CVE-2026-80590

Status: UPDATED  |  Advisory ID: CVE-2026-80590

Key Details

Affected Products, Subsystems & Sectors

What to Know

In the Linux kernel, the following vulnerability has been resolved:

inet: frags: strip GSO state from fragments before reassembly

A virtio_net_hdr (tun/tap, or AF_PACKET with PACKET_VNET_HDR) can mark

an IPv4 or IPv6 fragment as GSO; nothing relates gso_type to frag_off.

inet_frag_reasm_prepare()/inet_frag_reasm_finish() keep the first

fragment's skb as the head of the reassembled datagram, including its

shinfo->gso_size/gso_type/gso_segs, and chain the remaining fragments

on frag_list with whatever linear/paged layout they arrived with.

After ip_defrag() (ip_local_deliver(), nf_defrag_ipv4, ...) the

reassembled skb therefore still claims to be GSO (SKB_GSO_DODGY), and

the next software segmentation point - udp_rcv_segment() on local

delivery, validate_xmit_skb(), or the ip_finish_output_gso() slow

path - hands it to skb_segment(). skb_segment()'s frag_list walk

assumes GRO-shaped input and hits one of its BUG_ON()s. Two writes to

a tap by an unprivileged user in its own userns are enough:

kernel BUG at net/core/skbuff.c:4899!

Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI

CPU: 0 UID: 1000 PID: 82 Comm: poc Not tainted 7.2.0-pentest+ #2

RIP: 0010:skb_segment+0x20ca/0x48b0

Call Trace:

<TASK>

__udp_gso_segment+0x29a/0x27d0

udp4_ufo_fragment+0x458/0x6c0

inet_gso_segment+0x429/0x1340

skb_mac_gso_segment+0x233/0x4f0

__skb_gso_segment+0x308/0x660

udp_queue_rcv_skb+0x440/0xad0

udp_unicast_rcv_skb+0xc7/0x2c0

udp_rcv+0x16ce/0x2260

ip_protocol_deliver_rcu+0x197/0x2d0

ip_local_deliver+0x430/0x690

ip_rcv+0x16f/0x1f0

__netif_receive_skb_one_core+0x15e/0x1c0

__netif_receive_skb+0x1e/0x110

netif_receive_skb+0xf6/0x5c0

tun_rx_batched.isra.0+0x3ab/0x790

tun_get_user+0x17c3/0x3550

tun_chr_write_iter+0xba/0x1b0

vfs_write+0x646/0x1130

</TASK>

Kernel panic - not syncing: Fatal exception in interrupt

This runs with BH disabled, so it is a panic rather than an oops. The

same is reachable with CAP_NET_RAW in a netns where a defrag point

precedes a GSO point, and from a guest whose VMM forwards

virtio_net_hdr to a tap. The SKB_GSO_DODGY frag_list checks added by

commit 3dcbdb134f32 ("net: gso: Fix skb_segment splat when splitting

gso_size mangled skb having linear-headed frag_list") and by

commit 9e4b7a99a03a ("net: gso: fix panic on frag_list with mixed head

alloc types") do not cover it: page-backed heads skip them, and kmalloc

heads skip them when gso_size == skb_headlen(head), which the sender

controls.

An skb entering a frag queue is an IP fragment by definition and

cannot legitimately carry GSO state: GRO does not merge fragments and

the stack segments before it fragments, so only untrusted sources are

affected. This has been reachable since

commit f43798c27684 ("tun: Allow GSO using virtio_net_hdr"), the first

path that let userspace attach GSO metadata to an IP fragment. Reset

the GSO fields of every fragment as it is queued, in

inet_frag_queue_insert(), which IPv4, IPv6, nf_conntrack_reasm and

6lowpan reassembly share; then neither the head nor the frag_list

members of the reassembled skb carry them (the members matter too:

the ip_do_fragment()/ip6_fragment() fast paths send them out as they

are). The head may remain CHECKSUM_PARTIAL; that is already accepted

on receive and resolved by skb_checksum_help() in

ip_do_fragment()/ip6_fragment() on forward.

Tested on top of net.git (dc4b95b8fee9), x86_64: the tap reproducer

above, two further IPv4 frag_list geometries that reach

BUG_ON(i >= nfrags) and BUG_ON(!list_skb->head_frag), and an IPv6

fragment-header variant (udp6_ufo_fragment()) each panic the unpatched

kernel; with this patch all four datagrams are delivered intact and

nothing is logged. (NVD)

What to Do

Monitor Linux's web page for any future patch releases.

References