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inet: Avoid ehash lookup race in inet_twsk_hashdance_schedule()

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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: 3ab5aee7fe ("net: Convert TCP & DCCP hash tables to use RCU / hlist_nulls")
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20251015020236.431822-4-xuanqiang.luo@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Xuanqiang Luo 2025-10-15 10:02:36 +08:00 • committed by Jakub Kicinski
parent 1532ed0d07
commit b8ec80b130
1 changed files with 12 additions and 23 deletions
Download patch file Download diff file Expand all files Collapse all files

35
net/ipv4/inet_timewait_sock.c
View file

@ -88,12 +88,6 @@ void inet_twsk_put(struct inet_timewait_sock *tw)
}
EXPORT_SYMBOL_GPL(inet_twsk_put);
static void inet_twsk_add_node_rcu(struct inet_timewait_sock *tw,
struct hlist_nulls_head *list)
{
hlist_nulls_add_head_rcu(&tw->tw_node, list);
}
static void inet_twsk_schedule(struct inet_timewait_sock *tw, int timeo)
{
__inet_twsk_schedule(tw, timeo, false);
@ -113,13 +107,12 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
{
const struct inet_sock *inet = inet_sk(sk);
const struct inet_connection_sock *icsk = inet_csk(sk);
struct inet_ehash_bucket *ehead = inet_ehash_bucket(hashinfo, sk->sk_hash);
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.
Note, that any socket with inet->num != 0 MUST be bound in
binding cache, even if it is closed.
/* 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.
*/
bhead = &hashinfo->bhash[inet_bhashfn(twsk_net(tw), inet->inet_num,
hashinfo->bhash_size)];
@ -141,19 +134,6 @@ 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.
@ -163,6 +143,15 @@ 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);