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 6fd27ea ("net/smc: fix lacks of icsk_syn_mss with IPPROTO_SMC")
to avoid casting smc_sock to inet_connection_sock.

Reported-by: syzbot+f775be4458668f7d220e@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f775be4458668f7d220e
Tested-by: syzbot+f775be4458668f7d220e@syzkaller.appspotmail.com
Fixes: d25a92c ("net/smc: Introduce IPPROTO_SMC")
Signed-off-by: Wang Liang <wangliang74@huawei.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: D. Wythe <alibuda@linux.alibaba.com>
Link: https://patch.msgid.link/20251017024827.3137512-1-wangliang74@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Wang Liang

net/smc/smc_inet.c

@@ -56,7 +56,6 @@ static struct inet_protosw smc_inet_protosw = {
 	.protocol	= IPPROTO_SMC,
 	.prot		= &smc_inet_prot,
 	.ops		= &smc_inet_stream_ops,
-	.flags		= INET_PROTOSW_ICSK,
 };
 
 #if IS_ENABLED(CONFIG_IPV6)
@@ -104,27 +103,15 @@ static struct inet_protosw smc_inet6_protosw = {
 	.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)
 {
 	struct net *net = sock_net(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);
 }