CVE-2026-74612

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

veth: fix skb length accounting after XDP frag adjustment

veth exposes non-linear skb fragments through an xdp_buff. If an XDP

program adjusts the fragment area, vethxdprcv_skb() copies

xdpfragssize back to skb->data_len but leaves skb->len containing the

old fragment contribution.

After a fragment shrink, this makes skb_headlen() larger than the actual

linear area. In the reproduced UDP receive path, _skbdatagram_iter()

copied 1024 bytes past the actual linear tail to userspace, starting at

struct skbsharedinfo. The copied bytes included the affected skb's

nrfrags, xdpfrags_size, and a kernel pointer from

skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same

amount and truncated at the end.

Subtract the old datalen before replacing it and add the new datalen

afterwards, keeping skb->len and skb->data_len synchronized.

Additionally, bpfxdppulldata() can advance dataend while leaving

frags present. The skb is then still non-linear, so the old

_skbput(skb, off) triggers SKBLINEARASSERT().

Use skbsettail_pointer() and update skb->len explicitly instead,

following bpfprogrungenericxdp(). Unlike _skbput(),

skbsettail_pointer() does not require a linear skb.

A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by

1024 bytes from its fragment area. Before the fix, all 10 runs produced

corrupted payloads. After the fix, all 10 runs matched the expected

payload exactly. A forced-tailroom reproducer also exercises

bpfxdppull_data() with frags still present; the old code triggers

SKBLINEARASSERT(), while this fix passes 10/10 runs.