net: ipv4: Fix UDP length overflow with PMTU discover and big MTU

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Summary

CVECVE-2026-98393
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-10-14 14:17:50 UTC
Updated2026-10-19 10:18:42 UTC
DescriptionIn the Linux kernel, the following vulnerability has been resolved: net: ipv4: Fix UDP length overflow with PMTU discover and big MTU This commit bounds cork->base.fragsize to IP_MAX_MTU to avoid a possible overflow of UDP length that triggers a WARN in udp_set_len_short when setsockopt IP_MTU_DISCOVER is set to IP_PMTUDISC_PROBE, and a large packet is sent over a netdev with an unusually large MTU. Steps to reproduce: 1. Set device MTU bigger than IP_MAX_MTU + 20. cork->base.fragsize will be set to that MTU in ip_setup_cork. 2. Set IP_MTU_DISCOVER to IP_PMTUDISC_PROBE. It lets maxnonfragsize be set to device MTU (cork->fragsize) in __ip_append_data, rather than to IP_MAX_MTU. 3. Send 65528 bytes of payload (+8 bytes of UDP header, +20 bytes of IPv4 header). Device MTU allows it (it's only one byte bigger than IP_MAX_MTU + IPv4 header, 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. Note: IP_PMTUDISC_DO with IPv4 is safe, because ip_dst_mtu_maybe_forward always clamps at IP_MAX_MTU, unlike ip6_dst_mtu_maybe_forward. The Fixes tag points at the first commit where I could reproduce the overflow with IPv4 and IP_PMTUDISC_PROBE.

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected daba287b299ec7a2c61ae3a714920e90e8396ad5 83fedc4741210f75be8ca5b487985a242bf6f18d git Not specified
CNA Linux Linux affected daba287b299ec7a2c61ae3a714920e90e8396ad5 81f540827d243b8dad76c1f44e70ea31719f393c git Not specified
CNA Linux Linux affected daba287b299ec7a2c61ae3a714920e90e8396ad5 b83641e0ab8b20eefcc4cdc5a059f897375291a2 git Not specified
CNA Linux Linux affected 3.13 Not specified
CNA Linux Linux unaffected 3.13 semver Not specified
CNA Linux Linux unaffected 6.18.53 6.18.* semver Not specified
CNA Linux Linux unaffected 7.2.7 7.2.* semver Not specified
CNA Linux Linux unaffected 7.3 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/83fedc4741210f75be8ca5b487985a242bf6f18d 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/81f540827d243b8dad76c1f44e70ea31719f393c 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/b83641e0ab8b20eefcc4cdc5a059f897375291a2 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
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