In 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.
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.
Metrics
Affected Vendors & Products
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Fixes
Solution
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Workaround
No workaround given by the vendor.
References
History
Fri, 04 Sep 2026 19:45:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Weaknesses | CWE-787 |
Fri, 04 Sep 2026 16:00:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Description | In 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. | |
| Title | ipv6: seg6: clear IPv4 control block on IPIP decapsulation | |
| First Time appeared |
Linux
Linux linux Kernel |
|
| CPEs | cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* | |
| Vendors & Products |
Linux
Linux linux Kernel |
|
| References |
|
|
Projects
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MITRE
Status: PUBLISHED
Assigner: Linux
Published:
Updated: 2026-09-04T15:54:50.403Z
Reserved: 2026-08-26T14:34:25.796Z
Link: CVE-2026-80840
Vulnrichment
No data.
NVD
Status : Received
Published: 2026-09-04T16:18:12.440
Modified: 2026-09-04T16:18:12.440
Link: CVE-2026-80840
Redhat
No data.
OpenCVE Enrichment
Updated: 2026-09-04T19:30:04Z
Weaknesses