In 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.
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.
Metrics
Affected Vendors & Products
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Fixes
Solution
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Workaround
No workaround given by the vendor.
References
History
Fri, 23 Oct 2026 17:30:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Weaknesses | CWE-197 |
Fri, 23 Oct 2026 12:30:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Description | In 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. | |
| Title | flow_dissector: avoid u16 truncation of skb->len when computing thoff | |
| 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: 2026-10-23T12:19:15.287Z
Reserved: 2026-09-25T10:25:14.382Z
Link: CVE-2026-98955
Vulnrichment
No data.
NVD
Status : Received
Published: 2026-10-23T13:17:16.740
Modified: 2026-10-23T13:17:16.740
Link: CVE-2026-98955
Redhat
No data.
OpenCVE Enrichment
Updated: 2026-10-23T17:15:07Z
Weaknesses