CVE-2026-98954

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

net: always dissect GSO packets in virtio net hdr to skb()

Commit 9e8db5913264 ("net: avoid false positives in untrusted gso validation") added a '&& skb->network header' check before flow-dissecting GSO packets without VIRTIO NET HDR F NEEDS CSUM in virtio net hdr to skb(), because some callers (such as tun get user(), tun xdp one(), virtnet receive done(), and raw verify header()) called virtio net hdr * to skb() before initializing skb->network header and skb->dev.

Because alloc skb() and build skb around() zero-initialize skb->network header to 0 (unlike mac header and transport header which are initialized to ~0U), those four callers always had skb->network header == 0 and bypassed flow dissection in virtio net hdr to skb(). More generally, skb->network header is an offset from skb->head (where 0 is also a valid offset whenever skb headroom(skb) is 0), not a boolean flag.

Whenever the 'if (gso type && skb->network header)' branch was skipped, the fallback 'else if (gso type)' only pulled nh min len + thlen (40 bytes for TCPv4) without dissecting the packet, without validating ip proto or n proto, and without setting skb->transport header.

If the packet has a malformed network header, it is not rejected and a subsequent skb probe transport header() also fails, leaving skb->transport header at ~0U (0xffff). Similarly, if an IPv4 packet carries IP options (ihl > 5) or an IPv6 packet carries extension headers, pulling only nh min len + thlen can leave the TCP header outside skb->head. In both cases, tcp hdrlen(skb) in skb gso transport seglen() reads out-of-bounds:

BUG: KASAN: slab-out-of-bounds in skb gso transport seglen Read of size 2 by task poc/133 skb gso transport seglen (net/core/gso.c:155) skb gso validate mac len (net/core/gso.c:270) tbf enqueue (net/sched/sch tbf.c:260) dev qdisc enqueue (net/core/dev.c:4227) dev queue xmit (net/core/dev.c:4884)

In addition, checking virtio net hdr match proto() only inside 'if (!skb->protocol)' before flow dissection both skipped validation when skb->protocol was pre-set by the caller and rejected VLAN-tagged frames whose outer L2 protocol is ETH P 8021Q or ETH P 8021AD.

Fix this by: