Re: [PATCH net-next v4 3/3] inet: Avoid ehash lookup race in inet_twsk_hashdance_schedule()
From: Kuniyuki Iwashima <kuniyu@google.com>
Date: 2025-09-23 00:45:43
On Sat, Sep 20, 2025 at 4:00 AM [off-list ref] wrote:
From: Xuanqiang Luo <redacted>
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 replace under lock_sock(), so 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")
Suggested-by: Kuniyuki Iwashima <kuniyu@google.com>This is not needed. A pure review does not deserve Suggested-by. This is used when someone suggests changing the core idea of the patch.
quoted hunk ↗ jump to hunk
Signed-off-by: Xuanqiang Luo <redacted> --- net/ipv4/inet_timewait_sock.c | 13 ++++--------- 1 file changed, 4 insertions(+), 9 deletions(-)diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c index 5b5426b8ee92..bb98888584a8 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. */@@ -140,14 +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().@@ -162,6 +154,9 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw, */ refcount_set(&tw->tw_refcnt, 3);
I discussed this series with Eric last week, and he pointed out (thanks!) that we need to be careful here about memory barrier. refcount_set() is just WRITE_ONCE() and thus can be reordered, and twsk could be published with 0 refcnt, resulting in another RST.
+ 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