From: Jose Fernandez (Anthropic) Date: Thu, 30 Jul 2026 22:32:47 +0000 (+0000) Subject: bpf: tcp: Fix use-after-free in bpf_iter_tcp_established_batch() X-Git-Tag: v7.2-rc7~18^2~3 X-Git-Url: http://git.ipfire.org/gitweb/?a=commitdiff_plain;h=e5fd3f514e27db1f05fbd72ba615d74941e23c51;p=thirdparty%2Fkernel%2Flinux.git 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. Fixes: 04c7820b776f ("bpf: tcp: Bpf iter batching and lock_sock") Assisted-by: Claude:unspecified Signed-off-by: Jose Fernandez (Anthropic) Reviewed-by: Kuniyuki Iwashima Link: https://lore.kernel.org/bpf/20260730-bpf-iter-tcp-refcnt-v3-1-754b9c8a6717@linux.dev Signed-off-by: Kumar Kartikeya Dwivedi --- diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c index b8887cdd66c5..7f413f509d7d 100644 --- a/net/ipv4/tcp_ipv4.c +++ b/net/ipv4/tcp_ipv4.c @@ -3078,24 +3078,24 @@ 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; - - sock_hold(*start_sk); - iter->batch[iter->end_sk++].sk = *start_sk; + struct sock *sk = *start_sk; + unsigned int expected = 0; - 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; + expected++; + 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; } } @@ -3133,12 +3133,13 @@ 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 */ expected = bpf_iter_fill_batch(seq, &sk); - if (likely(iter->end_sk == expected)) + if (likely(!sk)) goto done; /* Batch size was too small. */ @@ -3157,7 +3158,7 @@ static struct sock *bpf_iter_tcp_batch(struct seq_file *seq) return NULL; /* Done */ expected = bpf_iter_fill_batch(seq, &sk); - if (likely(iter->end_sk == expected)) + if (likely(!sk)) goto done; /* Batch size was still too small. Hold onto the lock while we try @@ -3170,10 +3171,14 @@ static struct sock *bpf_iter_tcp_batch(struct seq_file *seq) return ERR_PTR(err); } - expected = bpf_iter_fill_batch(seq, &sk); - WARN_ON_ONCE(iter->end_sk != expected); + bpf_iter_fill_batch(seq, &sk); + WARN_ON_ONCE(sk); done: bpf_iter_tcp_unlock_bucket(seq); + if (unlikely(!iter->end_sk)) { + ++iter->state.bucket; + goto again; + } return iter->batch[0].sk; }