CVE-2026-98541

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

fou: reject omitted FOU_ATTR_IPPROTO on FOU_ENCAP_DIRECT

Commit 7a9bc9e3f423 (fou: Dont 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 Software

References

• https://git.kernel.org/stable/c/3bcc1624cc1cdcc05b566475d6e848093ab2a8f6
• https://git.kernel.org/stable/c/46b1ffb7e169c7e401dd890b29e55dbc261f8f01
• https://git.kernel.org/stable/c/520cf06b8ea0ba5bcc1b8b982ef07f02b5e56221
• https://git.kernel.org/stable/c/7f4ba56f5d310dd018972916167985010a5d66a1
• https://git.kernel.org/stable/c/8b43f40798bb2758489fbbcae7e841ad4443c213
• https://git.kernel.org/stable/c/b1229b210ef5a39c58be665e6798203751411842
• https://git.kernel.org/stable/c/d22609f3d13fc5baacd92c222731b03c593401db
• https://git.kernel.org/stable/c/fd20220f86947d66d4dba2d76f13a28dbd86ef30