reqsk_queue_hash_req() publishes a TCP_NEW_SYN_RECV request_sock onto the ehash chain (via inet_ehash_insert(), which drops the bucket lock on return) and only afterwards refcount_set()s rsk_refcnt to 3. Lockless readers such as __inet_lookup_established() account for this by using refcount_inc_not_zero(), but bpf_iter_tcp_established_batch() uses plain sock_hold() while holding the bucket lock, on the assumption that the lock guarantees sk_refcnt > 0. That assumption does not hold for request_sock: CPU 0 CPU 1 ----- ----- tcp_conn_request() reqsk_queue_hash_req() inet_ehash_insert(req) spin_lock(bucket) __sk_nulls_add_node_rcu(req) // rsk_refcnt == 0 spin_unlock(bucket) bpf_iter_tcp_established_batch() spin_lock(bucket) sock_hold(req) <-- addition on 0 spin_unlock(bucket) refcount_set(&req->rsk_refcnt, 3) // clobbers saturated value which surfaces as: refcount_t: addition on 0; use-after-free. WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x48/0x90, CPU#1 Call Trace: bpf_iter_tcp_established_batch+0x14e/0x170 bpf_iter_tcp_batch+0x53/0x200 bpf_iter_tcp_seq_next+0x27/0x70 bpf_seq_read+0x107/0x410 vfs_read+0xb9/0x380 refcount_warn_saturate() then saturates the count, the publishing CPU's refcount_set() clobbers it, and the socket is left one reference short. When the last legitimate owner drops its reference the reqsk is freed while still reachable, leading to use-after-free panics in e.g. inet_csk_accept() or inet_csk_listen_stop(). This reproduces in seconds with tcp_syncookies=0, a handful of threads doing connect()/close() to a local listener while others read an iter/tcp link in a tight loop. Use refcount_inc_not_zero() and skip the socket on failure, the same way every other ehash walker does. The listening hash is unaffected as listeners are always inserted into lhash2 with sk_refcnt >= 1, so bpf_iter_tcp_listening_batch() is left as-is. If every matching socket in a bucket is mid-init, end_sk can stay at 0; advance to the next bucket in that case rather than terminating the whole iteration on a stale batch[0]. Fixes: 04c7820b776f ("bpf: tcp: Bpf iter batching and lock_sock") Reviewed-by: Ben Cressey Assisted-by: Claude:unspecified Signed-off-by: Jose Fernandez (Anthropic) --- net/ipv4/tcp_ipv4.c | 35 ++++++++++++++++++++--------------- 1 file changed, 20 insertions(+), 15 deletions(-) diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c index fdc81150ff6c..92342dcc6892 100644 --- a/net/ipv4/tcp_ipv4.c +++ b/net/ipv4/tcp_ipv4.c @@ -3074,25 +3074,25 @@ static unsigned int bpf_iter_tcp_established_batch(struct seq_file *seq, { struct bpf_tcp_iter_state *iter = seq->private; struct hlist_nulls_node *node; - unsigned int expected = 1; - struct sock *sk; + unsigned int expected = 0; + struct sock *sk = *start_sk; - sock_hold(*start_sk); - iter->batch[iter->end_sk++].sk = *start_sk; - - sk = sk_nulls_next(*start_sk); *start_sk = NULL; sk_nulls_for_each_from(sk, node) { - if (seq_sk_match(seq, sk)) { - if (iter->end_sk < iter->max_sk) { - sock_hold(sk); - iter->batch[iter->end_sk++].sk = sk; - } else if (!*start_sk) { - /* Remember where we left off. */ - *start_sk = sk; - } - expected++; + if (!seq_sk_match(seq, sk)) + continue; + if (iter->end_sk < iter->max_sk) { + /* reqsk_queue_hash_req() inserts with sk_refcnt == 0 + * and refcount_set()s it after the bucket lock drops. + */ + if (unlikely(!refcount_inc_not_zero(&sk->sk_refcnt))) + continue; + iter->batch[iter->end_sk++].sk = sk; + } else if (!*start_sk) { + /* Remember where we left off. */ + *start_sk = sk; } + expected++; } return expected; @@ -3129,6 +3129,7 @@ static struct sock *bpf_iter_tcp_batch(struct seq_file *seq) struct sock *sk; int err; +again: sk = bpf_iter_tcp_resume(seq); if (!sk) return NULL; /* Done */ @@ -3167,6 +3168,10 @@ static struct sock *bpf_iter_tcp_batch(struct seq_file *seq) WARN_ON_ONCE(iter->end_sk != expected); done: bpf_iter_tcp_unlock_bucket(seq); + if (unlikely(!iter->end_sk)) { + ++iter->state.bucket; + goto again; + } return iter->batch[0].sk; } --- base-commit: 4549871118cf616eecdd2d939f78e3b9e1dddc48 change-id: 20260619-bpf-iter-tcp-refcnt-107d52b238da Best regards, -- Jose Fernandez (Anthropic)