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CVE-2026-97984

net: ipv6: Fix UDP length overflow with PMTU discover and big MTU PUBLISHED

Description (CNA)

In the Linux kernel, the following vulnerability has been resolved: net: ipv6: Fix UDP length overflow with PMTU discover and big MTU This commit bounds cork->base.fragsize to IP6_MAX_MTU for UDP sockets to avoid a possible overflow of UDP length that triggers a WARN in udp_set_len_short when setsockopt IPV6_MTU_DISCOVER is set to IPV6_PMTUDISC_DO or IPV6_PMTUDISC_PROBE, and a large packet is sent over a netdev with an unusually large MTU. Steps to reproduce (included in the new selftest): 1. Set device MTU bigger than IP6_MAX_MTU. cork->base.fragsize will be set to that MTU in ip6_setup_cork. 2. Set IPV6_MTU_DISCOVER to IPV6_PMTUDISC_PROBE or IPV6_PMTUDISC_DO. It lets maxnonfragsize be set to device MTU (cork->fragsize) in __ip6_append_data, rather than to IP6_MAX_MTU. 3. Send 65528 bytes of payload (+8 bytes of UDP header, +40 bytes of IPv6 header). Device MTU allows it (it's only one byte bigger than IP6_MAX_MTU, and the device MTU is bigger than that). 4. The UDP length in the built packet is 65536, which overflows the 16-bit length field and triggers the WARN in udp_set_len_short. To avoid breaking sending UDP jumbograms over raw IPv6 sockets, limit the change to UDP sockets only. The original overflow bug with IPv6 and IPV6_PMTUDISC_DO seems to predate git history (verified reproduction on 2.6.21), was fixed later, and then reappeared in commit 427faee167bc ("net: ipv6: introduce ip6_dst_mtu_maybe_forward"), which is chosen as the Fixes tag here. The overflow with IPV6_PMTUDISC_PROBE reproduces since its introduction in commit 628a5c561890 ("[INET]: Add IP(V6)_PMTUDISC_RPOBE").

Affected (CNA)

Vendor / productVersions
Linux / Linuxaffected 427faee167bce4ccb0b22f6815a9ac509e31d4c3 before 1dd562f3019019ffdf4b396570613053facda9ee; affected 427faee167bce4ccb0b22f6815a9ac509e31d4c3 before 1d56a00ec91f2885c64ea5b8cea18cb746b91bbb; affected 427faee167bce4ccb0b22f6815a9ac509e31d4c3 before ce66352d3eb21825410af8b91db7cc4bb8e22c98; affected 427faee167bce4ccb0b22f6815a9ac509e31d4c3 before e01b7c176d9af2b7c22bb3fe12cf9a7904f99fdc; affected 427faee167bce4ccb0b22f6815a9ac509e31d4c3 before 0ae10b6be49b425827659b23bcce498f80eb7182
Linux / Linuxaffected 5.15; unaffected 0 before 5.15; unaffected 6.6.158 through 6.6.*; unaffected 6.12.111 through 6.12.*; unaffected 6.18.53 through 6.18.*; unaffected 7.2.7 through 7.2.*; unaffected 7.3 through *

CWE

No CWE identifiers supplied.

References