smc: Use __sk_dst_get() and dst_dev_rcu() in smc_clc_prfx_match().

[ Upstream commit 235f810 ]

smc_clc_prfx_match() is called from smc_listen_work() and
not under RCU nor RTNL.

Using sk_dst_get(sk)->dev could trigger UAF.

Let's use __sk_dst_get() and dst_dev_rcu().

Note that the returned value of smc_clc_prfx_match() is not
used in the caller.

Fixes: a046d57 ("smc: CLC handshake (incl. preparation steps)")
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250916214758.650211-4-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: q2ven

net/smc/smc_clc.c

@@ -657,26 +657,26 @@ static int smc_clc_prfx_match6_rcu(struct net_device *dev,
 int smc_clc_prfx_match(struct socket *clcsock,
 		       struct smc_clc_msg_proposal_prefix *prop)
 {
-	struct dst_entry *dst = sk_dst_get(clcsock->sk);
+	struct net_device *dev;
+	struct dst_entry *dst;
 	int rc;
 
-	if (!dst) {
-		rc = -ENOTCONN;
-		goto out;
-	}
-	if (!dst->dev) {
+	rcu_read_lock();
+
+	dst = __sk_dst_get(clcsock->sk);
+	dev = dst ? dst_dev_rcu(dst) : NULL;
+	if (!dev) {
 		rc = -ENODEV;
-		goto out_rel;
+		goto out;
 	}
-	rcu_read_lock();
+
 	if (!prop->ipv6_prefixes_cnt)
-		rc = smc_clc_prfx_match4_rcu(dst->dev, prop);
+		rc = smc_clc_prfx_match4_rcu(dev, prop);
 	else
-		rc = smc_clc_prfx_match6_rcu(dst->dev, prop);
-	rcu_read_unlock();
-out_rel:
-	dst_release(dst);
+		rc = smc_clc_prfx_match6_rcu(dev, prop);
 out:
+	rcu_read_unlock();
+
 	return rc;
 }