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

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.