Re: [PATCH bpf] bpf: tcp: Fix use-after-free in bpf_iter_tcp_established_batch()

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Fri Jun 19, 2026 at 8:32 PM EDT, Jose Fernandez (Anthropic) wrote:
> 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 <ben@cressey.dev>
> Assisted-by: Claude:unspecified
> Signed-off-by: Jose Fernandez (Anthropic) <jose.fernandez@linux.dev>

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

> ---
>  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) <jose.fernandez@linux.dev>