net/smc: Avoid overwriting the copies of clcsock callback functions

The callback functions of clcsock will be saved and replaced during
the fallback. But if the fallback happens more than once, then the
copies of these callback functions will be overwritten incorrectly,
resulting in a loop call issue:

clcsk->sk_error_report
 |- smc_fback_error_report() <------------------------------|
     |- smc_fback_forward_wakeup()                          | (loop)
         |- clcsock_callback()  (incorrectly overwritten)   |
             |- smc->clcsk_error_report() ------------------|

So this patch fixes the issue by saving these function pointers only
once in the fallback and avoiding overwriting.

Reported-by: syzbot+4de3c0e8a263e1e499bc@syzkaller.appspotmail.com
Fixes: 341adee ("net/smc: Forward wakeup to smc socket waitqueue after fallback")
Link: https://lore.kernel.org/r/0000000000006d045e05d78776f6@google.com
Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/smc/af_smc.c

@@ -667,14 +667,17 @@ static void smc_fback_error_report(struct sock *clcsk)
 static int smc_switch_to_fallback(struct smc_sock *smc, int reason_code)
 {
 	struct sock *clcsk;
+	int rc = 0;
 
 	mutex_lock(&smc->clcsock_release_lock);
 	if (!smc->clcsock) {
-		mutex_unlock(&smc->clcsock_release_lock);
-		return -EBADF;
+		rc = -EBADF;
+		goto out;
 	}
 	clcsk = smc->clcsock->sk;
 
+	if (smc->use_fallback)
+		goto out;
 	smc->use_fallback = true;
 	smc->fallback_rsn = reason_code;
 	smc_stat_fallback(smc);
@@ -702,8 +705,9 @@ static int smc_switch_to_fallback(struct smc_sock *smc, int reason_code)
 		smc->clcsock->sk->sk_user_data =
 			(void *)((uintptr_t)smc | SK_USER_DATA_NOCOPY);
 	}
+out:
 	mutex_unlock(&smc->clcsock_release_lock);
-	return 0;
+	return rc;
 }
 
 /* fall back during connect */