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 <redacted>
Assisted-by: Claude:unspecified
Signed-off-by: Jose Fernandez (Anthropic) <redacted>
---
 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) [off-list ref]