Fixes deadlock described in this bug:
https://syzkaller.appspot.com/bug?extid=e953a8f3071f5c0a28fd.
Specific crash report here:
https://syzkaller.appspot.com/text?tag=CrashReport&x=14670e07980000.

DESCRIPTION OF ISSUE
Deadlock: sk_lock-AF_INET --> &smc->clcsock_release_lock --> rtnl_mutex

rtnl_mutex->sk_lock-AF_INET
rtnetlink_rcv_msg() acquires rtnl_lock() and calls rtnl_newlink(), which
eventually calls gtp_newlink() which calls lock_sock() to attempt to
acquire sk_lock.

sk_lock-AF_INET->&smc->clcsock_release_lock
smc_sendmsg() calls lock_sock() to acquire sk_lock, then calls
smc_switch_to_fallback() which attempts to acquire mutex_lock(&smc->...).

&smc->clcsock_release_lock->rtnl_mutex
smc_setsockopt() calls mutex_lock(&smc->...). smc->...->setsockopt() is
called, which calls nf_setsockopt() which attempts to acquire
rtnl_lock() in some nested call in start_sync_thread() in ip_vs_sync.c.

FIX:
In smc_switch_to_fallback(), separate the logic into inline function
__smc_switch_to_fallback(). In smc_sendmsg(), lock ordering can be
modified and the functionality of smc_switch_to_fallback() is
encapsulated in the __smc_switch_to_fallback() function.

Signed-off-by: Daniel Yang <danielyangkang@gmail.com>
Tested-by: Daniel Yang <danielyangkang@gmail.com>
Reported-by: syzbot+e953a8f3071f5c0a28fd@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e953a8f3071f5c0a28fd
---
 net/smc/af_smc.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
index 0316217b7..e04f132be 100644
--- a/net/smc/af_smc.c
+++ b/net/smc/af_smc.c
@@ -895,11 +895,15 @@ static void smc_fback_replace_callbacks(struct smc_sock *smc)
 	write_unlock_bh(&clcsk->sk_callback_lock);
 }
 
-static int smc_switch_to_fallback(struct smc_sock *smc, int reason_code)
+/* assumes smc->clcsock_release_lock is held during execution
+ * reason for separating locking is to give flexibility in
+ * lock ordering in functions wanting to call smc_switch_to_fallback
+ * so that deadlocks can be avoided.
+ */
+static inline int __smc_switch_to_fallback(struct smc_sock *smc, int reason_code)
 {
 	int rc = 0;
 
-	mutex_lock(&smc->clcsock_release_lock);
 	if (!smc->clcsock) {
 		rc = -EBADF;
 		goto out;
@@ -923,6 +927,13 @@ static int smc_switch_to_fallback(struct smc_sock *smc, int reason_code)
 		smc_fback_replace_callbacks(smc);
 	}
 out:
+	return rc;
+}
+
+static int smc_switch_to_fallback(struct smc_sock *smc, int reason_code)
+{
+	mutex_lock(&smc->clcsock_release_lock);
+	int rc = __smc_switch_to_fallback(smc, reason_code);
 	mutex_unlock(&smc->clcsock_release_lock);
 	return rc;
 }
@@ -2762,13 +2773,15 @@ int smc_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 	int rc;
 
 	smc = smc_sk(sk);
+	/* acquire smc lock before sk to avoid deadlock with rtnl */
+	mutex_lock(&smc->clcsock_release_lock);
 	lock_sock(sk);
 
 	/* SMC does not support connect with fastopen */
 	if (msg->msg_flags & MSG_FASTOPEN) {
 		/* not connected yet, fallback */
 		if (sk->sk_state == SMC_INIT && !smc->connect_nonblock) {
-			rc = smc_switch_to_fallback(smc, SMC_CLC_DECL_OPTUNSUPP);
+			rc = __smc_switch_to_fallback(smc, SMC_CLC_DECL_OPTUNSUPP);
 			if (rc)
 				goto out;
 		} else {
@@ -2790,6 +2803,7 @@ int smc_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 	}
 out:
 	release_sock(sk);
+	mutex_unlock(&smc->clcsock_release_lock);
 	return rc;
 }
 
-- 
2.39.2

From: Daniel Yang <danielyangkang@gmail.com>
Date: Mon, 30 Sep 2024 18:55:54 -0700
> Fixes deadlock described in this bug:
> https://syzkaller.appspot.com/bug?extid=e953a8f3071f5c0a28fd.
> Specific crash report here:
> https://syzkaller.appspot.com/text?tag=CrashReport&x=14670e07980000.
> 
> DESCRIPTION OF ISSUE
> Deadlock: sk_lock-AF_INET --> &smc->clcsock_release_lock --> rtnl_mutex
> 
> rtnl_mutex->sk_lock-AF_INET
> rtnetlink_rcv_msg() acquires rtnl_lock() and calls rtnl_newlink(), which
> eventually calls gtp_newlink() which calls lock_sock() to attempt to
> acquire sk_lock.

Is the deadlock real ?

From the lockdep splat, the gtp's sk_protocol is verified to be
IPPROTO_UDP before holding lock_sock(), so it seems just a labeling
issue.
https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/drivers/net/gtp.c?id=9410645520e9b820069761f3450ef6661418e279#n1674


> 
> sk_lock-AF_INET->&smc->clcsock_release_lock
> smc_sendmsg() calls lock_sock() to acquire sk_lock, then calls
> smc_switch_to_fallback() which attempts to acquire mutex_lock(&smc->...).
> 
> &smc->clcsock_release_lock->rtnl_mutex
> smc_setsockopt() calls mutex_lock(&smc->...). smc->...->setsockopt() is
> called, which calls nf_setsockopt() which attempts to acquire
> rtnl_lock() in some nested call in start_sync_thread() in ip_vs_sync.c.
> 
> FIX:
> In smc_switch_to_fallback(), separate the logic into inline function
> __smc_switch_to_fallback(). In smc_sendmsg(), lock ordering can be
> modified and the functionality of smc_switch_to_fallback() is
> encapsulated in the __smc_switch_to_fallback() function.