---
id: "CVE-2026-98503"
kind: "vulnerability"
aliases: ["CVE-2026-98503"]
title: "Linux kernel: AF_PACKET TX_RING slot can be reused while in flight"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["af_packet", "kernel", "linux", "runtime"]
not_affected_if: []
published_at: "2026-10-18T16:17:01+00:00"
first_seen_at: "2026-10-23T21:57:15+00:00"
updated_at: "2026-10-25T05:25:30+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98503", "first_seen_at": "2026-10-23T21:57:15+00:00"}]
---

# Linux kernel: AF_PACKET TX_RING slot can be reused while in flight

The Linux kernel's packet socket TX_RING code could release a ring slot while the sent skb still referenced its payload, because skb orphaning called the destructor early. The fix switches TX_RING to standard zerocopy completion so a slot is freed only after its payload references are released.

_Summary, check and fix written by a model from the sources; labelled as such._

## Timeline

- 2026-10-23T21:57:15+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-98503

## From the sources

In the Linux kernel, the following vulnerability has been resolved:

packet: use ubuf_info completion for TX_RING packets

tpacket_snd sends skbs with frags pointing into its ring slots. Slots
are released when skb->destructor is called.

A call to skb_orphan calls skb->destructor before the skb is freed.
This can cause the slot to be reused while still linked into the skb.

Switch to standard zerocopy completion (ubuf_info) so the slot is only
released once all references to the payload are freed or copied.
Restore skb->destructor to standard sock_wfree.

The ubuf_info completion callback can be called with a NULL skb, but
only from net_zcopy_put and related API, used by zerocopy implementations
that hold their own reference on the uarg, such as MSG_ZEROCOPY. This
uarg is only ever completed from skb_zcopy_clear, so skb is always set.

To prevent userspace from aliasing in-flight state on shared ring
slots, allocate tpacket_uarg per packet, rather than per slot. This
adds a small allocation to the transmit path. Use standard kmalloc to
allow backporting to stable kernels.

The uarg holds an sk_wmem_alloc reference, rather than an sk_refcnt
reference. packet_free_tx_ring waits on sk_wmem_alloc before freeing
the ring pages. Always allocate vec->deferred for tx_ring so page-backed
rings also wait on sk_wmem_alloc when skb_copy_ubufs drops page refs
before calling tpacket_ubuf_complete.

Drop the tx_ring.pg_vec test that tpacket_destruct_skb performed before
accessing the slot. The sk_wmem_alloc reference now guarantees that the
slot is valid. The test is also not sufficient by itself, as it reads
pg_vec without pg_vec_lock, so it can race with packet_set_ring.

As a result a slot is released when its payload is copied, which can
be before transmission (e.g., in skb_orphan_frags_rx). If copied
before skb_tx_timestamp() is called, no slot timestamp is recorded,
similar to when skb_orphan() was called early in the datapath before
this patch.

Revert the now unused previous skb_zcopy_.._nouarg infra.

Depends on commit 992cc9f94ca9 ("net/packet: defer vmalloc TX_RING
free until skbs finish").

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