net/smc: fix general protection fault in __smc_diag_dump

The syzbot report a crash:

Oops: general protection fault, probably for non-canonical address 0xfbd5a5d5a0000003: 0000 [#1] SMP KASAN NOPTI
KASAN: maybe wild-memory-access in range [0xdead4ead00000018-0xdead4ead0000001f]
CPU: 1 UID: 0 PID: 6949 Comm: syz.0.335 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
RIP: 0010:smc_diag_msg_common_fill net/smc/smc_diag.c:44 [inline]
RIP: 0010:__smc_diag_dump.constprop.0+0x3ca/0x2550 net/smc/smc_diag.c:89
Call Trace:
<TASK>
smc_diag_dump_proto+0x26d/0x420 net/smc/smc_diag.c:217
smc_diag_dump+0x27/0x90 net/smc/smc_diag.c:234
netlink_dump+0x539/0xd30 net/netlink/af_netlink.c:2327
__netlink_dump_start+0x6d6/0x990 net/netlink/af_netlink.c:2442
netlink_dump_start include/linux/netlink.h:341 [inline]
smc_diag_handler_dump+0x1f9/0x240 net/smc/smc_diag.c:251
__sock_diag_cmd net/core/sock_diag.c:249 [inline]
sock_diag_rcv_msg+0x438/0x790 net/core/sock_diag.c:285
netlink_rcv_skb+0x158/0x420 net/netlink/af_netlink.c:2552
netlink_unicast_kernel net/netlink/af_netlink.c:1320 [inline]
netlink_unicast+0x5a7/0x870 net/netlink/af_netlink.c:1346
netlink_sendmsg+0x8d1/0xdd0 net/netlink/af_netlink.c:1896
sock_sendmsg_nosec net/socket.c:714 [inline]
__sock_sendmsg net/socket.c:729 [inline]
____sys_sendmsg+0xa95/0xc70 net/socket.c:2614
___sys_sendmsg+0x134/0x1d0 net/socket.c:2668
__sys_sendmsg+0x16d/0x220 net/socket.c:2700
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xcd/0x4e0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
</TASK>

The process like this:

(CPU1) | (CPU2)
---------------------------------|-------------------------------
inet_create() |
// init clcsock to NULL |
sk = sk_alloc() |
|
// unexpectedly change clcsock |
inet_init_csk_locks() |
|
// add sk to hash table |
smc_inet_init_sock() |
smc_sk_init() |
smc_hash_sk() |
| // traverse the hash table
| smc_diag_dump_proto
| __smc_diag_dump()
| // visit wrong clcsock
| smc_diag_msg_common_fill()
// alloc clcsock |
smc_create_clcsk |
sock_create_kern |

With CONFIG_DEBUG_LOCK_ALLOC=y, the smc->clcsock is unexpectedly changed
in inet_init_csk_locks(). The INET_PROTOSW_ICSK flag is no need by smc,
just remove it.

After removing the INET_PROTOSW_ICSK flag, this patch alse revert
commit 6fd27ea183c2 ("net/smc: fix lacks of icsk_syn_mss with IPPROTO_SMC")
to avoid casting smc_sock to inet_connection_sock.

Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=f775be4458668f7d220e
Tested-by: [e-mail]
Fixes: d25a92ccae6b ("net/smc: Introduce IPPROTO_SMC")
Signed-off-by: Wang Liang <[e-mail]>
Reviewed-by: Kuniyuki Iwashima <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
Reviewed-by: D. Wythe <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/smc/smc_inet.c

···
 	.protocol	= IPPROTO_SMC,
 	.prot		= &smc_inet_prot,
 	.ops		= &smc_inet_stream_ops,
-	.flags		= INET_PROTOSW_ICSK,
 };
 
 #if IS_ENABLED(CONFIG_IPV6)
···
 	.protocol	= IPPROTO_SMC,
 	.prot		= &smc_inet6_prot,
 	.ops		= &smc_inet6_stream_ops,
-	.flags		= INET_PROTOSW_ICSK,
 };
 #endif /* CONFIG_IPV6 */
-
-static unsigned int smc_sync_mss(struct sock *sk, u32 pmtu)
-{
-	/* No need pass it through to clcsock, mss can always be set by
-	 * sock_create_kern or smc_setsockopt.
-	 */
-	return 0;
-}
 
 static int smc_inet_init_sock(struct sock *sk)
 {
···
 
 	/* init common smc sock */
 	smc_sk_init(net, sk, IPPROTO_SMC);
-
-	inet_csk(sk)->icsk_sync_mss = smc_sync_mss;
-
 	/* create clcsock */
 	return smc_create_clcsk(net, sk, sk->sk_family);
 }