Improper Update of Reference Count Affecting kernel-64k-core package, versions *

Note: Versions mentioned in the description apply only to the upstream kernel-64k-core package and not the kernel-64k-core package as distributed by Centos. See How to fix? for Centos:10 relevant fixed versions and status.

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.

• https://bugzilla.redhat.com/show_bug.cgi?id=2521378
• https://access.redhat.com/security/cve/CVE-2026-74714
• https://git.kernel.org/stable/c/7d2b60a4bc0499f62ff8520af6309bbe170882fd
• https://git.kernel.org/stable/c/97e74d3e45d653c07c2d406fc530a9bbe3df8396
• https://git.kernel.org/stable/c/cc0295f89296ed351fc4b0b48fee887ba02c5d24
• https://git.kernel.org/stable/c/cefcbbe20846a45f9a7dae868f7ef1000953e2df
• https://git.kernel.org/stable/c/ddbe966b5d1fe212ada749bc3d0b410f1a7dea74
• https://git.kernel.org/stable/c/e5fd3f514e27db1f05fbd72ba615d74941e23c51