0x222/0x480 net/socket.c:2303
__sys_setsockopt+0x1a0/0x230 net/socket.c:2328
__do_sys_setsockopt net/socket.c:2334 [inline]
__se_sys_setsockopt net/socket.c:2331 [inline]
__x64_sys_setsockopt+0xbd/0x160 net/socket.c:2331
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (rtnl_mutex){+.+.}-{4:4}:
check_prev_add kernel/locking/lockdep.c:3163 [inline]
check_prevs_add kernel/locking/lockdep.c:3282 [inline]
validate_chain kernel/locking/lockdep.c:3906 [inline]
__lock_acquire+0x249e/0x3c40 kernel/locking/lockdep.c:5228
lock_acquire.part.0+0x11b/0x380 kernel/locking/lockdep.c:5851
__mutex_lock_common kernel/locking/mutex.c:585 [inline]
__mutex_lock+0x19b/0xb10 kernel/locking/mutex.c:730
ipv6_sock_ac_close+0xd9/0x110 net/ipv6/anycast.c:220
inet6_release+0x47/0x70 net/ipv6/af_inet6.c:485
__sock_release net/socket.c:647 [inline]
sock_release+0x8e/0x1d0 net/socket.c:675
smc_clcsock_release+0xb7/0xe0 net/smc/smc_close.c:34
__smc_release+0x5c2/0x880 net/smc/af_smc.c:301
smc_release+0x1fc/0x5f0 net/smc/af_smc.c:344
__sock_release+0xb0/0x270 net/socket.c:647
sock_close+0x1c/0x30 net/socket.c:1398
__fput+0x3ff/0xb70 fs/file_table.c:464
task_work_run+0x14e/0x250 kernel/task_work.c:227
resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
exit_to_user_mode_loop kernel/entry/common.c:114 [inline]
exit_to_user_mode_prepare include/linux/entry-common.h:329 [inline]
__syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline]
syscall_exit_to_user_mode+0x27b/0x2a0 kernel/entry/common.c:218
do_syscall_64+0xda/0x250 arch/x86/entry/common.c:89
entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
rtnl_mutex --> sk_lock-AF_INET6 --> &smc->clcsock_release_lock

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&smc->clcsock_release_lock);
lock(sk_lock-AF_INET6);
lock(&smc->clcsock_release_lock);
lock(rtnl_mutex);

*** DEADLOCK ***

2 locks held by syz.4.1528/11571:
#0: ffff888077e88208 (&sb->s_type->i_mutex_key#10){+.+.}-{4:4}, at: inode_lock include/linux/fs.h:877 [inline]
#0: ffff888077e88208 (&sb->s_type->i_mutex_key#10){+.+.}-{4:4}, at: __sock_release+0x86/0x270 net/socket.c:646
#1: ffff888027f596a8 (&smc->clcsock_release_lock){+.+.}-{4:4}, at: smc_clcsock_release+0x75/0xe0 net/smc/smc_close.c:30

stack backtrace:
CPU: 0 UID: 0 PID: 11571 Comm: syz.4.1528 Not tainted 6.14.0-rc3-syzkaller-00267-gff202c5028a1 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
print_circular_bug+0x490/0x760 kernel/locking/lockdep.c:2076
check_noncircular+0x31a/0x400 kernel/locking/lockdep.c:2208
check_prev_add kernel/locking/lockdep.c:3163 [inline]
check_prevs_add kernel/locking/lockdep.c:3282 [inline]
validate_chain kernel/locking/lockdep.c:3906 [inline]
__lock_acquire+0x249e/0x3c40 kernel/locking/lockdep.c:5228
lock_acquire.part.0+0x11b/0x380 kernel/locking/lockdep.c:5851
__mutex_lock_common kernel/locking/mutex.c:585 [inline]
__mutex_lock+0x19b/0xb10 kernel/locking/mutex.c:730
ipv6_sock_ac_close+0xd9/0x110 net/ipv6/anycast.c:220
inet6_release+0x47/0x70 net/ipv6/af_inet6.c:485
__sock_release net/socket.c:647 [inline]
sock_release+0x8e/0x1d0 net/socket.c:675
smc_clcsock_release+0xb7/0xe0 net/smc/smc_close.c:34
__smc_release+0x5c2/0x880 net/smc/af_smc.c:301
smc_release+0x1fc/0x5f0 net/smc/af_smc.c:344
__sock_release+0xb0/0x270 net/socket.c:647
sock_close+0x1c/0x30 net/socket.c:1398
__fput+0x3ff/0xb70 fs/file_table.c:464
task_work_run+0x14e/0x250 kernel/task_work.c:227
resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
exit_to_user_mode_loop kernel/entry/common.c:114 [inline]
exit_to_user_mode_prepare include/linux/entry-common.h:329 [inline]
__syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline]
syscall_exit_to_user_mode+0x27b/0x2a0 kernel/entry/common.c:218
do_syscall_64+0xda/0x250 arch/x86/entry/common.c:89
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f8b4b38d169
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffe4efd22d8 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4
RAX: 0000000000000000 RBX: 00000000000b14a3 RCX: 00007f8b4b38d169
RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003
RBP: 00007f8b4b5a7ba0 R08: 0000000000000001 R09: 000000114efd25cf
R10: 00007f8b4b200000 R11: 0000000000000246 R12: 00007f8b4b5a5fac
R13: 00007f8b4b5a5fa0 R14: ffffffffffffffff R15: 00007ffe4efd23f0
</TASK>

Fixes: d25a92ccae6b ("net/smc: Introduce IPPROTO_SMC")
Reported-by: syzbot+be6f4b383534d88989f7@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=be6f4b383534d88989f7
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
---
net/smc/af_smc.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
index 3e6cb35baf25..3760131f1484 100644
--- a/net/smc/af_smc.c
+++ b/net/smc/af_smc.c
@@ -362,6 +362,9 @@ static void smc_destruct(struct sock *sk)
return;
}

+static struct lock_class_key smc_key;
+static struct lock_class_key smc_slock_key;
+
void smc_sk_init(struct net *net, struct sock *sk, int protocol)
{
struct smc_sock *smc = smc_sk(sk);
@@ -375,6 +378,8 @@ void smc_sk_init(struct net *net, struct sock *sk, int protocol)
INIT_WORK(&smc->connect_work, smc_connect_work);
INIT_DELAYED_WORK(&smc->conn.tx_work, smc_tx_work);
INIT_LIST_HEAD(&smc->accept_q);
+ sock_lock_init_class_and_name(sk, "slock-AF_SMC", &smc_slock_key,
+ "sk_lock-AF_SMC", &smc_key);
spin_lock_init(&smc->accept_q_lock);
spin_lock_init(&smc->conn.send_lock);
sk->sk_prot->hash(sk);
--
2.48.1