GHSA-56F8-X2FW-63M3
Vulnerability from github – Published: 2026-09-04 18:31 – Updated: 2026-09-04 18:31In the Linux kernel, the following vulnerability has been resolved:
ipv6: seg6: clear IPv4 control block on IPIP decapsulation
End.DX4 and End.DT4 decapsulate an IPv4 packet through decap_and_validate() and send it directly to IPv4 routing. The inner packet therefore bypasses ip_rcv_core(), which normally clears IPCB before IPv4 interprets skb->cb.
The skb instead retains IP6CB data from the outer packet. IP6CB and IPCB use the same skb->cb storage, so IP6CB(skb)->lastopt overlaps IPCB(skb)->opt.optlen and srr, while IP6CB(skb)->nhoff overlaps rr and ts.
The sender can make the stale optlen byte nonzero with a valid outer extension-header chain. The reproducers put an eight-byte Destination Options header immediately after the 40-byte IPv6 header and before the Segment Routing Header. ipv6_destopt_rcv() records the sender-controlled Destination Options offset in both lastopt and nhoff, setting them to 40. On the reproduced little-endian x86-64 kernel, IPv4 therefore sees optlen = 40 and rr = 40.
Both tcp_v4_save_options() and __ip_options_echo() skip option copying when optlen is zero. Here optlen is 40, so the TCP SYN path allocates room for 40 bytes of option data and calls __ip_options_echo(). The stale rr value makes that function read inner packet byte 41 as the Record Route option length. The reproducers set that sender-controlled byte to 255, so __ip_options_echo() copies 255 bytes into the 40-byte option-data area.
Separate End.DX4 and End.DT4 reproducers on the unpatched v7.2-rc5 kernel both produced:
BUG: KASAN: slab-out-of-bounds in __ip_options_echo() Write of size 255
The relevant End.DX4 call path is:
__ip_options_echo tcp_v4_route_req tcp_conn_request tcp_v4_conn_request tcp_rcv_state_process tcp_v4_do_rcv tcp_v4_rcv ip_protocol_deliver_rcu ip_local_deliver_finish ip_local_deliver input_action_end_dx4_finish input_action_end_dx4
The relevant End.DT4 call path is:
__ip_options_echo tcp_v4_route_req tcp_conn_request tcp_v4_conn_request tcp_rcv_state_process tcp_v4_do_rcv tcp_v4_rcv ip_protocol_deliver_rcu ip_local_deliver_finish ip_local_deliver input_action_end_dt4
tcp_v4_save_options() is inlined into the tcp_v4_route_req() path, so it does not appear as a separate frame.
When decap_and_validate() handles IPPROTO_IPIP, save the ingress interface from IP6CB, clear IPCB, and restore the saved value. Doing this in the common decapsulation path covers End.DX4, End.DT4, and End.DT46's IPv4 arm.
Use IP6CB(skb)->iif rather than skb->skb_iif. These actions run after l3mdev processing, which can replace skb_iif with the L3 master; IP6CB iif still records the receiving interface set at IPv6 ingress.
{
"affected": [],
"aliases": [
"CVE-2026-80840"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-09-04T16:18:12Z",
"severity": null
},
"details": "In the Linux kernel, the following vulnerability has been resolved:\n\nipv6: seg6: clear IPv4 control block on IPIP decapsulation\n\nEnd.DX4 and End.DT4 decapsulate an IPv4 packet through\ndecap_and_validate() and send it directly to IPv4 routing. The inner\npacket therefore bypasses ip_rcv_core(), which normally clears IPCB\nbefore IPv4 interprets skb-\u003ecb.\n\nThe skb instead retains IP6CB data from the outer packet. IP6CB and\nIPCB use the same skb-\u003ecb storage, so IP6CB(skb)-\u003elastopt overlaps\nIPCB(skb)-\u003eopt.optlen and srr, while IP6CB(skb)-\u003enhoff overlaps rr and\nts.\n\nThe sender can make the stale optlen byte nonzero with a valid outer\nextension-header chain. The reproducers put an eight-byte Destination\nOptions header immediately after the 40-byte IPv6 header and before the\nSegment Routing Header. ipv6_destopt_rcv() records the sender-controlled\nDestination Options offset in both lastopt and nhoff, setting them to\n40. On the reproduced little-endian x86-64 kernel, IPv4 therefore sees\noptlen = 40 and rr = 40.\n\nBoth tcp_v4_save_options() and __ip_options_echo() skip option copying\nwhen optlen is zero. Here optlen is 40, so the TCP SYN path allocates\nroom for 40 bytes of option data and calls __ip_options_echo(). The\nstale rr value makes that function read inner packet byte 41 as the\nRecord Route option length. The reproducers set that sender-controlled\nbyte to 255, so __ip_options_echo() copies 255 bytes into the 40-byte\noption-data area.\n\nSeparate End.DX4 and End.DT4 reproducers on the unpatched v7.2-rc5\nkernel both produced:\n\n BUG: KASAN: slab-out-of-bounds in __ip_options_echo()\n Write of size 255\n\nThe relevant End.DX4 call path is:\n\n __ip_options_echo\n tcp_v4_route_req\n tcp_conn_request\n tcp_v4_conn_request\n tcp_rcv_state_process\n tcp_v4_do_rcv\n tcp_v4_rcv\n ip_protocol_deliver_rcu\n ip_local_deliver_finish\n ip_local_deliver\n input_action_end_dx4_finish\n input_action_end_dx4\n\nThe relevant End.DT4 call path is:\n\n __ip_options_echo\n tcp_v4_route_req\n tcp_conn_request\n tcp_v4_conn_request\n tcp_rcv_state_process\n tcp_v4_do_rcv\n tcp_v4_rcv\n ip_protocol_deliver_rcu\n ip_local_deliver_finish\n ip_local_deliver\n input_action_end_dt4\n\ntcp_v4_save_options() is inlined into the tcp_v4_route_req() path, so\nit does not appear as a separate frame.\n\nWhen decap_and_validate() handles IPPROTO_IPIP, save the ingress\ninterface from IP6CB, clear IPCB, and restore the saved value. Doing\nthis in the common decapsulation path covers End.DX4, End.DT4, and\nEnd.DT46\u0027s IPv4 arm.\n\nUse IP6CB(skb)-\u003eiif rather than skb-\u003eskb_iif. These actions run after\nl3mdev processing, which can replace skb_iif with the L3 master;\nIP6CB iif still records the receiving interface set at IPv6 ingress.",
"id": "GHSA-56f8-x2fw-63m3",
"modified": "2026-09-04T18:31:28Z",
"published": "2026-09-04T18:31:28Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-80840"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/0e3f01fe2e704e76af4385b8a1742641885a191c"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/10fd1a8f58ac619a9e251f2858e2e2c8fd6cd667"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/3e4476e58343fb8f2fffced9e22d935376b17aaf"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/44930446dde45a7a90fe1446fa38eb0e2c561646"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/9039e4f3e1c0ffe2b575b655b3f58fdd10f7e40c"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/bf1c1151560d11036a144d917fa4c131831342d7"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/eb0f422487228e140f3d609b032ac61aedcab8fa"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/f4be3b391265e24c7720fc867c50062b436acf33"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/f52f1e75716d2ee49e013edf204ac92337c72fd8"
}
],
"schema_version": "1.4.0",
"severity": []
}
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