[PATCH net-next v5 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance_schedule()

[xuanqiang.luo@linux.dev]

From: Xuanqiang Luo <luoxuanqiang@kylinos.cn>

Since ehash lookups are lockless, if another CPU is converting sk to tw
concurrently, fetching the newly inserted tw with tw->tw_refcnt == 0 cause
lookup failure.

The call trace map is drawn as follows:
   CPU 0                                CPU 1
   -----                                -----
				     inet_twsk_hashdance_schedule()
				     spin_lock()
				     inet_twsk_add_node_rcu(tw, ...)
__inet_lookup_established()
(find tw, failure due to tw_refcnt = 0)
				     __sk_nulls_del_node_init_rcu(sk)
				     refcount_set(&tw->tw_refcnt, 3)
				     spin_unlock()

By replacing sk with tw atomically via hlist_nulls_replace_init_rcu() after
setting tw_refcnt, we ensure that tw is either fully initialized or not
visible to other CPUs, eliminating the race.

It's worth noting that we held lock_sock() before the replacement, so
there's no need to check if sk is hashed. Thanks to Kuniyuki Iwashima!

Fixes: 3ab5aee7fe84 ("net: Convert TCP & DCCP hash tables to use RCU / hlist_nulls")
Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
---
 net/ipv4/inet_timewait_sock.c | 24 ++++++++++--------------
 1 file changed, 10 insertions(+), 14 deletions(-)

diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
index 5b5426b8ee92..80b9ef3a128f 100644
--- a/net/ipv4/inet_timewait_sock.c
+++ b/net/ipv4/inet_timewait_sock.c
@@ -116,7 +116,7 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
 	spinlock_t *lock = inet_ehash_lockp(hashinfo, sk->sk_hash);
 	struct inet_bind_hashbucket *bhead, *bhead2;
 
-	/* Step 1: Put TW into bind hash. Original socket stays there too.
+	/* Put TW into bind hash. Original socket stays there too.
 	   Note, that any socket with inet->num != 0 MUST be bound in
 	   binding cache, even if it is closed.
 	 */
@@ -140,19 +140,6 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
 
 	spin_lock(lock);
 
-	/* Step 2: Hash TW into tcp ehash chain */
-	inet_twsk_add_node_rcu(tw, &ehead->chain);
-
-	/* Step 3: Remove SK from hash chain */
-	if (__sk_nulls_del_node_init_rcu(sk))
-		sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
-
-
-	/* Ensure above writes are committed into memory before updating the
-	 * refcount.
-	 * Provides ordering vs later refcount_inc().
-	 */
-	smp_wmb();
 	/* tw_refcnt is set to 3 because we have :
 	 * - one reference for bhash chain.
 	 * - one reference for ehash chain.
@@ -162,6 +149,15 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
 	 */
 	refcount_set(&tw->tw_refcnt, 3);
 
+	/* Ensure tw_refcnt has been set before tw is published.
+	 * smp_wmb() provides the necessary memory barrier to enforce this
+	 * ordering.
+	 */
+	smp_wmb();
+
+	hlist_nulls_replace_init_rcu(&sk->sk_nulls_node, &tw->tw_node);
+	sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
+
 	inet_twsk_schedule(tw, timeo);
 
 	spin_unlock(lock);
-- 
2.25.1