---
id: CVE-2026-98541
title: 'fou: reject omitted FOU_ATTR_IPPROTO on FOU_ENCAP_DIRECT'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  fou: reject omitted FOU_ATTR_IPPROTO on FOU_ENCAP_DIRECT

  Commit 7a9bc9e3f423 ("fou: Don't allow 0 for FOU_ATTR_IPPROTO.") added
  NLA_POLICY_MIN(NLA_U8, 1) to fou_nl_pol…
severity: none
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 <
    fd20220f86947d66d4dba2d76f13a28dbd86ef30
  - >-
    Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 <
    46b1ffb7e169c7e401dd890b29e55dbc261f8f01
  - >-
    Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 <
    7f4ba56f5d310dd018972916167985010a5d66a1
  - >-
    Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 <
    3bcc1624cc1cdcc05b566475d6e848093ab2a8f6
  - >-
    Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 <
    8b43f40798bb2758489fbbcae7e841ad4443c213
  - >-
    Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 <
    b1229b210ef5a39c58be665e6798203751411842
  - >-
    Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 <
    520cf06b8ea0ba5bcc1b8b982ef07f02b5e56221
  - >-
    Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 <
    d22609f3d13fc5baacd92c222731b03c593401db
  - Linux 3.18
published: '2026-10-18'
updated: '2026-10-19'
sourceUpdated: '2026-10-19T09:21:24.620Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-98541'
references:
  - url: 'https://git.kernel.org/stable/c/fd20220f86947d66d4dba2d76f13a28dbd86ef30'
  - url: 'https://git.kernel.org/stable/c/46b1ffb7e169c7e401dd890b29e55dbc261f8f01'
  - url: 'https://git.kernel.org/stable/c/7f4ba56f5d310dd018972916167985010a5d66a1'
  - url: 'https://git.kernel.org/stable/c/3bcc1624cc1cdcc05b566475d6e848093ab2a8f6'
  - url: 'https://git.kernel.org/stable/c/8b43f40798bb2758489fbbcae7e841ad4443c213'
  - url: 'https://git.kernel.org/stable/c/b1229b210ef5a39c58be665e6798203751411842'
  - url: 'https://git.kernel.org/stable/c/520cf06b8ea0ba5bcc1b8b982ef07f02b5e56221'
  - url: 'https://git.kernel.org/stable/c/d22609f3d13fc5baacd92c222731b03c593401db'
tags:
  - cve.org
ingestedAt: '2026-10-18T15:45:12.267Z'
---

## Overview

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

fou: reject omitted FOU_ATTR_IPPROTO on FOU_ENCAP_DIRECT

Commit 7a9bc9e3f423 ("fou: Don't allow 0 for FOU_ATTR_IPPROTO.") added
NLA_POLICY_MIN(NLA_U8, 1) to fou_nl_policy[FOU_ATTR_IPPROTO], which
rejects an explicitly supplied FOU_ATTR_IPPROTO == 0 attribute with
-ERANGE.

However, FOU_ATTR_IPPROTO is an optional netlink attribute. When a user
sends FOU_CMD_ADD with FOU_ATTR_TYPE set to FOU_ENCAP_DIRECT and omits
FOU_ATTR_IPPROTO entirely, nla_policy validation succeeds and
parse_nl_config() leaves cfg->protocol as 0 (from memset(cfg, 0,
sizeof(*cfg))). fou_create() then creates a FOU_ENCAP_DIRECT socket with
fou->protocol == 0.

In fou_udp_recv(), returning -fou->protocol to udp_queue_rcv_one_skb()
triggers IP protocol resubmission when fou->protocol > 0, whereas
returning 0 tells the UDP tunnel layer that the skb was consumed without
freeing it. When fou->protocol == 0, every packet received on the socket
returns 0 from fou_udp_recv() and leaks the sk_buff.

Reject FOU_ENCAP_DIRECT when !cfg->protocol in fou_create() so that
creating a direct encapsulation port without FOU_ATTR_IPPROTO fails with
-EINVAL while leaving FOU_CMD_DEL and FOU_CMD_GET (which share
parse_nl_config()) unaffected.

Tested in QEMU against Linux 7.3.0-rc3 by sending a FOU_CMD_ADD Generic
Netlink request with FOU_ATTR_PORT = 5555 and FOU_ATTR_TYPE =
FOU_ENCAP_DIRECT while omitting FOU_ATTR_IPPROTO. On the unfixed kernel,
FOU_CMD_ADD succeeds (err = 0), FOU_CMD_GET reports fou->type = 1 and
fou->protocol = 0, and sending 4000 UDP packets to 127.0.0.1:5555 leaks
all 4000 sk_buffs (SUnreclaim in /proc/meminfo grows from 41456 kB to
59008 kB, +17552 kB); with this patch applied, FOU_CMD_ADD is rejected
with -EINVAL (-22).

## Affected

- `Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 < fd20220f86947d66d4dba2d76f13a28dbd86ef30`
- `Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 < 46b1ffb7e169c7e401dd890b29e55dbc261f8f01`
- `Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 < 7f4ba56f5d310dd018972916167985010a5d66a1`
- `Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 < 3bcc1624cc1cdcc05b566475d6e848093ab2a8f6`
- `Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 < 8b43f40798bb2758489fbbcae7e841ad4443c213`
- `Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 < b1229b210ef5a39c58be665e6798203751411842`
- `Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 < 520cf06b8ea0ba5bcc1b8b982ef07f02b5e56221`
- `Linux >= 23461551c00628c3f3fe9cf837bf53cf8f212b63 < d22609f3d13fc5baacd92c222731b03c593401db`
- `Linux 3.18`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.