GHSA-935W-9G6M-G833
Vulnerability from github – Published: 2026-10-23 15:31 – Updated: 2026-10-23 15:31In the Linux kernel, the following vulnerability has been resolved:
flow_dissector: avoid u16 truncation of skb->len when computing thoff
__skb_flow_dissect() computes key_control->thoff using:
key_control->thoff = min_t(u16, nhoff, skb ? skb->len : hlen);
min_t(u16, ...) casts both arguments to u16 before comparing them. Whenever skb->len (or hlen) modulo 65536 is smaller than nhoff, the truncated length wins and thoff is set to a bogus small value, even when the dissection succeeded. For example, an IPv4/TCP frame with skb->len == 65540 and nhoff == 34 gets thoff == 4, so callers such as skb_probe_transport_header() point the transport header inside the Ethernet header.
Such skbs are not exotic: - At the time of commit d0c081b49137 ("flow_dissector: properly cap thoff field"), AF_PACKET with PACKET_VNET_HDR could already build GSO skbs larger than 64KB (MTU checks are skipped for GSO, and alloc_skb_with_frags() accepted up to MAX_SKB_FRAGS (17) order-0 pages on top of the linear part). - BIG TCP now makes skbs larger than 64KB common. - The following patch makes tun_get_user() dissect IFF_TAP frames before eth_type_trans() pulls the Ethernet header, so a GSO frame carrying a 65522..65535 byte L3 packet will be dissected with skb->len in [65536, 65549].
Compare as u32 instead. If the resulting offset cannot be represented in the u16 key_control->thoff, cap it to U16_MAX and report the dissection as failed rather than silently returning a wrong transport offset. thoff is still set on failure, as some callers (such as eth_get_headlen()) use it regardless of the return value.
{
"affected": [],
"aliases": [
"CVE-2026-98955"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-10-23T13:17:16Z",
"severity": null
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nflow_dissector: avoid u16 truncation of skb-\u003elen when computing thoff\n\n__skb_flow_dissect() computes key_control-\u003ethoff using:\n\n key_control-\u003ethoff = min_t(u16, nhoff, skb ? skb-\u003elen : hlen);\n\nmin_t(u16, ...) casts both arguments to u16 before comparing them.\nWhenever skb-\u003elen (or hlen) modulo 65536 is smaller than nhoff, the\ntruncated length wins and thoff is set to a bogus small value, even\nwhen the dissection succeeded. For example, an IPv4/TCP frame with\nskb-\u003elen == 65540 and nhoff == 34 gets thoff == 4, so callers such as\nskb_probe_transport_header() point the transport header inside the\nEthernet header.\n\nSuch skbs are not exotic:\n- At the time of commit d0c081b49137 (\"flow_dissector: properly cap\n thoff field\"), AF_PACKET with PACKET_VNET_HDR could already build GSO\n skbs larger than 64KB (MTU checks are skipped for GSO, and\n alloc_skb_with_frags() accepted up to MAX_SKB_FRAGS (17) order-0\n pages on top of the linear part).\n- BIG TCP now makes skbs larger than 64KB common.\n- The following patch makes tun_get_user() dissect IFF_TAP frames\n before eth_type_trans() pulls the Ethernet header, so a GSO frame\n carrying a 65522..65535 byte L3 packet will be dissected with\n skb-\u003elen in [65536, 65549].\n\nCompare as u32 instead. If the resulting offset cannot be represented\nin the u16 key_control-\u003ethoff, cap it to U16_MAX and report the\ndissection as failed rather than silently returning a wrong transport\noffset. thoff is still set on failure, as some callers (such as\neth_get_headlen()) use it regardless of the return value.",
"id": "GHSA-935w-9g6m-g833",
"modified": "2026-10-23T15:31:32Z",
"published": "2026-10-23T15:31:32Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98955"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/50d0cd575cab763f833d6ac6eb6550b23a0b3dae"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/52efce4755a4b8b82039caa773645ffc80b25362"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/9993e6a0ac1b5b3101ffba702cacb9f42b9909d8"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/99bda1ecbd56905f8bd934b9cdccd3bd251192b1"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/d9e08836d06c84abb18470c94b12b131eaccdc7d"
}
],
"schema_version": "1.4.0",
"severity": []
}
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