Description
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).
Published: 2026-10-18
Score: n/a
EPSS: n/a
KEV: No
Impact: n/a
Action: n/a
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History

Sun, 18 Oct 2026 15:30:00 +0000

Type Values Removed Values Added
Description 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).
Title fou: reject omitted FOU_ATTR_IPPROTO on FOU_ENCAP_DIRECT
First Time appeared Linux
Linux linux Kernel
CPEs cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Vendors & Products Linux
Linux linux Kernel
References

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Linux Linux Kernel
cve-icon MITRE

Status: PUBLISHED

Assigner: Linux

Published:

Updated: 2026-10-18T15:18:09.190Z

Reserved: 2026-09-25T10:25:14.356Z

Link: CVE-2026-98541

cve-icon Vulnrichment

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Weaknesses

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