bpf: tcp: Fix use-after-free in bpf_iter_tcp_established_batch()

Summary

In the Linux kernel, the following vulnerability has been resolved: bpf: tcp: Fix use-after-free in bpf_iter_tcp_established_batch() reqsk_queue_hash_req() publishes a TCP_NEW_SYN_RECV request_sock onto the ehash chain, drops the bucket lock, and only afterwards sets rsk_refcnt to 3. Lockless readers such as __inet_lookup_established() handle this with 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 The iterator's stolen reference is lost when the publishing CPU's refcount_set() overwrites the count, leaving the socket one reference short. When the last legitimate owner drops its reference the reqsk is freed while still reachable, leading to use-after-free. 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. A skipped socket is still part of the bucket, so keep counting it in expected. The reallocations are sized from expected, and a request sock whose refcount gets published while the lock is held across the last realloc must already have room. A skipped socket is counted in expected but never batched, so end_sk can be short of expected on a batch that is actually complete. Decide completeness by whether the walk left any socket behind instead. The WARN after the locked realloc checks the same, replacing an end_sk == expected check that could not hold on that path since commit cdec67a489d4 ("bpf: tcp: Make sure iter->batch always contains a full bucket snapshot"). If every matching socket in a bucket is mid-init (refcount 0), end_sk stays 0. Advance to the next bucket rather than returning a batch entry that was never filled this round.